Foundations of Science

Brigham Young University - Idaho

December 20, 2010

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c©2010 by Brigham Young University - Idaho. All rights reserved.

Contents

0.1 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x0.2 Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi0.3 How To Use This Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii0.4 Syllabus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix

0.4.1 Course Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix0.4.2 Grading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx0.4.3 Attendance and Participation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx0.4.4 Preparing for Class: Pre-class Readings and Quizzes . . . . . . . . . . . . . . . . . . . . . xxi0.4.5 Learning Groups and Weekly Participation Reports . . . . . . . . . . . . . . . . . . . . . xxi0.4.6 Homework Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii0.4.7 Exams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii0.4.8 Getting the Grade that You Really Want . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiii0.4.9 Other Miscellaneous Policies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiii0.4.10 Course Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxv

0.5 I-learn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxvi0.6 The BYU-Idaho Learning Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxvii0.7 The BYU-Idaho Honor Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxix0.8 Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxii

1 What is Science? 11.1 Truth: The Foundation of Correct Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Mormon Scientists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.3 The Characteristics of Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4 The Scientific Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.5 Climate Change: A Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2 The Universe 272.1 The Scale of the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.2 Time and Intuition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.3 Early Cosmological Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.4 The Big Bang Model: Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.5 The Big Bang Model: Part II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

3 Atoms 593.1 Where Do Atoms Come From? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.2 Atomic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703.3 Bonding and the Periodic Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.4 Radioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4 Earth 974.1 Uniformitarianism and Relative Dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 984.2 Absolute Dating and the Age of the Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1034.3 Plate Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1134.4 Earth Changes! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

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iv CONTENTS

5 Life 1315.1 Observations of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1325.2 Origin of Species: Early Ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1475.3 Genetics and DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1555.4 Molecular Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1625.5 Human Evolution I: Anatomical Evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1695.6 Human Evolution II: Anatomy and Genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

A Views on Science and Religion 207A.1 Reconciling Scientific and Religious Views of Nature . . . . . . . . . . . . . . . . . . . . . . . . . 207A.2 Making Sense of Scientific and Religious Assertions . . . . . . . . . . . . . . . . . . . . . . . . . . 210A.3 The BYU Evolution Packet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

B Study Helps 221B.1 How to Take Notes Effectively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221B.2 Good Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221B.3 Concept Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

C Image Licensing 223C.1 GNU Free Documentation License 1.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223C.2 Creative Commons Attribution 2.0 License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

Bibliography 235

Glossary 242

Index 243

List of Figures

1 Implementing the Learning Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxvii2 BYU-Idaho Learning Model: Student Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxviii3 Realizing the Mission: Developing Disciple-leaders . . . . . . . . . . . . . . . . . . . . . . . . . . xxix4 The Honor Code Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxxi

1.1 Candida Albicans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 An example of a model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.3 The process of science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.4 World ecologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211.5 Types of learners/scholars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.1 The Hubble Deep Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.2 An artist’s rendering of the Milky Way galaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3 The U.S.S. Constitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.4 A wristwatch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.5 A “natural” clock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.6 The evolution of the universe after the CMB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.7 Massive objects warp space time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.8 Time elapsed photo of the stars at night . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.9 A representation of the Geocentric model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462.10 Epicycles as a means of refining the Geocentric model . . . . . . . . . . . . . . . . . . . . . . . . 472.11 Epicycles associated with Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.12 A depiction of the Heliocentric model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.13 Doppler shifting of light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532.14 Redshift in the spectrum from a supercluster of distant galaxies (BAS11) . . . . . . . . . . . . . 542.15 The Wilkinson Microwave Anisotropy Probe (WMAP) image of the cosmic microwave background 55

3.1 A high resolution transmission electron microscopy image of atoms . . . . . . . . . . . . . . . . . 593.2 A depiction of an atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.3 The PP-I cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.4 The triple alpha reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.5 The CNO cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.6 s-process nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.7 A cathode ray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.8 The plum pudding model of the atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.9 The Rutherford gold foil experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733.10 The planetary model of the atom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.11 The Bohr model of the atom. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.12 Electron probability clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753.13 John Newland’s octaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.14 Dmitri Mendeleyev’s periodic table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 803.15 Dmitri Mendeleyev’s periodic table, with noble gasses . . . . . . . . . . . . . . . . . . . . . . . . 813.16 The periodic table of the elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

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3.17 Molecular structure of a nucleotide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.18 Alpha decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883.19 Beta minus decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.20 Beta plus decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.21 Electron capture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.22 Gamma decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.23 Radioactive decay series for 238U . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 923.24 Amount of parent and daughter nuclei as a function of elapsed half lives . . . . . . . . . . . . . . 943.25 Daughter-to-Parent ratio as a function of elapsed half lives . . . . . . . . . . . . . . . . . . . . . . 95

4.1 Earth, as seen from the Apollo 8 spacecraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 974.2 Geologic cross section from Glacier National Park . . . . . . . . . . . . . . . . . . . . . . . . . . . 1004.3 The geologic time scale, as determined by relative dating . . . . . . . . . . . . . . . . . . . . . . . 1024.4 Seasonal rings of a tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1054.5 Dendrochronology of overlapping tree rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1054.6 Seasonal layers in a glacier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1064.7 The geologic timescale, with times assigned by radiometric dating . . . . . . . . . . . . . . . . . . 1074.8 Radiocarbon calibration curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.9 Concordia curve for Uranium-Lead dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1114.10 Distribution of fossil record across ocean basins . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1154.11 A simplified representation of the interior of the Earth . . . . . . . . . . . . . . . . . . . . . . . . 1174.12 Earth’s major tectonic plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1174.13 Illustration of tectonic boundary types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.14 Epicenter locations of world earthquakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.15 The “geo-hour” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1244.16 The early precambrian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1254.17 Mesozoic life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5.1 Cyanobacteria, as seen under a microscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1315.2 Paleolithic paintings from the Lascaux caves, France . . . . . . . . . . . . . . . . . . . . . . . . . 1325.3 The hierarchy of life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345.4 The domains of life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355.5 A prokaryotic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1355.6 A eukaryotic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1365.7 An evolutionary tree of life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1375.8 An evolutionary history of modern elephants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395.9 Six extinct species of ancestral elephants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1405.10 Pakicetus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415.11 Ambulocetus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1415.12 Remingtonocetus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425.13 Protocetus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1425.14 Dorudon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1435.15 Basilosaurus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1435.16 The evolution of whale species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.17 Vestigial structures in modern whales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.18 “The Creation of Adam” by Michelangelo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1485.19 Fossil trilobites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.20 Georges Cuvier’s sketches of elephant and mammoth jaws . . . . . . . . . . . . . . . . . . . . . . 1505.21 The process of natural selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1535.22 A Punnett square . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1575.23 The double helix shape of DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585.24 The molecular structure of DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1585.25 Semi-conservative pattern of DNA replication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1605.26 DNA-mRNA transcription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

LIST OF FIGURES vii

5.27 Protein synthesis inside a ribosome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1645.28 The universal genetic code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1655.29 Phyletic gradualism v. punctuated equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1685.30 Knee and pelvis joints in primate species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1725.31 Ape feet compared to human feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1735.32 A phylogenetic tree or cladogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1745.33 Phylogenetic trees for hominid species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1765.34 Sahelanthropus tchadensis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1775.35 Ardipithecus ramidus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1795.36 Australopithecus afarensis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1815.37 Homo habilis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1835.38 Reconstructed skull of A. afarensis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1835.39 Homo ergaster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1855.40 Reconstruction of Homo ergaster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1855.41 Homo heidelbergensis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1875.42 Homo sapiens-neanderthal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1895.43 Comparison of Homo sapiens and Homo sapiens-neanderthal skulls. . . . . . . . . . . . . . . . . 1915.44 Whale skeletons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1955.45 Homologies in skeletal forelimbs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1995.46 Cervical vertebrae in giraffes and humans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2005.47 Phylogenetic tree of life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2015.48 The GULO gene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2025.49 Cladogram indicating the timing of the GULO mutation . . . . . . . . . . . . . . . . . . . . . . . 2035.50 Fetal development of human and chimp skulls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204

viii LIST OF FIGURES

List of Tables

1 Generic course schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxv

1.1 Attributes of learners/scholars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.1 Powers of ten greater than zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.2 Powers of ten less than or equal to zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.3 Original and scaled dimensions of the U.S.S. Constitution . . . . . . . . . . . . . . . . . . . . . . 32

3.1 Elements and atomic numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.2 Discovery of the Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783.3 Particles emitted in radioactive decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 913.4 Penetration and RBE of radioactive decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

4.1 Major isotopes used in radiometric dating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1064.2 Factors affecting global climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

5.1 Traits shared by all living things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.2 Characteristics of elephants and related species . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1385.3 Evolutionary trends in elephant species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395.4 Major anatomical trends in whale evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1455.5 Anatomical traits of the common ancestor of hominids and chimps . . . . . . . . . . . . . . . . . 1775.6 Anatomical traits of S. tchadensis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1785.7 Anatomical traits of A. ramidus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1805.8 Anatomical traits of A. afarensis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825.9 Anatomical traits of H. habilis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1845.10 Anatomical traits of H. ergaster. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1865.11 Anatomical traits of H. heidelbergensis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1885.12 Anatomical traits of H. neanderthalensis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1905.13 Anatomical traits of H. sapiens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1925.14 Vestigial structures in humans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1965.15 Similarity between DNA of humans and other selected species . . . . . . . . . . . . . . . . . . . . 1985.16 Primate pseudogenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

ix

x LIST OF TABLES

0.1 Acknowledgments

The material found in this text represents a great deal of time and effort from several BYU-Idaho faculty. Inparticular, we wish to acknowledge the following:

• John S. Griffith, Biology

• Evan D. Hansen, Physics

• Alan R. Holyoak, Biology

• Kevin Kelley, Physics

• Brian J. Lemon, Chemistry

• Christopher M. Lowry, Psychology

• Daniel K. Moore, Geology

• Dave Stricklan, Biology

0.2. PREFACE xi

0.2 Preface

A public that does not understand how science works can, all too easily, fall prey to those ignora-muses...who make fun of what they do not understand, or to the sloganeers who proclaim scientists tobe the mercenary warriors of today and the tools of the military. The difference...between...understandingand not understanding...is also the difference between respect and admiration on the one side, andhate and fear on the other.

- Isaac Asimov

The Lower Case “s” Scientist

Having a working knowledge of what science is and how it works will provide great benefits to many areas of yourlife: as a parent, voting citizen, member of a professional community, or member of the church. That’s why everystudent who comes to BYU-Idaho studies science. The Science Foundations and Issues in Science courses alongwith courses from the other foundations categories are designed to add strength and depth to your educationand to your testimony. Knowledge enhances your development as a disciple of Jesus Christ and simultaneouslypromotes development of attributes of Christ-like leadership. BYU-Idaho President Kim B. Clark refers to thatkind of leadership as lower case “l” leadership [Clark07]. A lower case “l” leader is someone who inspires thosearound them to do what is right whether or not that person is in a formal leadership position.

This Science Foundations course is similarly designed to help you move forward in your personal developmentsince it is designed to help you become a lower case “s” scientist. Being a lower case “s” scientist does notnecessarily mean that you will be a professional scientist, rather it means that you will develop an awarenessof and an interest in scientific topics and issues. It also means that you will learn and apply the critical andcreative thinking skills used by scientists. A great example of a lower case “s” scientist is Benjamin Franklin.

Benjamin Franklin was one of the most prominent founding fathers of the United States of America. He, likemany of you, came from a humble home, but unlike you he had little opportunity for formal education. After onlya few years Franklin was forced to leave school and become a printer’s apprentice3. Franklin spent his workingyears as a writer, printer, newspaper publisher, and postmaster. Throughout his life, however, he continued tolearn. He learned all he could about everything he could (he is a good example of acting on the instructiongiven in D&C 88: 78-79). He continued his education primarily by reading, writing and interacting with otherinformed people. He had a deep interest in science4, and he developed an uncommon ability to connect withpeople at every socio-economic level. Franklin retired from his profession at the age of 42 and looked forward topursuing his scientific interests full-time.

Franklin pursued his scientific interests full-time for only a few years, and his scientific work covered animpressive range of disciplines. Some of his accomplishments include the invention of the Franklin stove, bifocalglasses, improved methods of fertilization for agriculture, a description of the association between lightningand electricity, development of the lightning rod, an improved design for street lamps, a chart of the GulfStream, the development of the first battery and more3. His work on electricity and lightning rods made itsway to England and France, and made him perhaps the best known living scientist of his day4. By the way,Franklin also established the first lending library in America, the first police force and the first fire departmentin Philadelphia, and he founded the University of Pennsylvania5.

After only a few years of full-time scientific work Franklin set science aside in order to accept appointmentsas a public servant. He served as a statesman to England and France, a member of the Continental Congress,a signer of the Declaration of Independence, and was a prominent voice for the freedom and unification of theAmerican colonies. Still, with all that going on Franklin maintained contacts with scientific colleagues andcarried out scientific experiments. One author stated that “Franklin never gave up being a scientist; what hedid abandon was the career of a full-time scientist...and that...science was a subject to which he always gladlyturned in every odd day or hour of leisure, even in the midst of exacting duties and heavy responsibilities of hispublic career4.”

Do you need to develop the prominence of someone like Benjamin Franklin to become a small “s” scientist?Of course not, but we hope you will develop some of the attributes of Benjamin Franklin: become a life-longlearner with a continuous thread of interest in science.

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Truth is Learned Through Religion and Science

We are confident that each of you has had spiritual experiences in which you learned pure truth by personalrevelation. Those experiences may have occurred while you were praying, studying the scriptures, having a seriousdiscussion, or some at other time while you were pondering a particular question, topic, or problem. And, inthe middle of one of those acts you had an “a-ha” or other strong impression moment in which you received astrong spiritual confirmation pertaining to the topic or question at hand or by which you gained understanding.Some of the truths you received by revelation may have included things like a confirmation of the divinity andreality of our Heavenly Father and His son Jesus Christ, the reality and significance of the atonement of JesusChrist, that the Book of Mormon is true, that Joseph Smith, Jr., is a prophet, that your parents and family loveyou, etc. Those are the kinds of things that often form the core of our personal testimonies. They are eternaltruths that are obtained only by revelation, and upon which we should build our testimonies. Always hold fastto those truths (2 Timothy 3:14).

You may also have gained instruction, spiritual confirmation or illumination regarding a secular challenge orquestion. An experience like that was shared by Elder Russell M. Nelson, a heart surgeon by profession, regardinga particularly challenging operation that had never before been attempted. This experience was related in the173rd annual general conference of the Church of Jesus Christ of Latter-Day Saints in April 2003 [Nelson03].

Elder Nelson’s experience demonstrates that we can receive revelation and inspiration to learn truths aboutsecular things. It should be emphasized that only someone who was prepared to understand the message sent toElder Nelson could be edified by that message. That means that the knowledge needed to solve the previouslyinsurmountable problem was provided by revelation, Elder Nelson’s professional training and experience madeit possible for him to recognize the value and truth of that inspiration.

Like Elder Nelson, we too must obtain all the knowledge and experience we can so that when we need andreceive revelation we will be able to understand it and use it to bless the lives of others. In other words, it isimportant to realize there is an enormous amount of truth and knowledge that we must obtain that we cannotcount on receiving by personal revelation. That fact is underscored by Elder Marion G. Romney who stated, “Ibelieve in study. I believe that men learn much through study. As a matter of fact, it has been my observationthat they learn little concerning things as they are, as they were, or as they are to come without study. I alsobelieve, however, and know, that learning by study is greatly accelerated by faith [Romney68].”

Elder Rex E. Lee likewise taught that, “No matter how righteous you are, no matter how carefully youcultivate the companionship of the Holy Ghost, there are vast amounts of knowledge which you need to acquireand which you are not going to receive through revelation [Lee82].”

Does that mean that our spiritual and secular learning are separate and distinct when it comes to revelationand inspiration? No! Elder Henry B. Eyring taught, “[Our] faith will largely determine whether we learn hereby study and also by faith. As we do, we will attain academic excellence. We will not attain academic excellencewithout that faith of yours as students and those that follow to learn by study and by faith[Eyring01].”

In other words, we can maximize our learning and understanding as we combine study and faith. Since themajority of you already have a significant store of spiritual experiences there is little need to comment further onthat aspect of obtaining truth. There is, however, a need to consider what we should learn, and then explore howwe know what we know through scientific investigation. In the Doctrine and Covenants we are given instructionregarding what we should learn:

78. Teach ye diligently and my grace shall attend you, that you may be instructed more perfectly intheory, in principle, in doctrine, in the law of the gospel, in all things that pertain unto the kingdomof God, that are expedient for you to understand;

79. Of things both in heaven and in the earth, and under the earth; things which have been, thingswhich are, things which must shortly come to pass; things which are at home, things which are abroad;the wars and the perplexities of the nations, and the judgments which are on the land; and a knowl-edge also of countries and of kingdoms-

80. That ye may be prepared in all things when I shall send you again to magnify the calling whereuntoI have called you, and the mission with which I have commissioned you.

- Doctrine and Covenants 88:78-80

0.2. PREFACE xiii

When you look at the list of topics in the verses above we are directed to learn about, well, just abouteverything there is. That is one of the great strengths of the gospel, and of the Church of Jesus Christ ofLatter-Day Saints. It is a religion that encourages the people to learn all they can. That includes things welearn via science.

Does that mean that we will learn everything and understand how everything fits together while we are inthis life? Of course not, no one can know everything during the brief time we have here, but we should still doour best to learn all we can while we are here. Because our knowledge and learning is imperfect there will betopics that may appear to contain truth, but which we cannot reconcile with other truths that we have. In thatcase we urge patience. Do not toss the baby out with the bath when a perceived conflict appears. Keep thefaith, hold to the eternal truths you have acquired, and know that someday we will know how everything worksand how everything fits together into one complete understanding of truth. We know that will happen becausewe are taught that all things will be revealed when the Savior returns:

32. Yea, verily I say unto you, in that day when the Lord shall come, he shall reveal all things-

33. Things which have passed, and hidden things which no man knew, things of the earth, by whichit was made, and the purpose and the end thereof-

34. Things most precious, things that are above, and things that are beneath, things that are in theearth, and upon the earth, and in heaven.

- Doctrine and Covenants 101:32-34

And, like Elder Nelson’s experience, we will be prepared to receive and understand this information after wehave done our best to learn all we can.

Conflicts Between Science and Religion

As you begin your experience in FDSCI 101 you may wonder how science and religion mesh with each other.Can they? Should they? Will they? As you ponder these questions you need to remind yourself that you areon a quest to find and embrace truth as part of your experience in this life. Elder Richard G. Scott taught thattwo ways we can find truth are the scientific method and inspiration [Scott07]. Of course the methods used tofind truth via these two methods differ, but truths obtained by both methods increase our understanding of theuniverse and our place in it.

There are, alas, some people in the world who assert that science and religion are antagonists. People ofthis opinion believe that someone can be a person of science or a person of faith, but not both. They promotethe ideas that a person of science has no need for religion, or that a person of religion faces great personal riskof losing their faith by considering the things discovered by science. Rest assured that this line of reasoningrepresents a false dichotomy, and you do not have to choose between science and religion. You can, in fact, enjoythe luxury of obtaining truths obtained by inspiration and by scientific investigation, and that by doing so yourtestimony can be strengthened.

One of the preeminent scientists of the mid-20th century, the rocket scientist Werner von Braun, provides aninteresting perspective on the topic of science and faith:

Science and faith are the two dominant forces in this century. We must try to understand their natureif we are to comprehend some of the most serious problems of the era in which we live.

The mainspring of science is curiosity. Since time immemorial, there have always been men andwomen who desire to know what was under the rock, beyond the hills, across the oceans. Thisrestless breed now wants to know what makes an atom work, through what process life reproducesitself, or what is on the far side of the moon.

But, also, there would not be a single great accomplishment in the history of mankind without faith.Any man who strives to accomplish something needs a degree of faith in himself. And wheneverhe takes on a challenge that requires more moral strength than he can muster with his own limitedmental and spiritual resources, he needs faith in God.

xiv LIST OF TABLES

One of the most crucial issues of our time lies in the fact that modern science, along with miracledrugs and communications satellites, has also produced nuclear bombs. It cannot be denied thatscience has failed to provide a practical answer on how to cope with them. As a result, science andscientists have often been blamed for the desperate dilemma in which man finds himself today.

Science, by itself, has no moral dimension. The drug which cures when taken in small doses may killwhen taken in excess. The nuclear energies that produce cheap electrical power when harnessed ina nuclear reactor may kill when abruptly released in a bomb. Thus, it does not make sense to ask ascientist whether his poison or his nuclear energy is “good” or “bad” for mankind.

And, so, the realization that science is unable to control the possible abuse of the forces it has madeavailable, has led hundreds of millions in the world to a new interest in religion. This religious revivalshows that there is a widespread realization that in the nuclear age man has a desperate need forstronger ethical control of the immeasurable physical forces he has unleashed.

But many people find the churches, those old ramparts of faith, badly battered by the onslaught ofthree hundred years of scientific skepticism. This has led many to believe that science and religion arenot compatible, that “knowing” and “believing” cannot live side by side. Nothing could be fartherfrom the truth. Science and religion are not antagonists. On the contrary, they are sisters. Whilescience tries to learn more about the creation, religion tries to better understand the Creator. While,through science man tries to harness the forces of nature around him, through religion he tries toharness the forces of nature within him.

Science may not have a moral dimension. But I am certain that science, in its search for new insightsinto the nature of the creation, has produced new ethical values of its own. Most certainly sciencehas fostered veracity and humility. Again, it is a mark of all true science that its findings are validand objective for all times and all peoples; that these findings demand unconditional acceptance andthat once proved correct, they are universally embraced. If a man has ever come close to findingan answer to Pontius Pilate’s question, “What is truth?”, science has shown the way. Personally, Ibelieve in the ultimate victory of truth. I am confident that to the extent that we shall learn moreabout nature, we shall not only arrive at universally accepted scientific findings, but also at a set ofuniversally accepted rules and standards of human behavior.

The materialists of the nineteenth century and their Marxist heirs of the twentieth, tried to tell usthat, as science gives us more knowledge about creation, we could live without faith in a Creator.Yet, so far, with every new answer, we have discovered new questions. The better we understand theintricacies of the atomic structure, the nature of life, or the master plan for the galaxies, the morereason we have found to marvel at the wonder of God’s creation.

But our need for God is not based on awe alone. Man needs faith just as he needs food, water, or air.

With all the science in the world, we need faith in God, whenever faith in ourselves has reached itslimit. [Braun65] (Italics included in the original text)

Teachings of the Prophet Joseph Smith

As a member of the Church of Jesus Christ of Latter-Day Saints you should seek after knowledge of all things(D&C 88: 78-79). You can therefore be confident that seeking for truth obtained by inspiration and truthdiscovered by science is desirable. The prophet Joseph Smith, Jr. taught that all truth is part of the gospel ofJesus Christ, and that you are free to embrace all truth without limitation.

The following quotations are taken from chapter 22 of [Teachings07].“A man is saved no faster than he gets knowledge.”The gospel of Jesus Christ embraces all truth; the faithful accept the truths God has revealed and put aside

false traditions.“Mormonism is truth; and every man who embraces it feels himself at liberty to embrace every truth:

consequently the shackles of superstition, bigotry, ignorance, and priestcraft, fall at once from his neck; and hiseyes are opened to see the truth, and truth greatly prevails over priestcraft.... “...Mormonism is truth, in otherwords the doctrine of the Latter-day Saints, is truth.... The first and fundamental principle of our holy religionis, that we believe that we have a right to embrace all, and every item of truth, without limitation or without

0.2. PREFACE xv

being circumscribed or prohibited by the creeds or superstitious notions of men, or by the dominations of oneanother, when that truth is clearly demonstrated to our minds, and we have the highest degree of evidence ofthe same.”

In January 1843, Joseph Smith had a conversation with some people who were not members of the Church:“I stated that the most prominent difference in sentiment between the Latter-day Saints and sectarians was, thatthe latter were all circumscribed by some peculiar creed, which deprived its members the privilege of believinganything not contained therein, whereas the Latter-day Saints...are ready to believe all true principles that exist,as they are made manifest from time to time.”

“I cannot believe in any of the creeds of the different denominations, because they all have some things inthem I cannot subscribe to, though all of them have some truth. I want to come up into the presence of God,and learn all things; but the creeds set up stakes [limits], and say, ‘Hitherto shalt thou come, and no further’[Job 38:11]; which I cannot subscribe to.”

“I say to all those who are disposed to set up stakes for the Almighty, You will come short of the glory ofGod. To become a joint heir of the heirship of the Son, one must put away all his false traditions.”

“The great thing for us to know is to comprehend what God did institute before the foundation of the world.Who knows it? It is the constitutional disposition of mankind to set up stakes and set bounds to the works andways of the Almighty.... That which hath been hid from before the foundation of the world is revealed to babesand sucklings in the last days [see D&C 128:18].”

“When men open their lips against [the truth] they do not injure me, but injure themselves.... When thingsthat are of the greatest importance are passed over by weak-minded men without even a thought, I want to seetruth in all its bearings and hug it to my bosom. I believe all that God ever revealed, and I never hear of a manbeing damned for believing too much; but they are damned for unbelief.”

“When God offers a blessing or knowledge to a man, and he refuses to receive it, he will be damned. TheIsraelites prayed that God would speak to Moses and not to them; in consequence of which he cursed them witha carnal law.”

“I have always had the satisfaction of seeing the truth triumph over error, and darkness give way beforelight.”

Gaining knowledge of eternal truths is essential to obtaining salvation.“Knowledge is necessary to life and godliness. Woe unto you priests and divines who preach that knowledge is

not necessary unto life and salvation. Take away Apostles, etc., take away knowledge, and you will find yourselvesworthy of the damnation of hell. Knowledge is revelation. Hear, all ye brethren, this grand key: knowledge isthe power of God unto salvation.”

“Knowledge does away with darkness, suspense and doubt; for these cannot exist where knowledge is.... Inknowledge there is power. God has more power than all other beings, because He has greater knowledge; andhence He knows how to subject all other beings to Him. He has power over all.”

“As far as we degenerate from God, we descend to the devil and lose knowledge, and without knowledge wecannot be saved, and while our hearts are filled with evil, and we are studying evil, there is no room in our heartsfor good, or studying good. Is not God good? Then you be good; if He is faithful, then you be faithful. Add toyour faith virtue, to virtue knowledge, and seek for every good thing [see 2 Peter 1:5]...

“...A man is saved no faster than he gets knowledge, for if he does not get knowledge, he will be broughtinto captivity by some evil power in the other world, as evil spirits will have more knowledge, and consequentlymore power than many men who are on the earth. Hence it needs revelation to assist us, and give us knowledgeof the things of God.”

Joseph Smith taught the following in April 1843, later recorded in Doctrine and Covenants 130:18-19: “What-ever principle of intelligence we attain unto in this life, it will rise with us in the resurrection. And if a persongains more knowledge and intelligence in this life through his diligence and obedience than another, he will haveso much the advantage in the world to come.”

Joseph Smith taught the following in May 1843, later recorded in Doctrine and Covenants 131:6: “It isimpossible for a man to be saved in ignorance.”

We obtain knowledge of eternal truths through diligent study and prayer.George A. Smith, while serving in the First Presidency, reported: “Joseph Smith taught that every man

and woman should seek the Lord for wisdom, that they might get knowledge from Him who is the fountain ofknowledge; and the promises of the gospel, as revealed, were such as to authorize us to believe, that by takingthis course we should gain the object of our pursuit.”

xvi LIST OF TABLES

The Prophet Joseph Smith wrote the following to a man who had recently joined the Church: “You rememberthe testimony which I bore in the name of the Lord Jesus, concerning the great work which He has brought forthin the last days. You know my manner of communication, how that in weakness and simplicity, I declared toyou what the Lord had brought forth by the ministering of His holy angels to me for this generation. I pray thatthe Lord may enable you to treasure these things in your mind, for I know that His Spirit will bear testimonyto all who seek diligently after knowledge from Him.”

We gain knowledge of eternal truths a little at a time; we can learn all things as fast as we are able to bearthem.

“It is not wisdom that we should have all knowledge at once presented before us; but that we should have alittle at a time; then we can comprehend it.”

“When you climb up a ladder, you must begin at the bottom, and ascend step by step, until you arrive atthe top; and so it is with the principles of the gospel - you must begin with the first, and go on until you learnall the principles of exaltation. But it will be a great while after you have passed through the veil before you willhave learned them. It is not all to be comprehended in this world; it will be a great work to learn our salvationand exaltation even beyond the grave.”

Words in Season from the First Presidency

In summary, Joseph Smith taught that you should seek after and accept all truth. Even so, there have beentimes in Church history when there was much discussion and even heated debate about the Church’s positionon science. After repeated queries on this topic were sent to the First Presidency, the First Presidency releasedthe following statement that clarifies the Church’s position with respect to questions raised by science:

Diversity of opinion does not necessitate intolerance of spirit, nor should it embitter or set rationalbeings against each other. The Christ taught kindness, patience, and charity.

Our religion is not hostile to real science. That which is demonstrated, we accept with joy; but vainphilosophy, human theory and mere speculations of men, we do not accept nor do we adopt anythingcontrary to divine revelation or to good common sense. But everything that tends to right conduct,that harmonizes with sound morality and increases faith in Deity, finds favor with us no matter whereit may be found. [Words10]

This statement from the First Presidency (which represents the official position of the Church on this matter)indicates that the Church is not hostile to “real science” and “that which is demonstrated, we accept with joy.”At the same time they stated that you are not obligated to accept “vain philosophy, human theory, and merespeculations of men,” that is, ideas that in our modern scientific terminology represent untested or unfoundedhypotheses.

So as you embark on your study of some of the truths discovered by science you should be confident thatthe things we discuss in FDSCI 101 represent scientific discoveries that fall into the category of things that are“demonstrated,” and are things that we should “accept with joy.” This is true, since one of the goals of thiscourse is to give you opportunities to complement your current level of spiritual knowledge and understandingwith an increased level of understanding of scientific truths. This process of learning and increased understandingof truth can strengthen your testimony, and allows the shackles of superstition and unfounded tradition to fallaway, opening up a more complete understanding in your heart and in your mind.

As you exercise faith as you study and learn, be assured that we will avoid areas of speculation and will focusin discoveries and interpretations that are well founded and clearly demonstrated, and that we will avoid delvinginto areas of unfounded scientific and spiritual speculation. You should also keep in mind that our scientificunderstanding of all things is unavoidably incomplete, though ongoing advances in scientific discovery constantlyimprove and deepen our understanding of the nature of the physical universe. As for the areas in which ourunderstanding is still limited, well, Ralph Waldo Emerson said it best, and is a sentiment of faith that we shouldall adhere to. He said:

All I have seen teaches me to trust the Creator for all I have not seen.

0.3. HOW TO USE THIS TEXT xvii

0.3 How To Use This Text

Overview and Review Questions

At the beginning of each section you will find a tan box labeled “OVERVIEW”.Here you will find a brief summary of the section, a list of objectives, and anynew vocabulary found therein. The list of learning objectives tells you what, Overview boxesultimately, you are responsible for - the ideas or concepts that will be assessedin the exams. As you read the section and complete the assignments, you willwant to refer back to these objectives. Ask yourself: “am I ready to be testedon these ideas?” An example of such an overview follows:

OVERVIEW

Summary: This section provides a road map for how you should use thistext. It discusses the overview and assignment boxes, the helps and hintsfound in the margins, and recurring themes found throughout the text.

Learning Objectives:

• Understand what is expected of you as you use this text to preparefor each class period.

• Identify the various types of helps given in the margins.

• Value these resources as study aids.

Vocabulary:

• Overview box

• Review questions

• Margin definitions

• Margin hints

• Cautions

• Recurring themes

At the end of each section you will find a list of review questions for whichyou should write answers before attending your weekly learning group meeting.These questions are intended to help you reflect on what you have read and Review questionswhat we have discussed in class. It is highly recommended that you answerthese questions immediately after completing the reading assignment, and thenreview and revise after the class meeting. The review questions are found ingreen boxes, as follows:

REVIEW QUESTIONS

1. What is the significance of the blue, tan, and green shaded boxesfound in each section of this text?

2. Where can you look to find succinct definitions for new terms en-countered in the text?

3. What is the purpose of the caution statements that appear from timeto time in the margins?

4. What are you expected to do before coming to each class period?

5. How can you use the information in this section to help you under-stand the material?

For traditional, face-to-face classes, you will be assigned to read one sectionof this text and complete the assigned activities before each class period. Foronline courses, block courses, or courses taught only once each week, you willbe assigned two sections of the text.

xviii LIST OF TABLES

Helps and Hints

One thing that can make learning science difficult is the large number of termsused to relate ideas and concepts. The scientific language consists of words thathave very specific meanings, which in turn are based on common experiencesor other words similarly defined. While knowing the definitions of these termsdoes not constitute true understanding, it is nearly impossible to acquire un-derstanding without first knowing what the words refer to. For this purpose,important terms that are presented in the reading are accompanied by defini-tions in the margin, written in blue text. The hope is that by placing the terms

Definitions:Important terms are definedin the margin as well as inthe text.

in the margins, even when explanations may still appear in the body of the text,you will be able to quickly locate the definition of words that are unfamiliar toyou.

Additional notes are given in the margins which highlight important ideaspresented in the text. These notes are written in a magenta font, such as theImportant ideas highlightedone seen here.

For various reasons, there are some misconceptions about scientific ideas (orsometimes science itself) that seem to be common among most people. Whensuch misconceptions arise in these readings, you will find a caution statementin the margin, as you see here.

Caution statements such asthis alert the reader to com-mon misconceptions or otherconceptual difficulties.

A common human response when we encounter an idea that does not agreewith our own understanding is to simply discount the idea, without so much asconsidering that it might be our own understanding that is flawed. We sincerelyhope that as you study science this semester you will keep an open mind, andwhen you find that one of these caution statements applies to your currentmode of thinking, that you will take a critical look at the reasons why you arehaving trouble accepting new ideas.

0.4. SYLLABUS xix

0.4 Syllabus

For your convenience, a generic copy of the course syllabus is included here.Your instructor may provide you with a more detailed syllabus or additionalinformation as they deem fit.

0.4.1 Course Objectives

This course is an introduction to the nature, practice, power, and limitations ofscience, as well as relationships between science and religion, and science andsociety. These topics are explored through the study of selected episodes of sci-entific discovery that demonstrate methodical and creative aspects of scientificinquiry, and the self-correcting nature of science.

The main goal of this course is to provide you with a set of experiencesthat will enable you to think and act intelligently on science-related issues andconcerns you will face during your life. It should also prepare you for the Issuesin Science Foundations courses and other science courses you will take. Thespecific objectives for this course are as follows:

1. Understand that there are two paths that lead to truth, revelation andscientific inquiry, and appreciate that both paths can and should fit com-fortably into a gospel-centered life.

2. Understand the nature, practice, power, and limitations of science, in-cluding the following principles:

(a) That science can only address questions of an objective and empiricalnature.

(b) That there is a difference between scientific observations and scien-tific interpretations.

(c) That scientific interpretations are provisional and mutable ratherthan final and unalterable.

(d) That scientific investigation involves a deliberate strategy of formingand testing hypotheses to refine the theories and models we use tounderstand the world around us.

3. Have a basic introduction to these fundamental ideas of modern science:the big bang, atomic theory, deep time, plate tectonics, and evolution.

4. Appreciate the role that science plays in your life, and develop an ongoinginterest in science.

5. Introduce and apply the principles of the BYU-Idaho Learning Model.

In order to fulfill these objectives, it is expected that you will do the follow-ing:

• Fully invest yourself in this class.

• Complete the assigned reading and any other preparation prior to comingto class each day.

• Actively participate in your learning group.

• Actively participate in classroom activities.

In total, you should be prepared to spend about four hours in preparation andwork outside of the classroom each week.

xx LIST OF TABLES

0.4.2 Grading

Your grade in this course will be based on attendance and participation, classpreparation, quizzes, homework assignments, group meetings, and exams. Yourinstructor will provide you with information as to the weighting of variousgrading items and the assignment of final grades.

Extra credit opportunities may be made available at the discretion of yourinstructor, provided that the opportunity is extended equally to the entire classand at a reasonable time, and that extra credit opportunities do not exceed atotal of 2% of your final grade.

Inevitably at the end of each semester a few students will be very close toearning a higher grade. For example, a student might have a total of 898 pointsin the class, needing only two additional points to change their B+ grade intoan A-. In such situations students will almost always ask to have their gradesrounded up or to be provided with some sort of additional activity to boosttheir grade.

Such a request is completely unprofessional and unethical. It indicates thatthe student believes that they are entitled to some kind of accommodation thatis not being offered to the rest of the class. That kind of post-course gradeimprovement request, either via additional extra credit or simply expecting aprofessor to change a grade, is a worrisome evidence of academic entitlement,which is essentially a desire to receive the grade that the student wants, ratherthan the grade that the student earned.

Accommodations will always be made in the event of errors in data entryor grade calculations. Personal extenuating circumstances, such as prolongedillnesses or family emergencies, are generally covered by University policies anddo not require other accommodations.

Our hope for every student at BYU-Idaho is that they will work their hard-est, do their very best, and then feel a sense of satisfaction at having knownthat they did their very best. Then, having done their very best they shouldrealize that the grade assigned is an indication of how that particular student’sperformance in required activities stacked up against the set standards of grad-ing in the class in question, realizing that grades are based on performance, noton personal desires for a particular grade.

0.4.3 Attendance and Participation

You are expected to arrive on time to each class period, remain in the classroomuntil class is dismissed, and to be actively engaged throughout each class meet-ing. When every student in the class commits themselves to these behaviors,it fosters a classroom environment that facilitates deeper learning. It providesopportunities for students and faculty to teach and to learn from each other.

An i>Clicker is required forthis course.

In this class attendance and participation are tracked with i>Clickers. Thesewonderful devices help make class more interactive, help us perform real-timeassessment, and collect class opinions. The best part is, your responses are onlyknown by you and your instructor (and in some cases only by you). You willeach need to purchase an i>Clicker before the second day of class, and will beexpected to bring it (in working condition) to each class meeting. In order toreceive your participation points for a given class meeting you must respond toat least 50% of the i>Clicker questions.

Naturally, there may be a day or two that you are unable to attend class.Illnesses, family emergencies, and the like are a part of life. The dropped scores,as indicated in the grading schedule, should cover any such excused absences.You may also show up to class one day only to discover that the batteries inyour i>Clicker are no longer functional. The dropped scores should also coverthese situations.

If you arrive late to class or leave early, your instructor may dock some orall of your attendance points for the day.

0.4. SYLLABUS xxi

Please note that the instructor reserves the right to fail any student whohabitually does not come to class, regardless of any other points earned inthe course. In other words, if you get perfect scores on all of the quizzes andexams and meet regularly with your group, but you only attend half of the classmeetings, you can expect to receive a failing grade.

0.4.4 Preparing for Class: Pre-class Readings and Quizzes

Preparing for class is a critical part of your FDSCI 101 experience. It is oneof the fundamental ways in which you apply the BYU-Idaho learning model.Without pre-class preparation, we would not get through very much material(after all, we only have 24 one-hour class meetings). Furthermore, we wouldhave to resort to the traditional lecture style of instruction, and sitting andlistening to lectures is not an efficient way to learn. On the other hand, whenstudents come to class prepared, we can spend our time together clarifying dif-ficult concepts, discussing the ideas together, and engaging in other interactiveactivities.

Most importantly, preparing for class involves action on your part, andaction authorizes the Holy Ghost to teach.

This brings up the question: How does one prepare for class? To begin with,there will be a reading assignment for each class period, which may includeadditional activities that you should complete. These readings are provided inthis text. To ensure that your preparation is sufficient, there will be a quizon each reading assignment, which must be taken before the beginning of theassociated class period. These quizzes will be on I-learn. They are open bookand open note, but you should complete them on your own (in other words, it isnot appropriate to have another person help you with the quiz). You can takeeach quiz up to three times, which basically allows you an initial take (whichshould indicate what you still need to learn from the reading), a second take,and the possibility of a dropped Internet connection. The quizzes will closeapproximately 10 minutes before class begins. Under no circumstances will anystudent be allowed a late take on a reading quiz.

As with your class participation, the lowest four quiz grades will be dropped.This should accommodate any legitimate difficulties you encounter during thesemester.

At the end of each reading assignment, you will find a series of reviewquestions. You should write out answers to each of these review questions on apersonal blog on I-learn. As to how these review questions are graded, see thenext section.

0.4.5 Learning Groups and Weekly Participation Reports

An important part of the BYU-Idaho learning model involves students activelyteaching and serving each other. Meeting regularly with a study group to reviewmaterial and work on assignments together can be a powerful tool when it comesto improving your understanding of difficult concepts and opening your mindto other points of view.

Early in the semester you will be assigned to a learning group. Your groupwill need to decide on a regular time and place to meet each week. To eachof these meetings you will bring your written answers to the review questions(you can either print a hard copy or bring your laptop), which you will discusswith the members of your group. You may also be given other assignments oractivities to complete during these group meetings.

At the end of each week you will report, via a quiz on I-learn, your levelof preparation for and participation in these group meetings. This report,which must be completed no later than midnight on Saturday, consists of five

xxii LIST OF TABLES

questions (the possible responses are “yes” or “no”), which may be similar tothe following:

1. Did you complete all of the assigned reading this week?

2. Did you write out answers to all of the review questions associated withthe reading assignments?

3. Did you attend your group meeting?

4. Did you complete the reflection activity on the I-learn blog?

5. Did you actively participate in your group meeting?

All five questions are equally weighted. Notice that failure to attend any groupmeetings results in the loss of 150 points, or roughly one and a half letter grades.The two lowest reports are dropped, so if you are sick and cannot attend two ofyour group meetings, your grade should not be adversely affected. Late reportsare not accepted under any circumstances.

It is important to remember that the law of the harvest applies to yourlearning: “He which soweth sparingly shall reap also sparingly; and he whichsoweth bountifully shall reap also bountifully.” (2 Corinthians 9:6). If you onlyput a 50% effort into your work, you should only expect to understand about50% of the material.

0.4.6 Homework Assignments

In addition to your weekly class preparation and group meetings, there will betwelve homework projects assigned during the course, one every week. Theseassignments will be completed as part of your learning group meeting. Youmust complete the assignment individually before arriving at your group meet-ing, then during the meeting you will discuss the assignment, make correctionsas necessary, and then hand in one copy per group. These assignments will begraded. Should the responsible party in your group forget to bring the assign-ment to class, that individual will lose half of the points for that assignment(the rest of the group will not be held accountable).

0.4.7 Exams

Four regular exams will be administered in the testing center. These examswill consist of 25 multiple choice questions and will be worth 75 points. Withthe exception of the first exam, five of the questions on each of these examswill be comprehensive. The final exam, also administered in the testing cen-ter, will consist of 50 multiple choice questions, of which roughly half will becomprehensive. The final exam is worth 150 points.

The exams will be open for one day only. Exceptions must be approved ona case-by-case basis, and are subject to the following policies:

• In no case will you be allowed to take an exam early. This includes thefinal exam.

• Exams missed because of illness, injury, hospitalization, or Universityacademic excused absences may be taken up to two weeks late withoutpenalties or fees. You will need to provide some sort of documentation.

• Exams missed because of personal choice issues, including travel conflictsor negligence

– If arrangements are made before the exam, the test may be taken upto one week late and is subject to a 15% penalty and a $3 late takefee (paid to the testing center).

0.4. SYLLABUS xxiii

– If you simply forget to take an exam, or you did not make arrange-ments before the exam, you will forfeit all points for that exam.

• Exams missed because of funerals, weddings, and the like may be takenup to one week late, with penalties and/or late charges at the discretionof the instructor.

All late takes have to be arranged with the instructor. It is your responsibilityto make these arrangements!

0.4.8 Getting the Grade that You Really Want

In the final analysis, your grade will reflect three things: the amount of effortyou put into your work, how much you achieve, and how much knowledge youretain. Removing any of these three items will earn you a lower grade than youwould like. For example, you should not expect to receive an A in the class ifyou work hard and achieve knowledge but fail to retain that knowledge (andsubsequently perform poorly on the final exam).

This brings up the question, “What do I need to do to get an A in thisclass?” While there are many factors involved, thus making it difficult to layout a specific list of things you should “do”, there are some common character-istics possessed by outstanding students (A students) and average students (Cstudents).

The A Student: Attends class every day, and is actively (not superficially)involved in every class and group activity. They come to class prepared, theyoffer insights and ask questions in class, and they complete all their assignmentson time. Their assignments reflect independent effort, as opposed to relyingon others. They demonstrate curiosity and a willingness to go beyond therequirements of the class. They have determination and self-discipline, andthey are committed to their school work. Because of their work ethic andattitudes, they consistently do well on exams.

The C Student: Misses class a little more than they should, and whenthey are in class they generally participate only in a superficial manner. Theymay have all of their assignments in on time, and mostly correct, but they relya little too much on their peers to show them how to work the problems, or theyput the bare minimum of effort into their work. Their attitude is essentiallyone of just “getting through the class”, and they show little to no interest inlearning anything that is not explicitly covered in the course material. Theirefforts are often half-hearted and/or inefficient. Their exam grades are averageand/or inconsistent. They have some concept of what is going on, but have notmastered the material.

These are, of course, general characteristics observed in students, and donot guarantee a particular grade.

0.4.9 Other Miscellaneous Policies

Dress and Grooming

All aspects of the BYU-Idaho Honor Code will be observed throughout everypart of your experience in this class. This includes compliance with the Uni-versity dress and grooming standards, which are designed to help us developand maintain an environment where the Holy Ghost can enlighten all aspectsof our learning and teaching and where each class member will do nothing thatdetracts from that goal. Please note that if a student is habitually or flagrantlyin noncompliance with the dress or grooming codes, or the honor code, thatmatter will be brought to their attention. If that notification is not enoughto help student bring him/herself into compliance, that student’s name will beforwarded to the Honor Code Office for formal action.

xxiv LIST OF TABLES

Academic Integrity

You should make every effort to ensure that assignments, projects, exams, andother classwork submitted on your behalf are an accurate representation ofyour effort, work, and understanding. Anything less than this is a deception,and qualifies as academic dishonesty. Classic examples of academic dishonestyinclude cheating on exams, having someone else do your work for you, andplagiarism. Other more subtle examples could include reporting that you havecompleted a reading assignment when you only skimmed through it, doingsomewhat less than your fair share of a group project, or devising clever waysto boost your grade through loopholes. Academic dishonesty is not toleratedin this class, and may result in receiving no credit for a particular assignment,referral to the Honor Code Office, or dismissal from the class with a failinggrade.

Sexual Harassment

Title IX of the Education Amendments of 1972 prohibits sex discriminationagainst any participant in an educational program or activity that receivesfederal funds, including federal loans or grants. Title IX also covers student-to-student sexual harassment. If you encounter unlawful sexual harassment orgender-based discrimination, please contact the Human Resources Office (496-1130).

Students with Disabilities

Brigham Young University-Idaho is committed to providing a working andlearning atmosphere that reasonably accommodates qualified persons with dis-abilities. If you have any disability that may impair your ability to completethis course successfully, please contact the Services for Students with DisabilitiesOffice (496-1158). Services are coordinated with the student and instructor bythis office. Reasonable academic accommodations are reviewed for all studentswho have qualified, documented disabilities. If you need assistance or if youfeel you have been unlawfully discriminated against on the basis of disability,you may seek resolution through established grievance policy and proceduresthrough the Human Resources Office (496-1130).

Electronic Devices

The BYU-Idaho Learning Model envisions students who have come to classprepared to share ideas, rather than merely receive them. Learning occursthrough discussion in which each student listens carefully to the comments ofothers and seeks the opportunity to add, as inspired, to what is being said.Participating in such a discussion requires careful attention - as though onewere with a friend, one-on-one.

It is to promote such a learning environment that the University requires,as general policy, that electronic devices be turned off during class time. Thesedevices include cell phones, handheld devices, personal media players, and soforth.

The use of laptops will be left to the discretion of your instructor. If laptopuse is allowed, please make sure that you use your laptop appropriately.

Instructors may, for the sake of achieving special learning objectives or tomeet individual student needs, authorize the use of specific electronic devices intheir classrooms. However, it is recommended that the use of laptops for notetaking not be allowed except for occasional lectures. In the Learning Modelenvironment, thinking about what is being said in the classroom and seekingthe opportunity to add a comment is more important than transcribing thediscussion. Impressions that come in class can be noted by hand. When class is

0.4. SYLLABUS xxv

over, students will find that their handwritten notes, along with ideas broughtto remembrance by the Spirit, will allow them to write detailed reflections.Those reflections will be richer because of the student’s active participation inthe class discussion. (See http://www.byui.edu/it/electronicdevices.htm)

Amendments and Corrections

The instructor reserves the right to correct and/or amend this syllabus at hisor her discretion. Such amendments will be communicated to students by wayof email, I-learn, in class, or any combination of the above.

0.4.10 Course Schedule

Changes to this schedule (Table 1) will be announced in class and via campusemail. Reading assignments refer to sections in the text. Any additional classmeetings will be used as catch-up or review sessions. Your instructor can pro-vide you with additional details, including dates for the various class meetings.

Class Period Topic Section1 Introduction, Learning Model, I-learn (none)

Unit I: What is Science?2 Truth: the Foundation of Correct Decisions 1.13 Mormon scientists 1.24 The characteristics of science 1.35 The scientific process 1.46 Case studies: How science affects society 1.5

Unit II: The Universe7 The scale of the Universe 2.18 Time and intuition 2.29 Early cosmological models 2.310 The big bang model, part I 2.411 The big bang model, part II 2.5

Unit III: Atoms12 Where do atoms come from? 3.113 Atomic models 3.214 Bonding and the periodic table 3.315 Radioactivity 3.4

Unit IV: Earth16 Relative dating 4.117 Absolute dating 4.218 Plate Tectonics 4.319 Earth changes 4.4

Unit V: Life20 Observations of life 5.121 Origin of the species: early models 5.222 Genetics and DNA 5.323 Molecular genetics 5.424 Human evolution I 5.525 Human evolution II 5.6

Table 1: Generic course schedule.

xxvi LIST OF TABLES

0.5 I-learn

Online resources play a big role in this course. The textbook and lecture slidesare made available online. Reading quizzes are taken online. Weekly reportsare submitted online. You can check your grades online.

I-learn is the resource provided by the University that makes all of thispossible. It is accessed by pointing your browser to

http://www.byui.edu/onlinelearning

To log into I-learn, you will need to use the same credentials (username andpassword) that you use to access other university resources, such as my.byui.edu.Once you have logged in, you will see a list of your classes. When you click onthe appropriate link, you will access the I-learn site for this class.

On the first day of class your instructor will show you how to navigate theclass I-learn site.

0.6. THE BYU-IDAHO LEARNING MODEL xxvii

0.6 The BYU-Idaho Learning Model

It is our intent that the FDSCI 101 course be aligned with the BYU-IdahoLearning Model. The Learning Model is based on five fundamental principles.These principles are as follows:

Teachers and learners at BYU-Idaho:

1. Exercise faith in the Lord Jesus Christ as a principle of action and power;

2. Understand that true teaching is done by and with the Holy Ghost;

3. Lay hold upon the word of God as found in the holy scriptures and inthe words of the prophets in all disciplines; Principles of BYU-Idaho

Learning Model4. Act for themselves and accept responsibility for learning and teaching;

5. Love, serve, and teach one another.

The implementation of the learning model has, in turn, been broken intofive steps. The first two (specifying outcomes and designing course architecture)are the responsibility of the faculty. The latter three are of more interest tostudents. These three steps are:

Figure 1: Implementing the Learning ModelImplementing the LearningModelLet’s elaborate on these three steps, as they apply to you as a student.

First of all, you should prepare for each class period. There are severalaspects of preparation involved. You should prepare spiritually. This includesliving in a manner whereby the Holy Ghost can teach you. It means developingattitudes, characteristics, and habits that are conducive to the Spirit, and aban-doning those that are distracting to it. You also need to prepare academically,which means you read, study, complete assignments, and so forth. In summary,you are doing all that you can to prepare to learn.

Second, we all have a responsibility to teach one another. This involveslistening carefully and responding in the classroom. It involves forming andactively participating in study groups. It requires a safe classroom environmentwhere all ideas, opinions, and questions are shown respect.

Thirdly, the end of a class period should not mark the end of the learningprocess. You should ponder on your classroom experience, keep a record ofwhat you are learning, and in all ways prepare yourself to demonstrate yourknowledge.

The responsibilities of students with respect to the learning model are sum-marized in the following figure:

xxviii LIST OF TABLES

Figure 2: Student responsibilities in the BYU-Idaho Learning Model. Studentsshould consistently prepare before class, actively teach each other in class, andthen spend time pondering the material after class is over.

0.7. THE BYU-IDAHO HONOR CODE xxix

0.7 The BYU-Idaho Honor Code

Brigham Young University-Idaho is owned and operated by The Church of JesusChrist of Latter-day Saints. Its mission is to:

1. Build testimonies of the restored gospel of Jesus Christ and encourageliving its principles.

2. Provide a quality education for students of diverse interests and abilities.BYU-Idaho missionstatement

3. Prepare students for lifelong learning, for employment, and for their rolesas citizens and parents.

4. Maintain a wholesome academic, cultural, social, and spiritual environ-ment.

Two of the primary outcomes of this mission is to help students becomedisciple of Jesus Christ and refine their discipleship, and also to help thembecome leaders. Figure 3 illustrates how these outcomes are related to studenthonor.

Figure 3: Realizing the Mission: Developing Disciple-leaders. The major ob-jectives of the University are facilitated by the “Spirit of Ricks”, which in turnrelies on student honor.

As you can see, student honor is at the very center. From student honorcomes the Spirit of Ricks, which in turn facilitates inspired teaching and learn-ing, disciple preparation, and leadership development. Given the importance

xxx LIST OF TABLES

of student honor in this process, understanding the definition of student honoris of utmost importance.

Student Honor is following the path of discipleship and learning to be morelike Christ - learning to think, to feel, and to act more as He does.

Student honor:following the path of disci-pleship and learning to bemore like Christ.

Living a life of honor:

• Begins as we learn and live the baseline standards of the Honor Code,understand their purposes, and are true to the promises we have made.

• Continues as we heed promptings of the Spirit to raise our personal bar ofrighteousness and foster a spirit of integrity, sacrifice, consecration, love,service, and willing obedience as students and throughout our lives.Statement on student honor

• Prepares our hearts for devoted discipleship in the family, church, work,and community.

The baseline standards of the Honor Code, mentioned above, are as follows:

We believe in being honest, true, chaste, benevolent, virtuous, andin doing good to all men.... If there is anything virtuous, lovely, orof good report or praiseworthy, we seek after these things.

- Thirteenth Article of Faith

• Be honest

• Live a chaste and virtuous life

• Obey the law and all campus policies

• Use clean languageHonor Code standards

• Respect others

• Abstain from alcoholic beverages, tobacco, tea, coffee, and substanceabuse

• Participate regularly in church services

• Observe dress and grooming standards

• Encourage others in their commitment to comply with the Honor Code

Details on the various aspects of the Honor Code are available on the BYU-Idaho website (http://www.byui.edu/StudentHonor). You should familiarizeyourself with the letter of the Honor Code. However, living a life of honorgoes far beyond living these baseline standards. There is a particular spiritengendered within the honor code, and we should strive to follow that spirit inall situations. President Kim B. Clark has spoken of the interplay between theletter and spirit of the Honor Code, and summarized it graphically, as presentedin Figure 4 [Clark06]. We want to be in compliance with both the letter andthe spirit of the Honor Code, for that is where true disciple preparation takesplace. If one complies with the letter of the Honor Code but not the spirit of it,then one is acting in a hypocritical manner. Someone who desires to follow thespirit of the code but is not in compliance with the letter of it, then they are ina state of ignorance. Neither of the latter two places are good to be in. Lastly,one who does not comply with the spirit or the letter of the Honor Code is ina state of rebellion. President Clark suggested that those who are in this stateshould go somewhere else and make room for other students.

During your time here at BYU-Idaho, we hope that you will come to ap-preciate the Honor Code and adopt its precepts throughout your stay at thiscampus, and in your life afterwards.

0.7. THE BYU-IDAHO HONOR CODE xxxi

Figure 4: The Honor Code map. There are four quadrants one can be in, de-pending on their level of adherance to the spirit and the letter of the honor code.All persons at BYU-Idaho should strive to fall in the quadrant of discipleship.

xxxii LIST OF TABLES

0.8 Images

Some of the images found in this text are protected by copyright or so-called“copyleft” licenses, such as the GNU Free Documentation License or the Cre-

Some images in this text areprotected by copyright or byother restrictions. Pleaseabide by copyright and li-censing laws when reproduc-ing images or pages from thistext.

ative Commons Attribution License. The texts of these licenses have beenincluded in the appendices of this text.

Image credits are given in the image captions, and when the image is subjectto restrictions on reproduction and distribution, the appropriate copyright orlicense accompanies the image credit. Should you need to reproduce any ofthese images or pages in this text, please abide by the appropriate copyrightlaws and licensing agreements.

Chapter 1

What is Science?

Figure 1.1: Candida Albicans, a fungus found in the human body, is usually harmless. However, in certainabnormal conditions the fungus can rapidly multiply, resulting in a mucosal or skin infection called Candidiasis.Understanding how this microorganism and others function is one example of how science has had a positiveimpact on the lives of most people. (Image courtesy of U.S. Centers for Disease Control and Prevention)

In this chapter, we will explore the fundamental characteristics and processes that form the foundation ofscientific investigation. We will begin by addressing the question of how science interacts with religion, includingperspectives from well known LDS scientists. Following will be discussions about what constitutes a scientificquestion (what’s in, what’s out), the differences between observations and interpretations, and the power andlimitations of science. We will identify a pattern that is used in scientific discovery. Last of all, we will considersome specific examples of ways in which science can affect the lives of individuals and society as a whole.

1

2 CHAPTER 1. WHAT IS SCIENCE?

1.1 Truth: The Foundation of Correct Decisions

OVERVIEW

Summary: There are two ways to discover truth. One way is throughrevelation. The other is through science. The truth that we discoverguides us as we make decisions - some of which are critical to developingour character.

Learning Objectives:

• Define truth (D&C 93).

• Identify the two paths to truth.

• Identify two limitations of the scientific method.

• Identify two essential ingredients for revelation.

• Identify the pattern associated with revealed truth (need, prepara-tion, seeking).

• Explain the role of secular preparation and revelation in the contextof Elder Nelson’s experience.

The following message was delivered by Elder Richard G. Scott of the Quo-rum of the Twelve Apostles during the Sunday afternoon session of the 177thsemiannual general conference of the Church of Jesus Christ of Latter-DaySaints [Scott07]

Elder Richard G. Scott(1928 - ): Member of theQuorum of the Twelve Apos-tles of the Church of JesusChrist of Latter-Day Saints.Prior to his call as a gen-eral authority of the Church,Elder Scott worked as a nu-clear engineer. (image cour-tesy newsroom.lds.org)

Since truth is the only meaningful foundation upon which we can make wisedecisions, how then can one establish what is really true? Increasingly morepeople are finding that making wise decisions is becoming more and more dif-ficult because of the ultra-interconnected world in which we live. Constantlyforced into our consciousness is an incessant barrage of counsel, advice, andpromotions. It is done by a bewildering array of media, Internet, and othermeans. On a given subject we can receive multiple strongly delivered, carefullycrafted messages with solutions. But often two of the solutions can be diamet-rically opposed. No wonder some are confused and are not sure how to makethe right decisions.

To further complicate matters, others try to persuade us that our decisionsmust be socially acceptable and politically correct. Some pondering of thatapproach will reveal how wrong it is. Since social and political structures differwidely over the world and can dramatically change with time, the folly of usingthat method to make choices is apparent.

There are two ways to find truth - both useful, provided we follow the lawsupon which they are predicated. The first is the scientific method. It canTwo ways to find truthrequire analysis of data to confirm a theory or, alternatively, establish a validprinciple through experimentation. The scientific method is a valuable way ofseeking truth. However, it has two limitations. First, we never can be sure weLimitations of sciencehave identified absolute truth, though we often draw nearer and nearer to it.Second, sometimes, no matter how earnestly we apply the method, we can getthe wrong answer.

The best way of finding truth is simply to go to the origin of all truth andask or respond to inspiration.1 For success, two ingredients are essential: first,unwavering faith in the source of all truth; second, a willingness to keep God’scommandments to keep open spiritual communication with Him. Elder RobertEssential ingredients for in-

spiration D. Hales has just spoken to us about that personal revelation and how to obtainit.

1.1. TRUTH: THE FOUNDATION OF CORRECT DECISIONS 3

Scientific Approach2

What have we learned from the scientific approach to discovering truth?An example will illustrate. Try as I might, I am not able, even in the smallestdegree, to comprehend the extent, depth, and stunning grandeur of what ourholy Heavenly Father, Elohim, has permitted to be revealed by the scientificmethod. If we were capable of moving outward into space, we would first seeour earth as did the astronauts. Farther out, we would have a grandstand viewof the sun and its orbiting planets. They would appear as a small circle ofobjects within an enormous panorama of glittering stars. Were we to continuethe outward journey, we would have a celestial view of our Milky Way spiral,with over 100 billion stars rotating in a circular path, their orbits controlledby gravity around a concentrated central region. Beyond that, we could looktoward a group of galaxies called the Virgo Cluster, which some feel includesour Milky Way, estimated to be about 50 million light years away. Beyond that,we’d encounter galaxies 10 billion light years away that the Hubble telescope hasphotographed. The dizzying enormity of that distance is suggested by notingthat light travels 700 million miles an hour. Even from this extraordinaryperspective there would not be the slightest evidence of approaching any limitto God the Father’s creations.

As awe inspiring as this incredible view of the heavens would present, there isanother consideration equally capable of confirming the unfathomable capacitiesof our Father in Heaven. Were we to move in the opposite direction to explorethe structure of matter, we could get a close-up view of a double helix moleculeof DNA. That is the extraordinary, self-duplicating molecular structure thatcontrols the makeup of our physical body. Further exploration would bring usto the level of an atom, composed of the protons, neutrons, and electrons we’veheard about. Examples of truths learned

by scienceWere we to penetrate further into the mysteries of the most fundamental

makeup of creation, we would come to the limit of our current understanding.In the last 70 years much has been learned about the structure of matter. AStandard Model of Fundamental Particles and Interactions has been developed.It is based on experimentation that has established the existence of fundamentalparticles designated as quarks and others called leptons. This model explainsthe patterns of nuclear binding and decay of matter, but it does not yet providea successful explanation for the forces of gravity. Also, some feel that even morepowerful tools than those used to acquire our current understanding of mattermight reveal additional fundamental particles. So there are yet more of Fatherin Heaven’s creations to be understood by the scientific method.

We can see the scientific method has brought about an extraordinary ex-pansion of our understanding as the Lord has inspired gifted men who may notunderstand who created these things nor for what purpose. Many of these maynot even recognize such inspiration or give credit to God for the origin of theircontributions. I was comforted recently as President Henry B. Eyring sharedan experience that his gifted father had in a meeting with other outstandingscientists. He asked them if their research indicated the existence of a supe-rior organizing intelligence. They all confirmed their conviction that such anintelligence exists.

Limited as it is, our understanding of our Father’s creations indicates thatit is mostly vacant space. Even those things we consider as solid, firm, tangible,when viewed at enormous magnification in the heavens or in minute matter, aremostly vacant space that God, our Father, perfectly controls and uses for Hisexalted purposes.

Revealed Truth Approach

4 CHAPTER 1. WHAT IS SCIENCE?

What have we learned about truth through revelation? Centuries ago, Godthe Father permitted some of His prophets to view His vast creations perfectly,through the eye of the Holy Spirit. He also explained why He had created them:“For behold, this is my work and my glory-to bring to pass the immortality andeternal life of man.”3 Enoch was one of those prophets. He observed the Godof heaven weep as He saw how the power and influence of Satan had turnedmany on earth to evil.

Enoch declared:“How is it that thou canst weep, seeing thou art holy, and from all eternity

to all eternity?Examples of truths learnedby revelation

“And were it possible that man could number the ... millions of earthslike this, it would not be a beginning to the number of thy creations; and thycurtains are stretched out still; and yet ... thou art just; thou art merciful andkind forever;

“... And naught but peace, justice, and truth is the habitation of thy throne;and mercy shall go before thy face and have no end; how is it thou canst weep?

“The Lord said unto Enoch: Behold these thy brethren; they are the work-manship of mine own hands, and I gave unto them their knowledge, ... and ...gave I unto man his agency;

“And unto thy brethren have I ... given commandment, that they shouldlove one another, and that they should choose me, their Father; but behold,they are without affection, and they hate their own blood.”4

Well did God the Father say unto Moses:“Worlds without number have I created; and I also created them for mine

own purpose; and by the Son I created them, which is mine Only Begotten. ...“... There are many worlds ... , and innumerable are they unto man; but

all things are numbered unto me, for they are mine and I know them.”5

A knowledge of truth is of little value unless we apply it in making correctdecisions. Consider for a moment a man, heavily overweight, approaching abakery display. In his mind are these thoughts: The doctor told you not to eatany more of that. It’s not good for you. It just gives you momentary gratifica-tion of appetite. You’ll feel uncomfortable the rest of the day after it. You’vedecided not to have any more. But then he hears himself say, “I’ll have twoof those almond twists and a couple of those chocolate doughnuts. One moretime won’t hurt. I’ll do it just once more, and this will be the last time.”

Faith and Character

The process of identifying truth sometimes necessitates enormous effort cou-pled with profound faith in our Father and His glorified Son. God intended thatit be so to forge your character. Worthy character will strengthen your capacityto respond obediently to the direction of the Spirit as you make vital decisions.Righteous character is what you are becoming. It is more important than whatyou own, what you have learned, or what goals you have accomplished. It al-lows you to be trusted. Righteous character provides the foundation of spiritualstrength. It enables you in times of trial and testing to make difficult, extremelyimportant decisions correctly even when they seem overpowering.

I testify that neither Satan nor any other power can weaken or destroy yourgrowing character. Only you can do that through disobedience.

Understand and apply this vital principle to your life: Your exercise of faithbuilds character. Fortified character expands your capacity to exercise greaterfaith. Thus, your confidence in making correct decisions is enhanced. And thestrengthening cycle continues. The more your character is fortified, the moreenabled you are to exercise the power of faith for yet stronger character.

Our Father and His Son

1.1. TRUTH: THE FOUNDATION OF CORRECT DECISIONS 5

With the enormity of what we can in just the smallest way begin to under-stand and certainly in no way fully comprehend, how grateful we must be thatthis God of unfathomable capacities is our Father. He is a loving, understand-ing, compassionate, patient Father. He created us as His children. He treatsus as a beloved son or daughter. He makes us feel loved, appreciated, valuable,and dear to Him. He has given us His plan of mercy6 and equipped us, whenwe are obedient, to make correct decisions. He has provided through His holySon a means for us to live, to grow, to develop, and to place ourselves squarelyon the path to be eternally under His guidance and influence.

Tests For TruthHere are a few simplequestions that you can askto help evaluate whetheror not a particular idea istrue:Theology: Does the idearepresent an official doc-trine? Official doctrinesare those that are

• Found in the stan-dard works of theChurch,

• Sustained by theChurch in generalconference, or

• Taught by the FirstPresidency as apresidency

Science: While scientificideas can never be provenabsolutely correct, the fol-lowing guidelines can helpevaluate scientific claims:

• Can we be 95% con-fident or better thatthey are correct?

• Has it been throughthe peer review pro-cess?

• Has it been scruti-nized and acceptedby the scientificcommunity?

I love our Father in Heaven beyond my capacity to express. In all humility,I solemnly bear witness that this creative Master of unparalleled capacities isour compassionate, holy Father. His Beloved Son laid His life down in abso-lute obedience to His Father to break the bonds of death and to become ourMaster, our Redeemer, our Savior. While I do not fully comprehend all Theircapacities, I understand something of Their power to express intensely Theirlove. Humbly I bear solemn witness that They live and love us. In the name ofJesus Christ, amen.

Notes

1. See Jacob 4:8.

2. For further information see McGraw-Hill Concise Encyclopedia of Physics(2005); Philip Morrison and others, Powers of Ten (1982);www.particleadventure.org; and www.atlasoftheuniverse.com.

3. Moses 1:39.

4. Moses 7:29-33.

5. Moses 1:33, 35.

6. See Alma 42:31.

REVIEW QUESTIONS

1. What is the principal argument of Elder Scott’s talk?

2. What points does Elder Scott emphasize when speaking about sci-ence?

3. What points does Elder Scott emphasize when speaking about rev-elation?

4. What is the appropriate way to use these methods for finding truth?

5. What is the connection between faith and character?

Additional Resources

• Nelson, Russell M. 2003. “Sweet Power of Prayer.” 173rd annual GeneralConference of the Church of Jesus Christ of Latter-Day Saints Ensign,May 2003, 7.

6 CHAPTER 1. WHAT IS SCIENCE?

1.2 Mormon Scientists

OVERVIEW

Summary: This reading provides links to several online resources thatgive biographical sketches of well known LDS scientists, many of whomserved as general authorities.

Learning Objectives:

• Recognize that a person can be a member of the church in goodstanding and be a good scientist at the same time.

• Recognize that revealed truth and scientific truth can exist in har-mony.

• Identify some areas where there appears to be tension between sci-ence and religion, and explain why those tensions exist.

• Identify the source of all scientific truth.

• Identify the official position of the Church of Jesus Christ of Latter-Day Saints relative to scientific knowledge and theories.

Henry B. Eyring (1901 -1981 ): (image courtesywww.mormonwiki.com)

Elder Joseph F. Merrill(1869 - 1952 ): (image cour-tesy www.gapages.com)

Elder Scott, whom we heard from in the previous section, is not the onlywell known member of the LDS faith with a professional background in thesciences. In this section you will read about others.

Follow the links below to read the biographical sketches of the individualsbelow. You should also read at least one of the other documents listed. As youread the additional document, please focus on the ideas that are most relevantto this class.

Please note that many of the online sources referenced below are not officialchurch websites, and therefore are not authorized to present the official positionof the LDS church.

Henry B. Eyring

Henry B. Eyring was a world renowned chemist who developed the AbsoluteRate Theory, which applied the principles of quantum mechanics to molecularinteractions. He served on the Deseret Sunday School General Board between1946 and 1971, and was often called upon by the brethren as an unofficialspokesman and advisor on science-related issues.

(Biographical sketch: http://www.nap.edu/html/biomems/heyring.html) Moredetails on Henry Eyring’s service in the church can be found in the secondchapter of “Mormon Scientist” [Eyring07], provided online at

http://media.mormonscientist.org/files/morm-sci-chap-2-faith.pdf

Elder Joseph F. Merrill

Elder Joseph F. Merrill served as an Apostle from 1931 to 1952. He received aPh.D. in physics from Johns Hopkins University in 1899.

(Biographical sketch: http://www.gapages.com/merrijf1.htm) “CharacteristicDoctrines of Mormonism” (Conference address April 1837):

http://www.gapages.com/doctrines.htm

1.2. MORMON SCIENTISTS 7

Elder James E. Talmage

Elder James E. Talmage served as an Apostle from 1911 to 1933. He receiveda Ph.D. in 1896 from Illinois Wesleyan University after studying chemistry andgeology. He authored the books “Articles of Faith” and “Jesus the Christ”.

Elder James E. Talmage(1862 - 1933 ): (image cour-tesy www.ldsces.org)

(Biographical sketch: http://www.gapages.com/talmaje1.htm) “Science in theAssociations” (Address to the YMMIA Conference June 1888):

http://www.gapages.com/science.htm

Elder John A. Widtsoe

Elder John A. Widtsoe served as an Apostle from 1921 to 1952. His formalstudies were in chemistry (particularly in biochemistry) and in 1899 he receiveda Ph.D. from the University of Goettingen, Germany. His book “Dry Farming,A System of Agriculture for Countries Under Low Rainfall” stands as one ofthe definitive works on dry farming.

Elder John A. Widtsoe(1872 - 1952 ): (image cour-tesy www.ldsces.org)

(Biographical sketch: http://www.gapages.com/widtsja1.htm) “Knowledge MustBe Quickened and Made Alive” (Conference address April 1838):

http://www.gapages.com/quickened.htm

REVIEW QUESTIONS

1. Are science and religion ever at odds with each other? Explain.

2. Regardless of how you answered the previous question, what can bedone to reconcile the differences (or apparent differences) betweenscience and religion?

3. What did you learn from reading about the lives of Henry Eyring,Elder Merrill, Elder Talmage, and Elder Widtsoe?

4. What did you learn from the other document that you read?

8 CHAPTER 1. WHAT IS SCIENCE?

1.3 The Characteristics of Science

OVERVIEW

Summary: Science can only address questions that are objective andempirical in nature. Answering these questions relies on an experimentalprocess that is centered around a two hypothesis method. The hypothesesgenerated and the results of experimentation are then subjected to a rigor-ous critical review process to ensure that the methods are sound. Criticalreview helps to ensure that science is self correcting. Finally, scientificideas can be formulated as laws, theories, or models, and these ideas areranked by relative strength.

Learning Objectives:

• Identify whether a particular question can be addressed by the sci-entific method, and if not, explain why.

• For a given questions, identify an appropriate research hypothesisand an appropriate null hypothesis.

• Distinguish between scientific hypotheses, theories, models, andlaws.

• Identify whether a particular piece of evidence is empirical.

• Distinguish between accuracy and precision.

• Identify the criteria that must be satisfied before accepting or reject-ing a null hypothesis.

Vocabulary:

• Empirical evidence

• Anecdotal evidence

• Objective

• Subjective

• Hypothesis

• Research (or alternative) hy-pothesis

• Null hypothesis

• Accuracy

• Precision

• Reproducibility

• Critical review

• Law

• Theory

• Model

In this section we will consider some of the goals of science and explorethe nature of scientific thinking. Providing a list of the goals of science is adaunting and somewhat dangerous exercise since there are so many opinions onthe topic. One goal of science is to discover and describe the laws of nature.Goals of ScienceThe more such laws we can identify, the better we will be able to understandhow things work. Another goal of science is to dispel incorrect or incompleteexplanations of how things work. Results of scientific work can also be usedto improve technology. Improved technologies can then further the advance ofscience. Finally, science strives to provide the knowledge we need to addressthe problems we face.

Additionally, we might want to consider the nature of scientific thinking andhow that contrasts with non-scientific thinking. You have probably at somepoint in your education been introduced to some aspects of scientific thinkingin terms of the scientific method. It is valuable for us at this point to compare

1.3. THE CHARACTERISTICS OF SCIENCE 9

common-sense thinking with the scientific method of problem solving that wewill refer to as science-sense thinking.

Common sense is, as Lewis Wolpert states, “a complicated thing” [Wolpert92].Navigating this life and this world via common sense requires a great deal ofpractice and effort. First of all it’s usually easier to identify someone who lackscommon sense than someone who has it. These individuals don’t seem to beable to do the right thing at the right time. Being able to choose what to doamidst the constant bombardment of challenges, choices, and decisions is tough.Individuals who display a highly developed degree of common sense have ac-cumulated an immense store of experiences that they can pull on at moment’snotice. They use that store of observations and experiences to develop andapply specific and sometimes unique responses to the wide range of moment tomoment situations they face. In other words, common-sense thinking involvesusing a massive store of data to develop a massive number of potential responsesor answers to answer a massive number of challenges or questions.

Science-sense, on the other hand, has an entirely different purpose. Sciencesense, like common sense, involves the collection of huge numbers of observa-tions that are in turn used to develop answers to a huge variety of phenomena.Unlike common sense, which requires that a different response be used for eachspecific challenge, science sense attempts to group challenges or problems intoidentifiable classes of problems. Then, once related sets of problems have beenidentified, someone with science-sense uses their vast catalog of experiences todevelop only a few or perhaps one answer that addresses or explains as manyquestions as possible. Thus the goal and practice of science-sense thinking isalien to the way most people approach problem solving. Since the underlyinggoal of science is to develop science-sense explanations for things it is oftendifficult for non-scientists to grasp what scientists report to other science-sensethinkers. The philosophy and language of science also often alienates the non-scientist, and one of the purposes of this course is to provide you with enoughof an understanding of what science is and how it works that you will not becompletely frustrated when you are faced with scientific issues or explanations.You can then be empowered rather than confounded by science, how it works,and what it teaches us about our world.

It seems like almost every time you watch the news, listen to the radio,or read the newspaper, someone is talking about a science-based issue, a newscientific discovery, or a new interpretation of an old one. Why do we as a globalcommunity pay so much attention to discoveries and interpretations made bythe scientific community? Well, the short answer to that question is that thescientific approach to discovering new information is an extremely powerfulapproach to thinking and working. Let us, then, investigate the characteristicsand processes that make science such a valuable tool for learning about theuniverse we live in.

What’s In and What’s Out

A person could describe science as a way of answering questions. However,there are some questions that the scientific method cannot be properly used toaddress. So as a starting point, we need to determine what types of questionsscience can answer.

As a general rule, science can only address questions which are objectiveand empirical. Objective questions are questions that relate to the actual

Objective:based on a measurable prop-erty and not a question ofpersonal opinions or feelings.state of a thing, and not an individual’s personal opinions or feelings. When

a question is objective, every correct answer to the question will be the same.For example, a good objective question would be “how tall is the Empire Statebuilding?” There are many ways that a person could go about determining theheight of this structure, and if the measurements are performed properly, eachmethod will give the same answer (at least to within an appropriate degree of

10 CHAPTER 1. WHAT IS SCIENCE?

uncertainty). On the other hand, the question “Is the Empire State buildingpretty?” is a subjective question, meaning that the answer depends on the

Subjective:subject to an individuals per-sonal opinions, feelings, ortastes; the opposite of objec-tive.

individuals personal preferences.Additionally, scientific questions must be empirical , meaning that the an-

Empirical:based on measurement, asopposed to personal recollec-tion

swers to those questions must be based on measurements. For example, thequestion “How big is the Earth?” can be answered by making appropriatemeasurements and calculations. On the other hand, the question “Does Godexist?” does not qualify as an empirical question, because the supreme beingis not revealed (or perhaps chooses to not be revealed) by measurement. Alongthose same lines, the feelings and impressions a person receives from the min-istration of the Holy Ghost, though they are real, do not qualify as empiricalevidence. Why? The reasons include: they cannot be quantified (i.e. you can-not put a number on it), they may not be the same for all observers (i.e. theyare not objective), they are not reportable in an empirical way, they cannot bedemonstrated to another individual, and they are not collected by use of thefive senses (see 1 Corinthians 2:9-10).

The Two Hypothesis Method

Scientists begin their process of discovery by observing interesting features ofnature and then asking a question, stating a problem, or offering explanationsfor observations and cause and effect relationships. They then develop a hy-pothesis , or a possible explanation for the problem, question, or observation.

Hypothesis:a reasoned possible explana-tion for an observation or setof observations.

It is important to note that a hypothesis is a reasoned and educated attemptat explaining - not a mere whimsical guess. Next they make additional detailedobservations or run experiments that yield data. They then analyze those dataand use that analysis to help them make a decision about whether to accept orreject their hypothesis. It sounds simple enough, but there’s a bit more to thescientific decision making process than that.

Good science typically requires that two hypotheses be stated for each issueunder investigation. Those hypotheses are called the null hypothesis (H0) andthe research hypothesis (HA, which is also known as the alternative hypothesisin statistics). The research hypothesis, which outside of the scientific com-

Research or AlternativeHypothesis:a researcher’s best explana-tion for an observation.

munity is commonly referred to as “the hypothesis”, represents the researcher’sbest explanation for an observation in nature, a cause and effect relationship,or answer to the scientific question that is being investigated before any ex-periments are run. The null hypothesis is the prediction that states that

Null Hypothesis:A hypothesis that states thatthe explanation for the ob-servation is something otherthan the research hypothesis.

the explanation, cause and effect, or answer to the question is something otherthan HA - the null hypothesis covers all other possible explanations. You mayalso note that the research and null hypotheses are mutually exclusive, meaningthat if one is correct the other must be incorrect.

Here’s an example: there are many individuals who believe that a mercury-containing preservative found in some vaccines causes autism. It’s a very im-portant public health issue and something we must be informed about. Inthe context of this issue, one possible research hypothesis would be, “Mercury-containing vaccines cause autism.” The null hypothesis would be, “Autismis caused by something else.” Another way of expressing the two hypotheseswould be, “What’s the likelihood that autism is caused by mercury-containingvaccines? Is it the vaccine or something else?” Note that the two hypothesisare mutually exclusive, meaning that only one of them can be correct.Two hypothesis method

helps eliminate biasAbout now you may be thinking, “Why use two hypotheses?” The risk is

this: believe it or not, scientists are human. There is a natural tendency foranyone to want their explanation to be correct. The risk of a single-hypothesismethod is that researchers will be tempted to view their data in ways that puttheir own hypothesis in the best light, whether it is actually correct or not.

Under the two-hypothesis method researchers make observations or carryout experiments, collect data, analyze data, and make a decision to accept

1.3. THE CHARACTERISTICS OF SCIENCE 11

or reject the null hypothesis. That is the important part: under the two-hypothesis method a scientist must always decide whether to reject or acceptthe null hypothesis! The significance is that scientists conduct carefully craftedexperiments in order to narrow down or weed out various explanations. It’s aprocess of eliminating possibilities.

How confident must a researcher be before the null hypothesis is rejected?That’s a difficult question to answer, but you can be sure that a good scientistwill always tell you their level of confidence in the answer. A minimum thresholdfor confidence is usually 95% , but it may be significantly higher. Before you

95% confidence usually re-quired before accepting or re-jecting a hypothesisget too comfortable with that statement, however, I hope that at least some of

you are wondering about the other 5%. What does it represent? If a scientistis at least 95% confident in stating a scientific conclusion, it means that weare willing to accept up to a 5% chance of being wrong in our decision-makingprocess. That’s what Elder Scott referred to when he stated, “The scientificmethod is a valuable way of seeking truth. However, it has two limitations.First, we never can be sure we have identified absolute truth, though we oftendraw nearer and nearer to it. Second, sometimes, no matter how earnestlywe apply the method, we can get the wrong answer” [Scott07]. Elder Scottis correct that we can never be absolutely confident that we have identifiedtruth via science because there is always a possibility that we may make amistake in our decision about whether to accept or reject a hypothesis for aparticular question. He is also correct in saying that we can draw closer tothe truth through scientific research. That happens as we critically evaluateone another’s results and conclusions, do additional research, and review andrevise our conclusions based on additional work. In other words science is aself-correcting discipline.

When a researcher decides, based upon his or her experimentation, to rejectthe null hypothesis when it is actually correct, we refer to this as a type Ierror. These errors are also referred to as α-errors (alpha errors) or false

Type I Error:When the null hypothesis isrejected, even though it is ac-tually correct.positives.

The other error that can be made consists of accepting the null hypothesiswhen it is in fact incorrect. Such errors are called type II errors. Such errors Type II Error:

when the null hypothesis isaccepted, even though it isactually incorrect.

are also referred to as β-errors (beta errors) or false negatives.The scientific community ruthlessly casts aside explanations found to be

faulty or incomplete whenever more complete or better-supported explanationsare described. This attribute makes science different than virtually every otherdiscipline. The self-correcting nature of science and the difference betweenscience and other fields, such as the humanities, is summed up nicely by JohnA. Moore who stated, “Great art is eternal; great science tends to be replacedby greater science” [Moore93].

Theories, Laws, and Models

A scientific hypothesis generally falls into one of two categories. The first cat-egory involves the formulation of a general principle that describes what willhappen in a given situation. After hypotheses of this type are validated byexperimentation, they are referred to as scientific laws. A simple example of

Law:a generalized description ofobservations.a scientific law would be the principle commonly known as the “law of grav-

ity,” which states that an object that is dropped near the surface of the Earthwill fall. This is a general principle that describes a large set of observations.Note that the scientific law only says what will happen. It does not attempt toexplain the fundamental processes that cause the particular thing to happen.

A scientific theory, on the other hand is a concept or idea that attemptsto explain “how”. An example would be Albert Einstein’s general theory

Theory:an attempt to explain, ata more fundamental level,how or why a particular phe-nomenon happens.

of relativity, which explains the gravitational pull of the Earth in the contextof the bending and warping of spacetime by massive objects. Laws generalizewhat is, and theories attempt to explain those generalizations.

12 CHAPTER 1. WHAT IS SCIENCE?

Among the general public, the terms “hypothesis” and “theory” are oftenused interchangeably. This is not appropriate in the sciences, as a hypothesisrefers to a preliminary explanation based on prior knowledge and observation,but a theory is an idea that is supported by a wealth of empirical evidence.Theories vary in the degree of support they have, and a newly formed theoryIn science, the terms “the-

ory” and “hypothesis” arenot interchangeable

with relatively little supporting data will receive less general support from thescientific community than a theory that is supported by extensive amounts ofdata.

Another common misconception is that, with an appropriate amount ofdata, a theory will eventually become a law. Because theories and laws dealwith completely different types of ideas (explaining how something happensand explaining simply what will happen, respectively) it becomes apparentthat theories and laws are two completely different entities.

Theories never become laws. It is often useful to create some sort of representation of reality in order tounderstand a particular set of observations. These representations, or modelsModel:

a visualization or analoguethat helps a scientist under-stand a particular system.

provide us with visualizations or analogues to help us understand and describethe way in which nature works. A model airplane is a good example. Themodel is not an actual airplane, but can be used to understand many of thebasic features of an airplane, including how it can fly.

Experimentation

We are now to the point where we need to discuss the process by which ahypothesis is validated. As suggested earlier, it is important to note that allscientific conclusions must be based on empirical evidence . Empirical evi-

Empirical Evidence:evidence consisting of mea-surements and observationsusing the senses or in-struments that extend thesenses.

dence consists of measurements and observations using the senses or instrumentsthat extend the senses. If we cannot collect empirical evidence on a particu-lar phenomenon then that phenomenon falls outside of the range of scientificinvestigation.

Observing, by definition, refers to viewing or otherwise noting some factabout the way things are. Observations, in other words, are facts. When I crackopen a rock and see a fossil, that fossil is indisputably there. No other observercan deny its existence. Based on the observations we can make inferences (e.g.“these are the fossilized remains of a horse”), and those inferences may or maynot be correct. But the observation itself represents an absolute truth, and anysatisfactory explanation relative to the existence of that truth must account forthe observation.

When we measure something, we are quantifying some aspect, such as itssize, shape, volume, mass, etc... relative to some standard. Like observations,measurements also represent how a certain thing is, and thus contains elementsof absolute truth (see Doctrine and Covenants 93:24).

Science also relies on accuracy and precision. An experiment exhibits ac-curacy when it yields a similar set of data each time it is run, no matter how

Accuracy:when a measurement yields asimilar result every time it istaken.

many times it is run. When standard (or accepted) data or results are known,accurate experiments will agree with those standard values. On the other hand,precision is demonstrated when an observation falls within a narrow range ofPrecision:

when repeated measure-ments fall within a narrowrange of values.

values.

Accuracy and precision arenot the same thing!

Here’s an example of precision and accuracy. If we run an experiment wherewe shoot arrows at a target, the level of precision would be associated withthe grouping of the arrows. The tighter the groups, the higher the precision.Accuracy, on the other hand, is demonstrated when those groups center aroundthe bulls-eye. Note that it is possible for an experiment to yield results thatare accurate but not precise (wide grouping centered around the bulls-eye). Itis also possible for an experiment to yield precise results which are not accurate(tight groups centered at the edge of the target, for example). Of course, theoverall goal for the scientist, just as with the competitive archer, is to maximizeprecision and accuracy. The ideal result for the arrow-shooting experiment

1.3. THE CHARACTERISTICS OF SCIENCE 13

would be to produce data sets where the arrows that are tightly grouped andwhere that grouping is centered consistently in the middle of the bulls-eye.Maximizing precision and accuracy increases the level of confidence we have inour results.

Another important aspect of empirical data is that it is reproducible or in

Reproducibility:other researchers can per-form the same experimentand get the same resultother words, different people can make the same measurement using the same

method and get the same result. This consistency is crucial when one considersthe process of critical review.

At this point you may be wondering if there is such a thing as non-empiricalevidence. There is. We refer to it as anecdotal evidence . Anecdotal evidence

Anecdotal Evidence:Evidence consisting of infer-ences based on the chrono-logical relationship of eventsor personal firsthand or spir-itual experiences which havenot been or cannot be testedempirically.

consists of inferences based on the chronological relationship between events orpersonal firsthand or spiritual experiences which have not been or cannot betested empirically. For example, consider the following scenario, relating back tothe autism and vaccine debate mentioned earlier. A parent (or several parents)may note that their children begin exhibiting symptoms of autism shortly afterreceiving vaccinations. They then associate the vaccination with the symptoms,and infer that the vaccinations cause autism. To be fair, the chronologicalconnection is consistent with that assertion. However, the association of autismwith vaccinations does not pass the test for empirical evidence, because carefulmeasurements (which in this case would consist of controlled studies) have notbeen made. (In fact, such studies have been conducted and have revealed nocausal effect [Taylor99].)

This is not to say that anecdotal evidence is worthless. Indeed, anecdotalevidence can lead us to ask important questions. Whether or not vaccines cancause autism is an important thing to know! However, when making impor-tant decisions, a wise person will consider whether the evidence supporting aparticular option is strictly anecdotal, or whether it has been tested empirically.

The Critical Review Process

Another attribute of science is a healthy dose of skepticism. When someonepresents results of their scientific work, their conclusion is not immediatelyaccepted. The scientific community always subjects a conclusion to criticalreview before it is even tentatively accepted. That level of skepticism is vitalto maintaining the high standard of excellence that scientists expect from oneanother. Skepticism also helps offset researcher bias.

Critical review happens when someone presents their work at a scientificconference, when a paper is submitted for publication, and even after that paperappears in a professional journal. Please be aware that this does not mean that

Critical Review:The process whereby a re-searcher’s work is scrutinizedby the scientific community.all scientists are unfriendly cynics; it means that scientists are critical thinkers

who bring different perspectives to the review of new ideas being presented tothe scientific community. That is a very healthy thing for the entire discipline,as well as for you and me.

Given the importance of the critical review process, let’s explore in a littlemore depth how it works. What happens once a scientist reaches a conclusionwhich is based on rather extensive investigation and experimentation? Thescientist shares their conclusion with other scientists. The means of sharingthis information might initially be giving a talk at scientific meetings, but italmost always involves submitting a manuscript of their work to a professionaljournal for publication. Critical review process

What happens to that manuscript? The editor of the journal reviews thepaper to see if it matches the mission and standards of the journal in termsof the area of research and overall manuscript formatting. If the manuscriptappears to meet both of those criteria then the editor sends the manuscript outfor external review, which is a process where the editor invites scientists who areexperts in the field the manuscript addresses to review the manuscript. Thosereviewers (usually three or four of them) examine the manuscript to see if the

14 CHAPTER 1. WHAT IS SCIENCE?

work done meets the high standards of rigor expected for scientific investigationby checking to see if the work is presented in such a way that the backgroundof the problem, the methods used to carry out the project, the results thatwere collected, and the conclusions are clearly and succinctly stated. Eachexternal reviewer then makes a recommendation to the journal editor aboutthe manuscript, such as: 1) unacceptable and manuscript should be rejected;2) acceptable after major revision of the manuscript; 3) acceptable after minorrevision of the manuscript; or 4) acceptable without revision. The journal editorcollects comments and recommendations from the external reviewers and makesa final decision regarding the fate of the manuscript.

Let’s assume that the editor decides that the manuscript should be pub-lished. Then what? When the manuscript is published the scientist’s work isexposed to the critical scrutiny of the entire scientific community. In most casesother scientists working on the same or similar problems take the most time andeffort to ponder and respond to the conclusions in the manuscript. The dataare reviewed, the methods of data analysis critiqued, and each scientist comesup with her or his own evaluation of the manuscript. If they find a weakness ora hole in the research someone will almost always carry out a similar researchproject that will allow them to compare their own result with those from themanuscript (which is why it is important for the empirical evidence supportingthe conclusion to be reproducible). If the follow-up research supports the origi-nal manuscripts assertions, then the manuscript’s conclusions are strengthened;if not then the conclusions are weakened or rejected.

This process continues for an indeterminate length of time. Once the dustsettles, then the scientific community comes to an informal consensus regardingthe original conclusions they will be accepted or rejected. As a consequence ofthis rigorous process of preliminary and extensive review the scientific commu-nity reaches a general consensus on the topic at hand.

What this all means is that the significance and relevance of the conclu-sions presented in a research manuscript are subjected to a veritable onslaughtof critical review, not all of it friendly, which serves to refine the conclusionsin such a way that the validity of the conclusions are finally considered to bevalid and acceptable, or invalid and rejected. And that means that each pieceof information and each conclusion is evaluated and reevaluated so that falseclaims, errors and mistakes are frequently identified and eliminated. As a con-sequence, facts, trends, patterns, explanations, and conclusions developed viascientific investigation move closer and closer to the truth as time goes on, andthey therefore deserve our attention and consideration.

REVIEW QUESTIONS

1. Make a list of five questions that can be appropriately addressed byscientific inquiry. Also compose a list of five questions that cannotbe properly addressed by science.

2. Why does science utilize the two hypothesis method? Think of sev-eral examples of possible research hypotheses, and identify an ap-propriate null hypothesis.

3. Explain the differences between scientific hypotheses, theories, mod-els, and laws. Which of these correspond with scientific observations?Which correspond with interpretations?

4. Write down three examples each of empirical and anecdotal evidence.

5. In what ways does the critical review process ensure that science isself correcting?

1.4. THE SCIENTIFIC PROCESS 15

1.4 The Scientific Process

OVERVIEW

Summary: Science is not a set of random ideas and discoveries, butthere is a consistent underlying process that involves observation, modelbuilding, testing, and refinement.

Learning Objectives:

• Arrange in proper sequential order the activities in a scientific inves-tigation.

• Explain what a model is and why it must be predictive.

• Explain how the critical review process works.

Vocabulary:

• System • Predictions

Every scientific effort studies some portion of the real universe in which weexist. Scientists refer to this portion of the real world as a system . In the

System:The portion of the universeof interest in a scientific in-vestigation.natural course of their investigations, scientists come up with ideas, concepts,

and descriptions which represent that system being studied. In their infancy,these ideas, concepts, and descriptions are referred to as hypotheses. In order tohelp them understand the system, scientists develop models (discussed below).These models help the scientists visualize the workings of the system, and allowthem to make predictions about as-yet unobserved phenomena related to thesystem. Upon validation by experiment, the hypotheses may be formulated intotheories (if they attempt to explain how the system works), or laws (if they aregeneralized statements about how the system will behave).

It is important to recognize that scientific ideas are “invented by acts ofhuman imagination and intelligence” and are therefore “mutable and provisionalrather than final and unalterable” [Arons97]. Furthermore, the ideas we concoctto help us understand a particular physical system are not the system itself. Tohelp illustrate this point, consider the following example of a model.

When most people hear the word model, they may think of something likea model airplane or model ship. Those objects are representations of the real

Example of how scientificideas (models) are not theactual system they describething. A model airplane (see Figure 1.2) may have the same general shape and

proportions. It may even have propellers that spin or landing gear that movesup and down. In many respects, the model is very much like a real airplaneand shares many characteristics. It can be useful in describing or studying areal airplane - investigating how the shape affects air resistance, for example.

On the other hand, the model also lacks many of the characteristics of areal airplane. The model is not made of the same material. It does not havea gas engine. It does not have an electrical system. So while the model sharesmany of the same characteristics, it is also missing a few things.

You can certainly refine your model airplane to make it more like the realthing. You can scale it up to real size. You can put in a gas engine. Youcan put in the electrical system. You can make it out of the same materials.You might even be able to use your “model” airplane to transport people andgoods from place to place. At this point, isn’t your model the same as theactual system? No! It is still a separate entity from the system itself! (Thinkabout it this way: the actual system, the original airplane, could have an enginemalfunction. Your model, the new airplane, does not automatically have thesame malfunction.)

Scientific models work in the same way. We try to represent the system ofinterest as best as we can. As we find discrepancies between our models and the

16 CHAPTER 1. WHAT IS SCIENCE?

Figure 1.2: An example of a model. This airplane was designed to look andfunction much like a real airplane. It shares many features in common with thereal thing. However, no matter how complex the model becomes, it will neverbe the actual airplane that it is meant to represent.

real thing, we refine our models accordingly. But no matter how “complete”our models may become, they are still a separate entity from the actual sys-tem. In this respect, no model is ever truly complete, and, as we have alreadydiscussed, science cannot discern whether a particular model is absolutely “cor-rect”. However, as our models are continually tested and refined they containmore and more of the characteristics of the actual system, and in that respectapproach closer and closer to absolute truth.

The Process of Science

It is now time to discuss the process whereby scientific ideas are developed andrefined. This process is diagrammed in Figure 1.3.

The process starts with a question, usually evoked by an observation of somenatural phenomenon. The scientist who makes the observation (or poses thequestion) attempts to understand it in the context of things that they alreadyknow. They come up with an educated description or explanation - a researchhypothesis, and in conjunction formulate a null hypothesis.

An important aspect of any scientific hypothesis is that it is predictive,

Predictive:suggesting that there are ad-ditional phenomena whichcould be observed.

meaning that the hypothesis will suggest there are additional phenomenon thatcould be observed. The testing of hypotheses depends on this predictive na-ture. If there were no predictions, there would be nothing to test! The scientistcrafts experiments or plans observations to look for these additional observablephenomena. Based on the analysis of the results of these tests (the additionalmeasurements, observations, and data), the scientist must then make a decisionwhether to accept or reject the null hypothesis. If the analysis indicates greaterthan a 5% probability that the research hypothesis is incorrect, the researcher isforced to accept the null hypothesis and the develop a new research hypothesis.If the probability that the research hypothesis is incorrect is less than 5%, thenull hypothesis is rejected and the research hypothesis is given provisional con-sideration as a plausible explanation or general description of the phenomenon.In other words, with experimental backing it may now be considered a theoryor law (depending on whether it deals with explaining “how” the phenomenonoccurs or whether it attempts to generalize “what” happens within the system).

Usually at this point (if it has not happened already), the investigation willbe subjected to the peer review process discussed in the previous section.

Whether the null hypothesis was accepted or rejected, at some point addi-tional predictions will be made - whether in the context of the new researchhypothesis or within the context of the provisional theory/law. This dumpsthe process right back into the cycle. The prediction leads to additional exper-imentation. As more and more evidence supports a particular explanation, the

1.4. THE SCIENTIFIC PROCESS 17

Figure 1.3: The process of science begins with an initial question or observation,which leads the researcher to develop a predictive hypothesis. At this point,the process falls into cycle of prediction, experiment or observation, analysis,and refinement.

scientific community will yield greater confidence to the idea.This process continues indefinitely. Scientific ideas are continuously tested

and refined, so that our picture of how the universe works becomes progres-sively more accurate and more complete. With each iteration, a wider range ofphenomena is included in the scope of the model (and theory or law), and theconfidence in a particular idea increases.

An Example in Science

As an example of how this process works in science, consider the laws of motion.One of the earliest theories came from Aristotle, who postulated (circa 364 B.C)that there were two types of motion: “natural motion”, which was the result ofan object seeking it’s natural state; and “violent motion” or motion imposed onan object by some outside influence. For example, the natural state of a rockwas supposedly to remain at rest, and the “natural motion” of the rock was tofall to where it could be at rest (on the ground). When a person lifted a rock offthe ground, they were imposing a “violent motion” on the rock. Based on thesepremises, Aristotle developed a quantitative description of motion: objects willfall at a constant speed, and that speed will be proportional to their weight.The reasoning behind this theory is that heavier objects have a stronger affinitytowards their natural state.

Aristotle (384 - 322 B.C.):Greek philosopher who for-mulated one of the first the-ories about motion.

Aristotle’s theory makes some clear predictions. If I were to drop two rocks,one twice as heavy as the other, the lighter rock should take twice as long tohit the ground. Unfortunately, experimentation was not high on the prioritylist of the thinkers of the day, and it took nearly two thousand years beforeAristotle’s method was properly tested.

In the late 1500’s Galileo Galilei performed a series of experiments thatinvolved dropping objects of different weights (legend has it that these experi-

18 CHAPTER 1. WHAT IS SCIENCE?

ments were performed at the famous leaning tower of Pisa). Unless one objectwas particularly light (a feather, for example), both objects would reach theground at the same time. This result is clearly (i.e. with a confidence leveleasily better than 95%) contrary to Aristotle’s theory.

Galileo Galilei (1564 -1642 ): Italian physicist,mathematician, astronomer,and philosopher.

Galileo made a significant revision. He proposed that objects fall at a con-stantly increasing rate, so that the speed of the falling object after two secondsis twice as large as the speed of the falling object after one second. Further-more, the speed at which objects fall should be independent of their weight.Galileo conducted experiments to check his hypothesis, and all of the data hecollected supported his idea. Galileo also noted that an object in horizontal mo-tion would tend to maintain a constant speed. This description of horizontalmotion constituted a separate law.

About one century later, Isaac Newton extended Galileo’s ideas and devel-oped a unified description of motion in the context of three ideas which arenow known as Newton’s Laws. For centuries, every observable motion fit per-fectly with the laws Newton developed. No deviations were seen until the 20thcentury.

Isaac Newton (1643 -1727 ): Generalized allknown observations ofmotion into three laws.

Albert Einstein (1879 -1955 ): Formulated the the-ory of relativity, shown herein his official 1921 Nobelprize photo.

In the year 1905, Albert Einstein published his special theory of relativity.One of the predicted consequences of special relativity is that at speeds near thespeed of light motion should deviate from what Newton’s laws would predict.Experimental evidence collected afterward supported Einstein’s ideas.

The historical development of the laws of motion demonstrate the scientificprocess. An idea is developed and tested. When the experiments yield resultsnot in agreement with the idea, the idea is refined to take into account the newdata, and the theory or model is updated or improved.

We will see many additional examples of how this process has been used inthe sciences during the remainder of this course.

An Example from Everyday Life

We use the scientific process frequently in our lives, though we may not neces-sarily recognize it for what it is. We develop (but generally do not formalize)all sorts of theories, laws, and models to address questions such as

• What is the best way to make cookies?

• What is the fastest way to get to school?

• What is the best time to go to bed/wake up?

Often times these ideas are initially developed with little or no data. I mightassume, for example, that driving to school will be faster than walking. Thistheory may be reasonable in terms of my previous experience. But then I startto gather additional data. As I drive to my first day of school, I encounter atraffic backup at a few intersections. When I get to campus, I discover that theparking lot closest to my class is full. After driving around for an additionalten minutes looking for a place to park (when I could have easily walked to myfirst class from my apartment in 15 minutes), I revise my ideas.

The point is that the scientific method is not really anything new or foreignto you, it’s really just a systematic description of a process that you alreadyintuitively use!

1.4. THE SCIENTIFIC PROCESS 19

REVIEW QUESTIONS

1. Explain how and why scientific ideas are mutable and provisional.

2. Explain why a scientific model or theory is never exactly the sameas the system it describes.

3. What would happen if any of the steps of the scientific process wereremoved?

4. How does the story about Aristotle, Galileo, Newton, and Einsteinillustrate the process of science?

5. Write down at least three examples of times in your life when youhave used the scientific process. What was your model? Did youhave to revise it based on additional observations?

20 CHAPTER 1. WHAT IS SCIENCE?

1.5 Climate Change: A Case Study

OVERVIEW

Summary: The results of scientific investigation can have a large impacton individuals and society. In this section we present the topic of climatechange as an example. The ramifications of global warming - both envi-ronmental and economic could be immense, and as such has become animportant discussion in public policy. Understanding the scientific aspectsof this topic can aid society in making proper decisions.

Learning Objectives:

• Know a few facts about climate change.

• Practice finding reliable sources of scientific information.

• Identify when the scientific method has been properly used and whenthe critical review process is functioning properly.

• Value the role of lower case “s” scientists.

Vocabulary:

• Climate change

• Greenhouse effect

• Carbon emissions

• Cap and trade

It is important to clarify a few points before we consider the topic of cli-mate change. One of the first things we need to understand is the relationship

Climate change:A change in the average tem-perature, humidity, weather,etc... of a region. between weather and climate. Weather is a description of short-term atmo-

spheric conditions and events occurring at a particular location at a particulartime: conditions that are subject to change hour-by-hour and day-by-day. Forexample, when you check the weather you may learn that it’s 64◦F, the windis blowing at 5 mph, and it is partly cloudy with a 20% chance of rain. Afew hours later the temperature may have dropped to 58◦F, cloud cover hasincreased, and it’s raining. Climate, on the other hand, is a description of thelong-term averages of weather-related factors such as temperature, humidity,and precipitation. Depending on the question a climatologist is asking, thelong-term averages they consider may include annual averages, decade-long av-erages, century-long averages, and even longer time frames. This means thatwhen we walk outside on a spring morning and find snow and colder than av-erage temperatures that is a weather event, which by itself does not indicate achange in local or global climate.

The climate of a region determines what kind of plant and animal com-munities exist there, e.g., desert, forest, grassland. This is the case becausethe ecology of any region is sensitive to climate change. For example, an in-crease in water temperature puts stress on aquatic communities. Changes inair temperature and precipitation affect the length of growing season and theamount of water available in the ground, and consequently the ability of plantsto carry out photosynthesis. Living things in turn affect the composition ofthe atmosphere, resulting in a feedback loop that can change local ecosystemstructure.

Earth is no stranger to climate change, including local and global climatechange, as we will see in chapter four. Generally, mass extinctions of life onearth are associated with major changes in Earth’s biosphere and are ofteninduced by climate change. For this reason, among others, many people areconcerned with data that show an increase in the average global temperature.

1.5. CLIMATE CHANGE: A CASE STUDY 21

Figure 1.4: World ecologies, which are sensitive to changes in climate. (Imagecourtesy of Duy Duc (used under the terms of the GNU Free DocumentationLicense)

The Greenhouse Effect

One of the primary factors affecting global climate is the greenhouse effect.

Greenhouse effect:When light energy is easilytransmitted into an object,but heat energy is retained.Most of us are familiar with this effect. One example of a greenhouse effect

occurs when you leave your car outside for a few hours on a sunny day withall of the windows rolled up. The result is that the inside of your car getsquite warm. This happens because light from the sun easily passes through thewindows of your car and then strikes the upholstery and other surfaces insidethe car where light energy is transformed into heat (infrared rays). This heatdoes not pass back through the windows of your car as easily as other forms oflight, so heat builds up inside the car faster than it can be released. This is, ofcourse, also how greenhouses work, thus the name - greenhouse effect.

A similar effect happens in our atmosphere, and indeed has happened sincethe Earth was formed. Our atmosphere is much like the windows of your car.Light can pass readily through the atmosphere, and it then strikes the surfaceof the Earth where it is transformed into heat. This heat is trapped and heldtemporarily by water, earth, and gases in the atmosphere. All solar energy thatstrikes the Earth eventually radiates back into space. When heat from solarenergy radiates back into space more slowly than new solar energy enters theatmosphere the earth warms up. When heat radiates back into space fasterthan solar energy enters the system the Earth cools. And, when the amount ofenergy entering and leaving the Earth is the same global temperature does notchange.

This natural process of our planet’s atmospheric greenhouse effect turns outto be a very good thing. Space is really quite cold, and the Sun is far away.Were it not for the greenhouse effect, the Earth would be a much colder placethan it is today. How much colder? According to some estimates the averagetemperature of the earth would be about 30◦C colder than it is today. FYI- The average temperature on Earth today is about 57◦F (14◦C). So withoutthe greenhouse effect the average temperature on earth would be about 3◦F(-16◦C).

Just how fast heat can escape back into space depends on the compositionof the atmosphere. There are many kinds of gasses in the atmosphere that can

22 CHAPTER 1. WHAT IS SCIENCE?

trap and hold heat. Some gases trap heat more effectively than others. Gassesthat are particularly effective at trapping heat are called greenhouse gasses.Some of the more common greenhouse gasses are water vapor, carbon dioxide,methane, and halocarbons (e.g., CFCs). If the concentrations of these gassesin the atmosphere are altered, then the ability of the atmosphere to trap andhold heat changes as well, and this can cause global climate change.

Climate Forcings

Earth’s climate has fluctuated a lot during its history, including warm and coldperiods. There are many factors that drive shifts in global climate. These fac-tors are referred to as climate forcings. Some forcing factors include things likethe intensity of solar radiation, the shape of the orbit of the Earth and the tilt ofthe Earth’s axis, both of which vary predictably over long periods of time, andother effects such as the amount of cloud cover, volcanic activity, the prevalenceof forest fires, the surface of the Earth covered by snow and ice, etc. These typesof forcings are referred to as natural forcings, because they occur whether hu-

Natural forcings:Climate forcings (factorsthat influence climate) thatoccur independent of humanactivity.

mans are present or not. Anthropogenic forcings, however, are factors that

Anthropogenic forcings:Climate forcings that occuras a result of human activity.

are caused by humans that can have an effect on the planet and cause climatechanges. Historically, natural forcings have dwarfed anthropogenic forcings, butthe development of modern technology has reached the point where evidencesuggests that anthropogenic forcings have become significant and are having aneffect on global climate. The most notable anthropogenic forcing is the mining,drilling, and then burning of fossil fuels.

Humans use fossil fuels (e.g., coal, oil, and natural gas) for heat and light,and we have done so for thousands of years. At the time of the industrial revolu-tion, mankind began to rapidly develop fossil fuel resources and the technologiesneeded to acquire them as quickly as possible in order to satisfy the demandsfor electricity, transportation, and industry. Our dependence on fossil fuels hasgrown immensely in the intervening years. All fossil fuels are made of hydrocar-bons, which are molecules containing primarily carbon and hydrogen. When afossil fuel is burned, molecules combine with oxygen to form water and carbondioxide (CO2), releasing a great deal of energy in the process. Thus burningfossil fuels unavoidably releases CO2 (a greenhouse gas) into the atmosphere,and this release is what we refer to as carbon emissions.

Carbon emissions:Carbon dioxide gas releasedinto the atmosphere. “So what?” you may ask. “Doesn’t the same thing happen when we burn

wood?” Yes, and no. It is true that when both fossil fuels and wood are burnedCO2 is produced and released, but there is a significant difference betweenburning wood and fossil fuels.

Wood is made of carbon-bearing compounds, and when it is burned CO2 isreleased into the air, but the CO2 used to make that wood was captured fromthe atmosphere by the plant that made the wood in the first place. So whenthat wood is burned there is no net change to the total amount of carbon inthe biosphere, and there is no net increase in CO2 concentration in the globalcarbon cycle. Essentially the CO2 is being released into the atmosphere andtaken up by plants in equilibrium. Fossil fuels are different. Fossil fuels aremade of plants and animals that lived long ago, and their bodies (includingthe carbon that was in them) was covered by sediments after they died. Thiscarbon was effectively removed from the global carbon cycle, and eventuallytransformed into fossil fuel. This means that when a fossil fuel is mined orpumped out of the ground and then burned the CO2 that is released increasesthe total amount of CO2 in the global carbon cycle, including increases in theatmospheric concentration of this gas.

The consensus of the vast number of climate scientists is that the measurableincrease in CO2 emissions due to anthropogenic activities (burning fossils fuels,among other things) is responsible for recent increases in the average global tem-perature. In order to avoid potential changes to Earth’s ecology stemming from

1.5. CLIMATE CHANGE: A CASE STUDY 23

higher average global temperatures, climate scientists concluded that mankindshould reduce its carbon emissions. Many people do not seem willing to ac-cept these conclusions and make changes on their own that would result involuntary reductions in fossil fuel use that would produce lower anthropogeniccarbon emissions. After all, energy is still quite inexpensive and supports ourhigh quality of living. It has therefore been suggested that governments shouldtake action on this issue to mandate reductions in carbon emissions in an effortto minimize global climate change effects resulting from anthropogenic forcings.

One way to do this is through a process called cap and trade where a limit

Cap and trade:a system where total carbondioxide emissions are lim-ited, and entities are allowedcarbon emissions throughtheir purchase of “carboncredits.”

or cap is placed on the total amount of CO2 that a nation is allowed to emit in agiven year, based on population size. The allowed emissions are then distributedto energy producers and industrial entities in the form of “carbon credits.” Anentity that exceeds its emission cap would be required to pay significant fines.Entities that release lower amounts of CO2 than their allocated limit couldsell there excess emission credits on the market to others needing additionalemissions capacity. Those who purchase carbon credits would then legally beable to emit more CO2, but at an added financial cost.

The cap and trade approach to the managing of carbon emission creditsincreases the cost of energy production and large scale manufacturing to reflectthe actual and ecological costs of continuing to emit CO2 from fossil fuel use.These costs would almost certainly be passed down to consumers, and thisadded cost will help to increase personal awareness and motivate innovationand development of alternative strategies to meeting our energy needs withoutcontinuing to emit greenhouse gasses into the atmosphere.

Are Humans Really At Fault?

A minority of climate scientist assert that the current observed rises in globaltemperature are not due to human activities, and that these global temperatureincreases can be accounted for strictly by natural forcings. The importance of the cli-

mate change questionThe question of whether or not humans are changing Earth’s climate is an

important one. A change in climate could have significant impacts on suchthings as sea level rise and its effect on coastal areas and populations (presently40% of the global population lives within 60 miles of the coast), and the abilityto continue to grow crops in currently productive regions. People tend to dis-agree on the severity of these effects. Reducing anthropogenic carbon emissionsin order to slow global warming will mitigate some problems, but doing so willalmost certainly have a significant impact on national and global economies,and on our individual standards of living.

The magnitude and importance of the question of global climate change hascaused this problem to become a big political issue. As with all political issues,we find numerous voices in government and media attempting to persuade us tobelieve one way or another on this topic. It is important to note, however, thatthe scientific question of global climate change is an objective one. Either peopleare driving climate change or they are not. It is also an empirical question inthat we can collect objective and reproducible data that allow this question tobe addressed by scientific methods.

Because the question of global climate change is an objective one, the an-swers does not depend on what an individual person may believe, or wantsothers to believe. There is an overwhelming mass of observations that havebeen collected objectively and empirically that we must take into account if weare truly on a quest to obtain an understanding of the truth related to thisquestion. Since this is the case, each of us need to take a serious look at whatwe think and why, and then consider whether we are on a quest for truth, or aquest to support our opinion on this topic regardless of what evidence has beendiscovered.

Questions like ones related to global climate change are helpful in giving us

24 CHAPTER 1. WHAT IS SCIENCE?

an opportunity to assess the kind of learners we are. As we discussed earlier thissemester, we should be on a quest for truth and understanding, and completeunderstanding is possible only in light of truth. With that thought in mindponder on where you stand in terms of your pattern and process of learning byconsidering Figure 1.5 and Table 1.1 below.

Figure 1.5: Types of learners/scholars in relation to their degree of discipleshipand scholarship. (Image courtesy of Dr. Dan Moore, BYU-Idaho, Dept ofGeology)

The information in Table 1 provides additional information that is helpfulin defining attributes of people who fall into each quadrant in Figure 1.5. Youare encouraged to take a sincere, personal look at yourself to determine whichof the four quadrants you best fit into at present. This is a difficult personalassessment, but it is one worth pondering.

Now that you have considered your personal placement on the chart shownin Figure 1.5 by using the information provided in Table 1.1 (a difficult thing todo objectively, by the way), you are ready to complete this reading assignment.

The last thing to do to complete this reading assignment is to do some re-search about the question of whether humans are having an effect on globalclimate. You should spend a minimum of 30 minutes searching for informationon the topic of anthropogenic climate change. You should search for informa-tion on both sides of this issue and make sure that you record the sites whereyou found your information. As you do this on-line research you should lookcarefully for indications of the whether the information you are accessing rep-resents solid scientific information or if it is little more than personal opinionon this topic. Some questions you can ask to help you make this distinctioninclude the following:

• Is the scientific method being used?

• Are steps taken by the author to reduce or eliminate bias?

• Are confidence levels reported, and if so, are the reported confidence levelssufficient to accept or reject a given hypothesis?

• Is the critical review process active and working properly?

• Is the information from a credible source (publication in a scientific jour-nal, reliable web-site, etc.)?

As you carry out this investigation you need to keep a healthy dose of skepti-cism as you review materials. That is, if you are a healthy skeptic on the search

1.5. CLIMATE CHANGE: A CASE STUDY 25

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26 CHAPTER 1. WHAT IS SCIENCE?

for truth you should realize the importance of the ongoing self-correcting natureof science as you ask questions, seek for meaningful observations and data, care-fully consider well-supported conclusions, reject unsupported ideas, and acceptthe best-supported and demonstrated ideas even when those ideas may not fitcomfortably with your preexisting opinion or position on a particular topic. Atthe same time we must all be on guard that we do not become denialists as weseek for answers to questions. A denialist is, sadly, someone who is not on aquest for truth. A denialist is typically on a quest to find ways to support theircurrently existing opinion or position rather than someone who is on an honestquest for truth.

REVIEW QUESTIONS

1. Summarize how the greenhouse effect works in causing climatechange.

2. What is the role of carbon dioxide? Where does it come from? Whatpolicies have been proposed with respect to it?

3. In what ways has scientific investigation been used with respect tothe issue of climate change?

4. Is the critical review process functioning properly?

5. Why might it be important for the general public to understand theanswers to these questions?

Chapter 2

The Universe

Figure 2.1: A portion of the composite Hubble deep field image. Between December 18 and December 28, 1995,the Hubble telescope peered at a dark section of sky near the constellation Ursa Major - a section comprising2.5 arcminutes across. (This is roughly equivalent to the size of a tennis ball located 100 meters away, or asmall square half the width of a dime on each side, held at arm’s length - about 0.00002% of the entire sky).Incredibly, of the 3,000 objects found in the image (of which this is only a portion), almost all are distant galaxies,each of which contains tens to hundreds of billions of stars. The Hubble deep field image illustrates the vastenormity of our Universe and the innumerable quantity of stars, and potentially solar systems, found therein.More importantly, since it takes a very long time for light to traverse these large distances, it gives us a glimpseof how the universe looked billions of years ago. (Image courtesy of NASA)

In this chapter, we will see how the scientific process has been used to find out and learn truths about ouruniverse.

Our foray into this subject, as with the other case studies in the chapters that follow, will be historical. First,however, we will think critically about human intuition, and see why it fails to give us understanding in realmsthat are outside our personal experience. At that point we will consider some simple observations any personcan make relative to the motion of stars and planets, and work our way through the succession of cosmological

27

28 CHAPTER 2. THE UNIVERSE

models that follow.It is important to note that in this chapter (and those that follow), you may come across some scientific

ideas that seem to be at odds with your religious views. This apparent conflict drives some students away fromscience. Others may find themselves drawn away from religion. Neither of these two outcomes is by any meansdesirable. Remember that our objective for this course is for you to understand, appreciate, and value the rolethat the scientific process has in your life. If you find yourself struggling with the question of whether you shouldaccept the scientific or religious explanation of any aspect of the world we live in, I would suggest that you takea few minutes to ponder again the message of Elder Scott in section 1.1, as well as the material in appendix Aat the back of this book.

2.1. THE SCALE OF THE UNIVERSE 29

2.1 The Scale of the Universe

OVERVIEW

Summary: The universe is a really big place, and the numbers used todescribe are often beyond our comprehension. Scientists use two tools tohelp them grasp just what these numbers mean. These tools are embodiedin scientific notation and a process called “scaling”.

Learning Objectives:

• Convert numbers from standard to scientific notation and vice versa.

• Be able to identify the sizes of the Earth, Sun, and other celestialobjects on a few different scales.

• Perform scaling exercises.

• Explain what “lookback time” is, and the effect that it has on ourcosmological observations.

Vocabulary:

• Scientific notation

• Base

• Power

• Exponent

• Mantissa

• Scaling

The universe is a really big place. For example, our Milky Way galaxy,which is hardly even a speck in the vast cosmos, is a whopping six hundredthousand trillion miles across. While many of us have a good feel for what onethousand is, we may not have much intuition when it comes to numbers likeone trillion. Why is this? Because we rarely deal with numbers of this size inour everyday life, and when we do, it is usually in the form of some type ofsound bite (think national debt) as opposed to anything that we immediatelyconnect with our personal existence. In order to really appreciate what suchnumbers mean, one needs to devise a means whereby they understand them interms of things they are already familiar with.

Figure 2.2: An artist’s rendering of the Milky Way galaxy. (Image courtesy ofNASA)

Mathematics and science have devised two tools for doing just this. Thefirst tool is scientific notation, which helps us express such large numbers in asomewhat more compact form. The second tool is called “scaling”, which helpsus to put relative sizes or distances in perspective.

30 CHAPTER 2. THE UNIVERSE

Scientific Notation and Powers of Ten

Going back to the size of the milky way galaxy (six hundred thousand trillionmiles), how does one go about writing such a number without using words? Instandard format, it looks like this:

600,000,000,000,000,000 miles

It is terribly inconvenient to have to write numbers in this format, not tomention ink consuming. A more compact notation, called scientific notation,would express this number as

Scientific notation:a compact way of writingnumbers that are not easy toexpress in standard format. 6.0× 1017 miles

The primary benefit of this notation is immediately obvious! Let’s explain howit works.

First of all, consider the second portion: the 1017. What exactly doesthis represent? It basically says you take seventeen “10”’s and multiply themtogether. The number 10 is called the base , and the number seventeen is called

Base:The integer that serves as thebasis for a number system. the power or exponent. This representation works for other powers of tenPower or exponent:A number representing howmany times the base shouldbe multiplied by itself.

as well, as shown in Table 2.1. The zeroth power is a special case. Any baseraised to the zeroth power is defined as one (i.e. 100 = 1).

Power of Ten Multiplied Out Standard Notation101 10 10102 10×10 100103 10×10×10 1,000104 10×10×10×10 10,000105 10×10×10×10×10 100,000106 10×10×10×10×10×10 1,000,000

Table 2.1: Powers of ten greater than zero. The first column gives the so-called“power of ten” in scientific notation. The second column demonstrates how youwould multiply the base the appropriate number of times. The third columnwrites the result using standard notation.

As you read through Table 2.1, you may have noticed a trend: when youraise ten to some power, the resulting number is a “1” followed by a numberof zeroes equal to the power. For example, 105 is a “1” followed by five zeroes.Equivalently, if you are starting with a “1”, the power of ten tells you how manytimes you need to move the decimal place to the right. This trend holds truefor all powers of ten greater than or equal to zero.

We can also define powers of ten for smaller numbers. In such cases, thepower or exponent is negative. For some examples, see Table 2.2. Notice thatthe same rules apply as before: the exponent tells us how many times we needto move the decimal place to the right. However, since the exponent is negative,we are actually moving the decimal place to the left.

Now let’s go back to the size of the Milky Way galaxy: 6.0×1017 miles. Wenow understand what the second part of this number means (a “1” followed byseventeen zeroes). What about the first half, which is called the mantissa ?

Mantissa:The portion of a numberexpressed in scientific nota-tion that tells you informa-tion beyond the appropriatepower of ten.

As the notation suggests, we simply multiply the power of ten by the mantissa.Let’s run through a few quick examples of how one goes about convert-

ing numbers in standard notation to scientific notation and vice versa. First,Converting standard formatto scientific notation

consider two cases where we want to convert scientific notation to standardnotation. Let’s start by converting 2.763 × 107 to standard notation. First,we need to evaluate the exponent. 107 is a “1” followed by seven zeroes, or10,000,000. When we multiply this exponent by the mantissa (2.763), we get27,630,000.

2.1. THE SCALE OF THE UNIVERSE 31

Power of Ten Standard Notation100 110−1 0.110−2 0.0110−3 0.00110−4 0.000110−5 0.00001

Table 2.2: Powers of ten less than or equal to zero. The first column gives theso-called “power of ten” in scientific notation. The second column writes theresult using standard notation.

What about when the exponent is negative? Consider the number 5.342×10−4. Just as in the previous example, we start by evaluating the exponent. Istart with a “1”, and then move the decimal place to the left four times, whichgives me 0.0001. Now when I multiply this exponent by the mantissa (5.432),I get 0.0005432.

Now let’s go the other way: converting standard notation to scientific no-tation. Suppose I wanted to express the number 345,210 in scientific notation. Converting standard format

to scientific notationThe first thing I need to do is choose an appropriate mantissa. I will select3.4521, since 345,210 is equal to 3.4521 times 100,000. Having selected the ap-propriate mantissa, I only need to express the number 100,000 as a power often. Since the decimal place is five spaces to the right of the “1”, the appropri-ate power is five. Thus, I get 3.4521 × 105. Note that in some instances it isappropriate to truncate the mantissa to fewer digits. For example, had I beengiven the number 345,211.2342, unless there was some particular need to keepten decimal places of precision, I might just write this number as 3.45× 105.

Now for one final example. Consider the number 0.0834. The process Igo through to convert this number into scientific notation is similar to that ofthe previous example. I first must choose an appropriate mantissa - like 8.34.This particular mantissa is chosen because I can write the number 0.0834 as8.34×0.01. All I need to do now is express the 0.01 as a power of ten. Since thedecimal place is two spots to the left of the “1”, I know that the appropriatepower is -2. Therefore, I write this number as 8.34× 10−2.

Scaling

If you have ever read a map before, you are familiar with what scaling is allabout. The map is simply a “scaled down” representation of the lay of the land,where, for example, one inch on the map may represent one mile on the Earth’ssurface. The process of scaling is all about figuring out how to represent the

Scaling:Representing a system at asize other than its actualsize.

sizes of various objects in such a scenario.The process of scaling involves choosing an appropriate size for representing

whatever it is you wish to represent. If I wanted to represent the entire state ofIdaho on a standard piece of paper, I might want to choose a scale where oneinch represented 50 miles. On the other hand, if I was attempting to representjust the BYU-Idaho campus, I might choose a scale where one inch represented1,000 feet.

To demonstrate the process of scaling in more detail, consider the following:Suppose I was building a scale model of the U.S.S. Constitution (see Figure2.3). The actual dimensions of the ship are shown in Table 2.3. Furthermore,suppose that my model needed to be of a scale where the mainmast should be23 cm high. How large should the other dimensions of the ship be?

The first thing I need to do is calculate a scaling factor. This is done bydividing my desired mainmast height (23 cm) by the actual mainmast height

32 CHAPTER 2. THE UNIVERSE

(220 feet). My scaling factor is therefore

S =23 cm220 ft

= 0.10455 cm/ft

Now all that I need to do to scale down the other dimensions is to multiplythem by this scaling factor. For example, the length at the keel should be

(150 feet)× (0.10455 cm/ft) = 15.682 cm

I repeat this process for all of the other lengths, and obtain the following forthe scaled-down dimensions of the U.S.S. Constitution the results in Table 2.3

Figure 2.3: The U.S.S. Constitution. (Image courtesy of U.S. Navy)

Dimension Original ScaledLength: Billet head to taffrail 204 feet 21.327 cm

At waterline 175 feet 18.295 cmAt keel 150 feet 15.682 cm

Beam (width): 43 feet, 6 inches 4.5477 cmDraft: Forward 19 feet, 2 inches 2.0038 cm

Aft 22 feet, 9 inches 2.3784 cmMast heights: Foremast 198 feet 20.700 cm

Mainmast 220 feet 23.000 cmMizzenmast 172 feet, 6 inches 18.034 cm

Table 2.3: Original and scaled dimensions of the U.S.S. Constitution. Actualdimensions obtained from http://michaelthompson.org/ironsides/.

In the review questions at the end of this section you will be given theopportunity to practice this process of scaling, in addition to the conversion ofstandard format numbers to scientific notation. It is recommended that youtry to complete these exercises on your own, and then review them during yourgroup meeting. You will also be given an additional opportunity to practice onone of your homework assignments. Understanding how to do these things isimportant if one is to understand the relative sizes and distances encounteredin astronomy.

Time as a Measurement of Distance

In astronomy and cosmology - the two fields of science most interested in theUniverse as a whole - we deal with incredibly large distances. Scientists havedeveloped an interesting way to describe these distances, and it is based on howlong it takes light to travel those distances.

2.1. THE SCALE OF THE UNIVERSE 33

Light travels through empty space at an amazingly fast speed (300,000,000meters each second, or 671,000,000 mph)! What’s equally astounding is thatlight always travels at this same measured speed through empty space, no mat-ter who measures it or when. The constancy of the speed of light allows usto equate time with distance: I know that in each second the light will travel300,000,000 meters through empty space!

Consider how long it takes light to traverse everyday distance. If you switchon a light bulb that is ten feet from your eye it takes 0.000000102 seconds toget to you. That’s just over one billionth of a second. What about the timelag for light travel over a longer distance? Let’s say that you are 20 miles westof Rexburg, looking back toward town, and the lights of the Rexburg Templeare turned on. How long will it take for that light to reach you? If you do themath you will discover that it takes light 0.000108 seconds to cover 20 miles.Concluding that the speed of light is more or less instantaneous makes sensein the scale of the every day, because the scale of the every day is made up ofrelatively short distances (inches, feet, miles, centimeters, meters, kilometers).However, at the scale of the solar systems, galaxies, and the universe, thedistances are much, much larger, and consequently it takes much longer forlight to traverse those distances.

Even at tiny astronomical distances like the Earth-Sun distance (a mere 93million miles) the travel time for light is significant (8.3 minutes). As a result,there is no way to know what is happening on the Sun at a given moment,because what we see was happening on the sun 8.3 minutes ago.

In 1977 NASA launched Voyager 1, a spacecraft that is traveling away fromthe Earth at a speed of about 50,000 mph. Now, over 30 years later, Voyager 1is just over nine billion miles from us. Imagine that you are sitting at Voyager 1mission control and you have to change its course, i.e., drive it. You have datashowing that if you do not change the course of Voyager 1 that it will collidewith something in around 20 hours. What will happen if you use your intuitionbased on the scale of the everyday that light travels more or less instantaneouslyto its destination, and you send the radio signal (which travels at the speed oflight) for the course correction about one hour before the collision is predictedto occur? If you do the math you will find that when you divide 9 billion milesby the speed of light that it actually takes over 13.4 hours for the signal to reacha point 9 billion miles away. Imagine driving a vehicle that only allows you tosee what was in front of you 13.4 hours ago, and only responds 13.4 hours afteryou tell it to do something. That’s a 27 hour lag time between observation andreaction. If you or I tried to drive a vehicle like that to the grocery store, we’dbe paying much more for our insurance! Fortunately, space is essentially emptyso there are exceptionally few obstacles to avoid.

You can now see how problems are generated by assuming that we interactwith light on the scale of the very large like we do in our day-to-day lives.Even so, the distances we’ve considered so far are relatively minuscule in thecosmological scale. Imagine the nature of a ‘conversation’ between someone onEarth and someone on a planet orbiting another star. It would take severalyears for messages to be transmitted between us and a planet orbiting thenearest stars, and much longer for anything else.

A light year is defined as the distance that light will travel through

Light year:The distance that light willtravel through empty spaceover the course of one year.empty space over the course of one year. Considering how fast light travels

(671,000,000 mph) this is quite a large distance (5.88 trillion, or 5.88 × 1012,miles). Using this standard of measurement, we would say that the distance be-tween the Earth and our nearest neighboring star, Alpha Centauri (25.7 trillionmiles), is around 4.4 light years.

34 CHAPTER 2. THE UNIVERSE

REVIEW QUESTIONS

1. Why do scientists use scientific notation?

2. Why do scientists use scaling?

3. Convert the following numbers into standard notation: (a) 4.34×103,(b) 7.22× 10−1, (c) 1.45× 100, (d) 8.99× 1015, (e) 2.25× 10−9.

4. Convert these numbers into scientific notation: (a) 18,900 (b)548,900,000 (c) 0.002964 (d) 3.24 (e) 0.000000993

5. In the following you are given several actual lengths of objects youmight encounter in a typical U.S. city. Suppose you wished to scaleall of these lengths down so that the car is 2.000 cm long. Determinethe scaled down lengths of the other given sizes. The objects andtheir sizes are: (a) a shoe which is 0.280 meters long, (b) a personwho is 1.676 meters tall, (c) a car that is 5.126 meters long, 1.892meters wide, and 1.562 meters high, (d) a building that is 25.40meters tall, and (e) a sidewalk that is 250.0 meters long.

6. Polaris, the North Star, is approximately 2.5 quadrillion miles (2.5×1015 miles) from the Earth. How long ago did the light we willobserve from the North Star tonight actually leave Polaris?

2.2. TIME AND INTUITION 35

2.2 Time and Intuition

OVERVIEW

Summary: “Time” is a word we use frequently. It seems sometimesthat our life is governed by the passage of time. Yet all of our experiencewith time is on the human scale - most of us have only directly observedthings that are roughly our size and have temporal durations ranging fromseconds to centuries. As you move into other scales of nature, our intuitiveunderstanding of time is no longer valid. To accurately measure time, youmust have two things: a rate-constant process and some way of knowingwhen that process began.

Learning Objectives:

• Explain where our intuition comes from.

• Identify the two characteristics that must be possessed by any objector process that is to be used as a clock.

• Define the following terms: sequence, duration, rate.

• Identify the characteristics of time in everyday life, the scale ofthe universe, at very high speeds, and on the scale of atoms andmolecules.

Vocabulary:

• Sequence

• Duration

• Rate

• Cyclicity

• Rate-constant process

• Deep time

• Spacetime

You learned about time early in your life. When you were an infant youinnately knew when it was time to eat, sleep, etc., and you have spent yourlife surrounded by time-keeping devices that help you mark the passing of time.Over the years you also gained insights into how the passage of time affects yourlife and things around you. These kinds of experiences give you an intuitiveunderstanding of what the passing of times means.

Since you have so much experience with time-keeping devices and the pas-sage of time, wouldn’t you think that you’d have a sound understanding of whattime is? Well, chances are that you don’t. Few people do. Surprised? OK,don’t get defensive; it’s just that most people haven’t seriously considered thenature of time itself. Time is an extremely rich subject; so rich that sciencehas yet to develop a definition that correctly and completely explains all of theobservable characteristics of time.

Sequence:the ordering of events intime.Duration:the length of an event or timeinterval between events.Rate:How quickly a process pro-ceeds.Cyclicity:the characteristic and fre-quency of repetition in a pro-cess.

The goal of science is to explain how the natural world works. As such, sci-entific explanations help you to understand things that you cannot understandby personal intuition alone. Time is a scientific topic where personal intuitionalone does not help you understand what time is and how it works at all of thedifferent scales of the physical world. The purpose of this particular reading isto help you increase your understanding and expand your intuition with respectto a fundamental aspect of the physical world - TIME.

Before getting into specifics there are some time-related terms that youneed to be acquainted with: sequence, duration, rate, and cyclicity. Sequencerelates to the ordering of events, and duration describes the length of an eventor the interval between events. Sequential time allows us to describe cause-and-effect relationships and to describe the direction in which time passes. The

36 CHAPTER 2. THE UNIVERSE

rate of a process describes how quickly something changes through time, andcyclicity refers to the characteristic of repetition or periodicity of a process orhow frequently a process happens.

Measuring Time

There are many devices that can measure the passage of time, or, perhaps,your passage through time. Any measurable, calibrated, rate constant processcan do this. A rate constant process is a process that happens regularly

Rate constant process:a process that proceeds al-ways at the same rate andwith regular cyclicity. and repeatedly. The apparent motion of the sun across the sky exhibits this

characteristic. The passage of the sun across the sky lasts a certain consistentperiod of time (duration), and it travels at a constant, predictable and mea-surable rate. Lastly, the sun follows this path daily (cyclicity). Because themovement of the sun has these characteristics, we can use a sundial as a time-keeping device. Some other things that have been used to track time include

Examples of rate constantprocesses used as the basisfor clocks hourglasses, candles or sticks of incense that burn at a constant rate, and, of

course, clocks and watches. Mechanical clocks keep time as a spring drives amechanism of wheels that move clock hands at a constant rate. Even betterwatches and clocks keep time based on the accurate and consistent vibrations ofquartz crystals (32,000 times/second) or cesium atoms (9 billion times/second).The best mechanical clocks (spring-driven) are accurate to within 2-3 secondsper day, quartz watches (like the ones most of you use) are accurate to within0.5 seconds per day, and a cesium clock (atomic clock) is accurate to within onesecond per millions of years!

Figure 2.4: A wristwatch is a typical time measuring device. (Image is in thepublic domain.)

It should be fairly obvious that none of the above mentioned devices couldfunction as a clock unless it somehow recorded the number of ticks that hadpassed. Imagine a device based on the oscillation of strontium atoms thatClocks must record the num-

ber of ticks that have passed accurately and precisely caused an LED bulb to flash once every second. UnlessI carefully watched and recorded the number of flashes myself (which I reallydo not want to do), this device could not be used to keep time. Indeed, arate constant process is useful only for keeping time inasmuch as the numberof cycles that have passed are recorded.

The time-keeping methods mentioned above are helpful in monitoring theshort-term passage of time, but a different class of clocks is needed to measurethe passage of extremely long periods of time - periods of time where humanswere not present to record the occurrence of events. These events include thingslike the formation of the earth, the ages and durations of ice ages, etc. Clocksthat can measure these kinds of events are sometimes called geologic clocksbecause they keep track of time and events that occurred before human-recordedGeologic clockshistory. Even so, geologic clocks must also be rate constant and record thenumber of “ticks” that have passed. Fortunately, geologic clocks with thesecharacteristics exist. Examples of geologic clocks include annual tree rings,

2.2. TIME AND INTUITION 37

the accumulation of annual ice layers in glaciers, and the predictable decay ofradioactive atoms trapped in different kinds of materials. These geologic clocksallow you to glimpse what happened hundreds, thousands, millions, and evenbillions of years ago. The topic of geologic time will be addressed in greaterdetail later in the course.

Figure 2.5: Seasonal growth rings in trees constitute a “natural” clock. Theyare based on a rate-constant process (regular change in the seasons), and therings constitute a record of the number of ticks. (Image is in the public domain.)

Strange Time: When Intuition Fails

There are three main levels or scales of nature, and science has discovered thattime acts differently at each of these scales. This is why individuals will oftencome to the conclusion that time is strange. The three levels of nature arethe scale of the very small (e.g., the scale of subatomic particles, atoms, andmolecules - where things happen much faster than you would expect), the scaleof the every day (e.g., individual organisms, populations, and societies - whereyou are familiar with the behavior of time), and the scale of the very large (e.g.,planets, galaxies, and the universe - where expanses of time are astoundinglylarge). Time also varies according to speed - a strange and fascinating truthuncovered early in the 20th century.

Your experience and intuition is unavoidably based on your personal ex-periences in the scale of the every day. It is therefore not unusual for you toassume that things at the scales of the very large and very small happen moreor less the same way that they do at the scale of the every day. This is, alas,

Intuition is based on day-to-day experience, and thusfails in realms outside our ex-perience.a false assumption, and when you start to learn about how things happen at

these other scales you may be confused, at least at first. Fortunately one of thepowerful aspects of science is its ability to help you develop and extend yourintuition of how the physical world works at all scales of nature.

At the end of section 2.1, we saw an example of how intuition can fail onscales outside of our own experience. How many movies have you seen where aspaceship communicates instantaneously with planets that are many light yearsaway?

The rest of this reading is devoted to giving you some insights into how timeworks at different scales, and also how attempting to use our intuition of timeat these scales can lead us to some faulty conclusions. In particular, we willinvestigate deep time (extremely long time intervals); relativistic time (the roleof time in the large-scale organization of the universe); and atomic time (howtime acts at the scale of the very small).

Deep Time

“Deep Time” is a term used to describe ages of very old things and enormousexpanses of time that extent as far back as the origin of the universe. Questions

38 CHAPTER 2. THE UNIVERSE

about origins are an integral part of being human. There is something insideof us that makes us wonder and want to know about when, where, and howthings came into being: the universe, the earth, and ourselves.

Scientific investigation into the age of the universe, galaxies, atoms, andthe Earth, indicate that these things are all extremely old. The astronomerEdwin Hubble observed that all galaxies are moving away from each other.Based on this observation and the physics of Albert Einstein, Georges Lemaıtrehypothesized that the universe began long ago as a singularity (an infinitelydense point containing all energy and matter) that began to inflate and cool,and this ongoing inflation produced the universe we observe today. (This modelof the universe is commonly referred to as “The Big Bang” and will be exploredin more depth at the end of this chapter.) The presence of cosmic microwaveradiation (left over heat from the big bang) led to the conclusion that theuniverse is about 13.7 billion years old. This age of the universe is also supportedby other lines of evidence including the proportion of types of atoms in theuniverse, the ages of stars and galaxies, and the decay rates of radioactiveatoms.

Figure 2.6: A depiction of the evolution of the universe after it became trans-parent to light. This “first light” is the cosmic microwave background (CMB)that we observe today. (Image courtesy of NASA)

Radiometric dating of meteorites, the moon, and rocks on Earth indicatethat our solar system, including Earth, is about 4.5 billion years old.

Big numbers like those needed to describe the age of the universe or the ageof the Earth go beyond our personal experience and intuition. This is whatmakes them difficult to comprehend!

Relativistic Time (and Space)The life of Albert Einstein

As a schoolboy, young Einstein showed ability in mathematics and physics, buthe loathed the rote learning used in school at the time. As a result he did notearn good grades, and one teacher even told him that he would never amountto anything. Yet, Einstein was a deep thinker. During his teenage years hecame upon a question that occupied his thoughts throughout much of his life.This question was, “What you would observe if you could run alongside a beamof light?” Anyway, he attempted to skip secondary school and go directly tocollege. He scored very well in mathematics and physics, but failed the otherportions of the college entrance exams, so he was forced to return to secondary

2.2. TIME AND INTUITION 39

school. He finished secondary school and then went to college, graduating witha degree as a high school math and science teacher. Jobs were hard to come by,and Einstein moved from one menial job to another. He eventually secured aposition as a patent clerk in Bern, Switzerland. During all of this time he hadvirtually no contact with the scientific community, yet he continued to ponderthe questions of the universe.

In 1905, at the age of 26, Einstein published four papers. 1905 has sincebeen referred to as “the miracle year,” because his four papers changed thecourse of modern physics. These papers addressed the photoelectric effect,Brownian motion, special relativity, and the equivalence of matter and energy(E = mc2). The quality and scope of his work was so important that theUniversity of Zurich awarded him a PhD that same year. In 1916 his theoryof general relativity was published, and several years later empirical evidencesupporting his conclusions were discovered. A few years later he received theNobel Prize, and from his unremarkable academic beginnings Einstein becameone of the most notable scientists of all time.

Einstein’s theories of relativity describe the nature of time and space in twosituations: at very high relative speeds, and in the vicinity of massive objects.These theories also suggest that time and space are part of a four-dimensionalentity called spacetime which makes up the fabric of the universe. Both of

Spacetime:The four dimensional fabricof our universe in which timeand space are intrinsicallylinked together.

the situations Einstein described are outside of your day-to-day experiences, soyour intuition will not serve you well as you try to understand relativity andspacetime. The essential conclusion about Einstein’s theories of relativity isthat spacetime is plastic, i.e., an inch or a second in one situation is not thesame as an inch or a second in another situation.

For example, the special theory of relativity states that at speeds approach- Special relativitying the speed of light, both space and time contract; in other words, movingclocks tick more slowly than stationary clocks, and moving rulers are physicallyshorter than stationary rulers. This is true of any objects moving relative toeach other at any speed, though at small speeds, like those in the scale of theeveryday, the differences are extremely small.

The general theory of relativity states that massive objects curve or warp General relativityspacetime. This is true for masses of any size, though the effects of an objecton spacetime are significant only for extremely massive objects. An adageattributed to John Wheeler summarizes this relationship succinctly: “Spacetells matter how to move; matter tells space how to curve.” While this maysound bizarre at first, it can be imagined in this way. Any object, such as anasteroid, can move through spacetime only in a straight line. When an asteroidmoves toward a massive object it encounters the curved space produced bythat massive object, the asteroid appears to change course. Why? The asteroidactually continues to move in a straight line, but now it is moving in a straightline through curved spacetime. Similarly, the Earth moves in a straight linethrough spacetime that is curved by the sun. The curvature of spacetime iswhat Newton called gravity. According to Einstein there is no such thing asgravity, there is only the presence or absence of curved spacetime. Where thereis no curvature of spacetime there are no measurable gravitational forces! Sincethis is the case, what would happen to the path of the Earth through spacetimeif the sun were to spontaneously disappear?

Einstein’s theories of relativity and what they tell us about spacetime are notjust hypotheses. They have been and are being tested, and have been confirmedby empirical observations. One set of observations was made using two identical Empirical evidence support-

ing the theory of relativityatomic clocks and one supersonic aircraft. The clocks were synchronized and onestayed on the ground while the other clock went on an airplane ride. Afterwardthe two clocks were compared to each other, and the clock that stayed on theground ticked more times than the clock that traveled at supersonic speeds- and the difference between the clocks was consistent with what you wouldpredict in the context of special relativity. There is also a conceptually simple,

40 CHAPTER 2. THE UNIVERSE

Figure 2.7: According to general relativity, massive objects, such as the Earth,warp space time in their near vicinity, and the gravitational interactions withother nearby objects can be understood in terms of the object traveling instraight lines through curved spacetime. (Image courtesy of NASA)

but technologically complex experiment designed to collect data that will eithersupport or reject Einstein’s conclusions about the effects of massive objects onspacetime. This is called the Gravity-Probe B (GB-P) project being conductedby Stanford University and NASA. Watch the following videos to learn moreabout the GP-B experiment:

1. A simple experiment (1 min 8 seconds)http://einstein.stanford.edu/Media/Simple Expt Anima-Flash.html

2. Gravity in Newton’s Universe (1 min 32 seconds)http://einstein.stanford.edu/Media/Newtons Universe Anima-Flash.html

3. Gravity in Einstein’s Universe (1 min 32 seconds)http://einstein.stanford.edu/Media/Einsteins Universe Anima-Flash.html

4. Measuring Spacetime Curvature with Orbiting Gyroscopes (3 mins)http://einstein.stanford.edu/Media/Rel gyro expt-anima-flash.html

5. Testing Einstein’s Universe (26 mins - but worth it!)http://einstein.stanford.edu/Media/Testing Einsteins Universe-Flash.html

To wrap up this section on relativity and spacetime, consider this. If youbuilt a spaceship and you got in and flew away at speeds approaching the speedof light, and you then turned around and flew back to Earth at near the speedof light, time would have passed much slower for you than it did for everyoneon Earth. According to Einstein’s theories, it would be entirely possible for youto travel at an extremely fast speed for a long time, and because of the rate atwhich you pass through spacetime you would age more slowly than everyone athome on Earth. So your family and friends would age at the regular rate andwould be very old or perhaps even dead by the time you got home, even thoughit didn’t feel to you like much time had passed at all. Seems strange!? That’srelativity! An the reason it seems strange is because our personal experiencesdo not include the realms of existence where the effects of relativity becomeobvious.

Atomic Time

There is one last scale to consider in terms of time: the scale of the verysmall. While the world of the very small is a miniature world, it is not justa miniaturized world of the scale of the every day. Again, be sure that youdo not rely too heavily on the intuition you have gained from your day-to-day

2.2. TIME AND INTUITION 41

experiences as you try to make sense of the scale of subatomic particles, atoms,and molecules.

Einstein spent the majority of his life working on questions related to relativ-ity, but one of the papers from “the miracle year” provided the physics neededto confirm the existence of atoms. Other researchers, such as Max Planck andNeils Bohr, pushed this work further. Of course atoms are very small, but yourintuition alone cannot help you comprehend just how small they are. Even so,here’s an example: the width of a strand of hair is about 700,000 atoms wide.So, if the period at the end of this sentence were the size of an atom, then astrand of hair would be about 350 yards wide!

Many of you may think about subatomic particles such as electrons as beinglittle spheres that hurtle through space, collide with each other, and sometimesstick together. While this model is helpful in some ways, it is quite inaccurate.Scientists have discovered that electrons and other particles sometimes act likethese particles, but other times they act like waves. The only way to trulyunderstand them is to account for both their particle-like and wave-like proper-ties! Because of this odd nature, referred to as wave-particle duality, electronsand other subatomic particles simply do not behave like the things we are usedto interacting with.

The scientist Max Born stated, “At every instant a grain of sand has adefinite position and velocity. This is not the case with an electron.” We candetermine the location of an electron or its rate of movement, but we cannotidentify its location and rate of motion simultaneously. And it’s not just becausethey are moving too fast or are too small. There is something intrinsic to theirwavelike behavior that makes it impossible to pin down both an electron’sposition and speed at the same time.This is one of the basic conclusions of theuncertainty principle. If this doesn’t make sense, well, the only way we canmeasure the location of something is to expose atoms to light or other formsof energy and use the effects of the interaction between light or energy andatoms to determine their locations. But, because atoms are small, using lightor energy to determine their location and rate of movement would be kind oflike trying to determine the location of a desk by bouncing cannon balls off ofit. The effects of the cannon balls make it impossible to determine the actualposition and direction of motion of your neighbor. So the best we can do inthis case is to develop a series of probabilities that explain where an atom isand what it is doing at a given time.

Frankly, the world of the very small, a world that is currently best describedby quantum theory, is much, much richer than the overly simple explanationprovided here. What we do know is that atoms are real, and the electronicworld in which we live is founded on atoms and what they do.

End Note

The topic of time is an interesting and sometimes confusing one. Confusionusually occurs when we try to use intuition that is meaningful only in the scaleof the every day to understand what is going on in the scales of the very largeand the very small. While you may suffer some initial confusion as you do yourbest to tackle the topic of time at these other scales, it is our hope that youwill pose questions, seek to find answers to them, and do your best to cometo an understanding of what science tells you about time. As you do so yourincreased exposure to discoveries and ideas about time may yield unexpectedunderstanding, or at least ideas, when you read and ponder scriptures that referto time, e.g., D&C 88:110, Alma 40:8. While we offer no explanations of whatthese scriptures mean, you may have ideas stemming from today’s topic thatare well worth consideration.

Last of all, we want to emphasize that although some of the ideas presentedin this reading seem fairly wild and crazy, they are ideas that can and have

42 CHAPTER 2. THE UNIVERSE

been tested scientifically!

REVIEW QUESTIONS

1. What is meant by “rate constant process”?

2. Why is it necessary for any clock to record the number of “ticks”that have passed?

3. Make a list of three naturally occurring processes that could be usedas clocks.

4. As you read about some of the more extreme examples of time (deeptime, relativistic time, atomic time) some things may have seemedcounter intuitive? Why do you think that is?

2.3. EARLY COSMOLOGICAL MODELS 43

2.3 Early Cosmological Models

OVERVIEW

Summary: Early models of the universe were formulated from the (rel-atively primitive) data that could be collected at the time. The Geo-centric model of the universe proposed by Claudius Ptolemy could ex-plain many celestial observations, including the retrograde motion of theplanets. Later observations, especially the phases of the planet Venus,indicated that the Sun did not orbit around the Earth. Over time the sci-entific community came to accept the Heliocentric, or Sun-centered, modelof Nicolaus Copernicus. As additional observations were made, this modelalso needed refinement.

Learning Objectives:

• Identify the important characteristics and aspects of the geocentric(with and without epicycles) and heliocentric models.

• Identify which pieces of data support or conflict with various earlycosmological models.

• Discern between observations and interpretations in the context ofthese models.

• Explain what “retrograde motion” and “phases” are, and how theycan be explained in the context of these early models (including allof the appropriate key terms).

Vocabulary:

• Geocentric model

• Crystal spheres

• Retrograde motion

• Epicycles

• Phases

• Heliocentric model

Hundreds of years ago, if not thousands of years ago, man looked into theheavens and observed the motion of celestial objects. His natural curiosityled him to question the mechanisms by which these motions took place. Theproduct of these investigations was a model which described where the variouscelestial objects were and how they moved with respect to each other. As newdata has been acquired over the years, that model has been refined. Even so,pretty much everything we know about the Solar System and the Universe mustbe gleaned from Earth or near-Earth based observations!

For this class period, we are going to consider the data that was available tothose early astronomers and see why they arrived at their models Throughoutthis process, it will be helpful if you can consider the observations from theperspective of a person living hundreds of years ago - a person who had neverseen pictures of the solar system, galaxies, and the like.

Installing Astronomy Software

We do not have hours and days to spend collecting data (and it really takes thatlong to collect astronomical data). Instead, we will use computer software tosimulate the motion of celestial objects in the sky. The software we will use iscalled Stellarium, and can be downloaded for free at http://www.stellarium.org.

The instructions that follow assume that you are using Stellarium version0.10. At the top of the above mentioned web page, on the right side, you will seelinks for downloading the program for Linux, Windows, and Mac OS X. Click

44 CHAPTER 2. THE UNIVERSE

Figure 2.8: A time-elapsed photo of the stars at night (Image courtesy of SteveRyan (licensed under CC SA-BY))

on the link appropriate for your operating system, and follow the instructionsfor installing the software.

Some students have experienced crashes while attempting to install or usethis software. If this happens to you, you can either borrow someone else’scomputer, or try installing an older version of the software.

A Quick Tutorial on Using Stellarium

When Stellarium starts up, you should see a view of the sky from Paris, France.The program can be controlled via the menus on the left and bottom edges ofthe screen, or by keyboard commands. For the purposes of this activity, you willneed to use the following keyboard shortcuts. (Note: because Mac computersKeyboard shortcuts for Stel-

larium have assigned other behaviors to the function keys, Mac users may find thatsome of these shortcuts do not work. All functions can still be performed bylocating the appropriate item in the menus, or you can check the softwaredocumentation.)

• F6: Brings up the location dialog box. In this menu, you can select yourlocation from a list (Rexburg is on that list, incidentally), simply clickon the world map, or specify a latitude and longitude. For portions ofthis activity, it is desirable to set our location at the North Pole. Thisreduces the amount of transient motion due to the Earth’s rotation. Theeasiest way to do this is to enter N 90◦00’00.00” for the latitude and E00◦00’00.00” for the longitude. You should probably set the altitude to 2meters.

• F3: Brings up a dialog box that allows you to search for an object in thesky. Simply enter the name of the object and push the “Enter” key.

• Space: Centers the view on the selected object.

• Arrow keys: change the direction in which you are looking.

• A: Toggles the atmosphere on and off. For our purposes, you should toggleit off.

• F: Toggles the fog on the horizon on and off. Again, it is better for us ifthis is turned off.

• G: Toggles the ground on and off. Depending on what object it is you aretrying to observe, you may need to turn the ground off.

• L: Increase the rate at which time passes. This allows us to collect monthsworth of observations in just a few minutes.

2.3. EARLY COSMOLOGICAL MODELS 45

• J: Decrease the rate at which time passes.

• K: Set the time rate to it’s actual value.

Observation 1: The Motion of the Stars and Planets

Using Stellarium, make the following observations and answer the questions thatfollow (each numbered item below has one question that needs to be answered).You should complete this exercise before coming to class!

1. Set your location to Rexburg and turn off the atmosphere and fog (seeinstructions above). Increase the time rate by pressing “L” several times.Watch the stars. What can you say about their motion?

2. Based on this motion, how would you say the stars are moving relative tothe Earth?

3. Do the stars appear to move separately or together?

4. Based on the motion of the Sun, how would you say it is moving relativeto the Earth? (You may need to use the arrow keys to get the Sun intoyour field of vision.)

5. Based on the motion of Venus, how would you say it is moving relativeto the Earth?

6. Based on the motion of Mars, how would you say it is moving relative tothe Earth?

7. Taking in account all of these motions together, develop a model thatexplains why the various celestial objects appear to move in the way thatthey do.

8. The previous observations formed the basis for the earliest cosmologicalmodel. As technology improved, astronomers were able to observe themotion of celestial objects relative to each other. In Stellarium, pressthe F6 key and set your location to the North Pole, as explained in theinstructions in the previous subsection. Turn the ground off by pressing“G”. Press the F3 key and select the planet Jupiter by typing “Jupiter”into the search field. When you press “Enter”, Jupiter should be at thecenter of your screen. Press the space key once to keep your field of visioncentered on Jupiter. Now, speed the time up by pressing “L” severaltimes (enough that Jupiter appears to move quickly relative to the stars).What can you say about Jupiter’s motion relative to the stars?

9. Is this new data consistent with the model you developed? Explain.

Geocentrism and its Demise

As you just observed, all celestial objects that are observable with the nakedeye appear to move in circles around the Earth. This set of observations ledClaudius Ptolemy (c.85 - 165 A.D.) to develop a model of the universe where theEarth was at the center, with all of the other celestial objects in orbit aroundthe Earth, as in Figure 2.9. This type of model is called a Geocentric model,or Earth-centered model.

Geocentric Model:A cosmological model wherethe Earth is at the center ofthe Universe.

A major test of the Geocentric model came with the observation of retro-grade motion of the planets. You observed this motion in item 8 above. The

Retrograde motion:The apparent periodic back-ward motion of the planetsrelative to the stars.

planets, for the most part, move uniformly relative to the background of stars,except occasionally they reverse direction for a short period of time. This mo-tion cannot be explained in the context of a simple Geocentric model. In orderto account for this motion, Ptolemy suggested the following: the motion of the

46 CHAPTER 2. THE UNIVERSE

Figure 2.9: A representation of a Geocentric model based on that of ClaudiusPtolemy. This particular model was included in Peter Apian’s Cosmographia(Antwerp, 1539). (Image is in the public domain.)

planets was due primarily to the rotation of a sphere (called the deferent) asin a simple Geocentric model,but embedded in this sphere was another smallersphere, called an epicycle (see Figure 2.10. The planet itself was fixed to the

Epicycle:An additional orbital path ofplanets in geocentric mod-els used to explain retrogrademotion.

epicycle, and when both the deferent and the epicycle rotate it will appear toan Earth bound observer that the planet travels backward for a space of time.Additionally, the deferent needed to rotate around a point somewhat distantfrom the center of the Earth to better account for planetary motion. For helpin visualizing this process, refer to the animation found at

http://astro.unl.edu/naap/ssm/animations/ptolemaic.swf.

As astronomical measurements improved, it was discovered that one epicyclefor each planet was not sufficient to fully describe the motion of celestial bodies.Further refinements were made as epicycles were built on top of epicycles. Theoverall effect was to clutter the model. Each deferent and epicycle had to bedescribed by a distance (radius) and a rotational speed. As more and moreepicycles were introduced, more and more parameters were included in themodel. Figure 2.11, which represents a model for the motion of the planetMercury, gives a flavor for just how complex things became.

With the introduction of the telescope by Galileo, further observations weremade that could not be explained by the Geocentric model. In particular,Galileo was able to observe the phases of Venus. You are probably familiar

Phases:Refers to the amount of a ce-lestial body’s surface illumi-nated by the Sun and visibleto an Earth-based observer.

with phases in terms of the phases of the Moon. The appearance of the Moonchanges from day to day with respect to how much of its surface we can seewith the unaided eye. The reason for the different phases is associated with therelative positions of the Moon and the Sun. The Moon does not emit light ofits own. The light we see from the Moon is in fact sunlight reflecting from itsPhases of the Moonsurface. Similarly, the planet Venus does not emit light. The light we see fromVenus is reflected sunlight, and therefore Venus should exhibit phases as well.Phases of Venus

In the context of a Geocentric model, Venus should only exhibit a partialset of phases ranging from a “half-Venus” where we see half of the planet’ssurface, to a “new Venus” where we see none of the surface at all. We saw these

2.3. EARLY COSMOLOGICAL MODELS 47

Figure 2.10: Epicycles were introduced as a means of refining the Geocentricmodel to account for retrograde motion. The motion of the planet was governedby two spheres. The outer sphere (deferent) maintained a constant rotationabout a central point located next to the Earth (the “×”), and the smallersphere (epicycle), to which the planet was fixed, rotated about the deferent.(Image is in the public domain.)

Figure 2.11: A model for the appearances of Mercury, attributed to Ibn al-Shatir (14th century), which demonstrates the complexity of multiple epicycles.(Image is in the public domain.)

48 CHAPTER 2. THE UNIVERSE

predicted phases at the end of our last class period. For review, you may wishto revisit the animation found at

http://astro.unl.edu/classaction/animations/renaissance/ptolemaic.html.

Now, let’s make the actual observation.

Observation 2: The Phases of Venus

Start the Stellarium program, and set your location to wherever you wish. Turnoff the atmosphere, fog, and ground by pressing the “A”, “F”, and “G” keys.Press “F3” to search for the planet Venus. The field of view should center onthe planet. You may also want to speed up time by pressing the “L” key. Priorto the invention of the telescope, this is how Venus appeared to astronomers.Can you say anything conclusive about it’s phases?

The Stellarium program has the ability to zoom in and out. This is accom-plished by pressing the “Page Up” and “Page Down” keys. This ability to zoomin on a particular place in the sky is akin to improving your telescope. Zoomin on the planet Venus until you are able to observe its phases, then answer thequestions below:

1. Does your observation of the phases of Venus agree with the predictionsof the geocentric model? Explain.

2. If the observations agree with the geocentric model, make a suggestion asto how we could further test the model. If the observations do not agreewith the model, can you think of a minor refinement to the model thatwould explain the new observations?

Refinement: The Heliocentric Model

Nicolaus Copernicus (1473-1543) suggested that it was not the Earth that oc-cupied the space at the center of the universe, but the Sun. All other celestialbodies maintained circular orbits around the Sun. This idea, referred to as aHeliocentric model was not exactly met with open arms. For one thing, the

Heliocentric model:A cosmological model wherethe Sun is at the center of theUniverse.

Geocentric model was very much aligned with the popular theology of the day.The Heliocentric model, represented in Figure 2.12 was able to account for

the retrograde motion of the planets without introducing additional parameters,as needed to be done in the Geocentric model. As long as the orbital periodof the planets differ, each planet will periodically appear to move backwardrelative to the background of distant stars. To help visualize this process, seethe animation found at

http://astro.unl.edu/classaction/animations/renaissance/retrograde.html

In this sense, the Heliocentric model was an improvement - it could explainall of the old observations and the new measurements, including the phases ofVenus. To see how this model explains the phases of Venus, see the animationfound at

http://astro.unl.edu/classaction/animations/renaissance/venusphases.html

Several centuries have passed since the acceptance of the Heliocentric model,and the technology we can use to view the cosmos has grown leaps and bounds.Are the additional observations we have made consistent with such a model?

2.3. EARLY COSMOLOGICAL MODELS 49

Figure 2.12: A depiction of the Heliocentric model, from Nicolaus Copernicus.(Image is in the public domain.)

Observation 3: Beyond our Solar System

It is now time for you to make a few more virtual observations, using theStellarium software, and answer this question for yourself. Start the Stellariumprogram, and set your location to wherever you wish. Turn off the atmosphere,fog, and ground by pressing the “A”, “F”, and “G” keys. Recall that you can“look” in different directions by using the arrow keys. Notice the positions ofthe stars. Are there some places in the sky where you see more stars thanothers?

Press “F3” and search for the object “M31”. Your field of view will centeron what seems to be an empty piece of sky. Now start zooming in. You mightalso search for the objects “M10”, “M22”, and “M110”.

Based on your observations, answer the following questions before comingto class:

3. Based on your observations in Stellarium, would you say that the starsare uniformly distributed throughout the sky?

4. What conclusions can you draw based on these observations? Do youneed to refine the Heliocentric model? If so, how?

5. What do the objects “M31”, “M10”, “M22”, and “M110” appear to be?

6. Are these objects consistent with the Heliocentric model? If not, howcould you refine the model?

50 CHAPTER 2. THE UNIVERSE

REVIEW QUESTIONS

1. What observations served as the basis for early cosmological models?

2. Why did early observers end up with a “wrong” model? Was it a caseof basing a conclusions on wrong assumptions? A case of insufficientdata?

3. How is retrograde motion accounted for in the Geocentric and He-liocentric models? (Note: it is not sufficient to just know the termi-nology. Could you explain this to, say, a child in the fourth grade?)

4. What observation or observations led science to reject the Geocentricand Heliocentric models of the universe?

5. In what ways does the development of cosmological models providean example of the scientific process?

6. People clung tenaciously to the Geocentric model because it wasaligned with the popular theology of the day. Do we see similarthings happening today? If and when they do happen, what is theappropriate course of action?

2.4. THE BIG BANG MODEL: PART I 51

2.4 The Big Bang Model: Part I

OVERVIEW

Summary: Improved technology, coupled with an improved understand-ing of physics on the atomic and subatomic scales, has led to significantchanges in our cosmological models. All the relevant science has culmi-nated in the so-called “big bang” model of the universe.

Learning Objectives:

• Identify the lines of evidence used as a basis for the theory of anexpanding universe.

• Identify other observations that can be explained by the big bangmodel, but could not be explained (or were in conflict with) previousmodels.

• Discern between observations and interpretations in the context ofthe big bang model.

Vocabulary:

• Quantum mechanics

• Special relativity

• General relativity

• Doppler shift

• Emission spectrum

• Redshift

• Big bang theory

• Cosmic microwave back-ground

• Big bang nucleosynthesis

• Galactic evolution

In this section we will present the scientific model that represents our cur-rent evidence-based understanding of our universe. As you read through thismaterial, think about other ways that the observations could be interpreted.Ask yourself how a person might empirically distinguish between these otherhypotheses and the big bang theory.

A New Theoretical Framework for Gravitation

The early 20th century saw two major innovations in our understanding of thephysical world. First, there was the discovery that things we first thought ofas particles, or tiny indivisible objects, sometimes behaved like waves, and thatthings we originally thought of as waves (like light) would sometimes behaveas though they were particles. These observations led to the development ofquantum mechanics , which describes all atomic and subatomic entities as

Quantum mechanics:A theory which describessubatomic particles as local-ized waves.

though they were some sort of localized wave. Experiments to date supportthis description, including all of its strange implications (which we will notgo into - although one should remember that the reason quantum mechanicsseems strange to most people is because of a lack of intuition, which in turn isdriven by a lack of experience at the subatomic scale). This improvement inour understanding made possible many new technologies which you may takefor granted. These include fluorescent lighting, light sensors, and silicon-basedmicrochips among others.

The second revolution in our understanding came with Albert Einstein’stheory of special relativity. Prior to Einstein, our understanding of motion,forces, and gravity was based on laws formalized by Isaac Newton. In thisreading we want to specifically focus on Newton’s universal law of gravitation,

52 CHAPTER 2. THE UNIVERSE

which up until the 20th century described all of the gravitational interactionsever observed. Newton’s universal law of gravitation states that the attractivegravitational force between any two objects is directly proportional to the massof each object and inversely proportional to the square of the distance betweenthem. Mathematically, this is written as

F = Gm1m2

r2

where m1 and m2 are the masses of the two objects and r is the distance betweenthem. The proportionality constant G has a value of 6.67 × 10−11 N·m2/kg2.(Note: the equation is given here for reference only - you will not be expectedto use it in this course.) This law was able to explain why an apple will falltoward the Earth, why the Moon orbits the Earth, and why each of the planetsmaintains a stable elliptical orbit around the Sun.

In 1905 Albert Einstein published his theory of special relativity , which

Special relativity:A theory based on twopremises: that the laws ofphysics are the same forall observers, and that lighttraveling through vacuumhas the same measured speedfor all observers - regardlessof their motion relative tothe source.

consists of two axioms. First, the laws of physics are the same for all observers(the relativity principle). Second, light traveling through vacuum has the samemeasured speed for all observers, regardless of their motion relative to thesource (constancy of the speed of light). This theory (which, incidentally, haswithstood a century of rigorous experimental tests) describes how space andtime change when objects travel at speeds approaching the speed of light.

After publishing this theory, Einstein recognized that the theory could begeneralized to describe a wider range of phenomenon. This expansion of thetheory, called general relativity , described gravitational interactions in terms

General relativity:An extension of special rela-tivity that includes gravita-tional interactions.

of the curvature of space-time. Like quantum mechanics, the theory of relativitymakes some predictions that, to us, seem somewhat counter intuitive. Yetexperiment after experiment indicates that relativity is a valid description ofthe way nature works. Furthermore, we have made observations that are easilyexplained in the context of general relativity that simply cannot be explainedusing Newtonian gravitation!

The Observable Universe is Red

Quantum mechanics and relativity, in conjunction with each other, have ledus to some fascinating interpretation of our modern observations about theuniverse. First of all, there is the phenomenon known as redshift.

At some point in your life you have observed the Doppler shift. When a

Doppler shift:The shifting of frequencies ofsound when the source of thesound is moving relative toan observer.

train blares its whistle while passing by, you hear a distinct change in the toneof the whistle. It is higher pitched as the engine approaches you, and lowerpitched when it has passed. The reason for this apparent shift in frequency isthat the sound waves emitted by the whistle get bunched up in front of thetrain and stretched out behind. The distance between the waves, called thewavelength, is what determines the frequency of the sound.

A similar thing happens with light, and it is described by the theory ofspecial relativity. When something is moving toward you, the light waves itemits in the forward direction get bunched up, and accordingly the frequencyof that light increases. Higher frequency visible light is blue, and thus wesay that the light in front of the object is blue shifted. Correspondingly, thelight waves emitted in the backward direction get somewhat spread apart. Thefrequency is lowered, so that the light is more red than it would be were thesource object stationary.

Doppler effect for light: ob-jects moving toward you areblue-shifted, objects mov-ing away from you are red-shifted.

In our everyday experience we do not easily observe these shifts in color. Thereason for this is that the effects of special relativity only become apparent atspeeds close to the speed of light - 300,000,000 meters every second. The speedsof objects in our everyday world do not even come remotely close to the speed oflight, and hence the color shifts are so small as to be practically unobservable!However, the effect has been verified in carefully controlled experiments.

2.4. THE BIG BANG MODEL: PART I 53

Figure 2.13: Doppler shifting of light. When an object is moving toward you,the light that it emits is more blue than it would be if the object were stationary.If the object is moving away from you, the light is more red. The effect is greatlyexaggerated in this image. (Image courtesy of Ales Tosovsky, licensed underthe GNU Free Documentation License.)

One of the reasons this effect is so difficult to observe is that light is emittedfrom or reflected off of most objects in many different colors. For example,consider the reflection of white light. All of the colors are present in whitelight, so if the light were to be shifted just a little to the blue, we would still seepractically all of the same colors, and would be unable to distinguish any sortof shift. But what if the light emitted by the object were just a single color?

It turns out, and quantum mechanics describes this phenomenon very well,that if you take a rarefied (meaning not very dense) gas of a single element andget it really hot, it will emit only a few specific wavelengths of light. This setof wavelengths, called the emission spectrum of the gas, is like a fingerprint

Emission spectrum:The unique set of specificwavelengths emitted by a hotrarefied gas.for the gas - each type of gas has a different spectrum. It is for this reason that

we can observe the light from the Sun and deduce what the Sun is made of.When you have light of a single frequency, it is not very difficult to measure

its wavelength. A simple prism or diffraction grating, which spreads whitelight out into its constituent colors, is sufficient. The moral of this story isthat if a hot, rarefied gas is moving towards us or away from us, even if it’sspeed is somewhat slow relative to that of light, we can observe the shift inthe wavelength of the light it emits and therefore deduce whether it is movingtowards us (in which case the light will shift to the small wavelength or blueend of the spectrum) or away from us (the light shifts to the long wavelengthor red end of the spectrum), and how fast it is going.

Redshift and blueshift can beused to measure motion andspeedGalaxies are made primarily of hot rarefied gases, which are found in stars.

Because most stars are made primarily of hydrogen (and even if they are madeof other things, there is always hot hydrogen on the outside), we can look for thehydrogen spectrum lines in distant galaxies, observe the shift, and determinetheir speed and direction of motion relative to us. The curious thing is thatwhen we make these observations, only a few of the closest galaxies (those inour own local cluster) show a shift to the blue. The vast majority of the galaxieswe observe have a spectrum that is shifted to the red (for example, see figure2.14).

Furthermore, the more distant the galaxy is, the more redshifted its spec-trum. This fact was discovered in 1929 by Edwin Hubble, for whom the Distance, speed, and redshift

of galaxies are all relatedHubble telescope is named. The distance to galaxies can be inferred from therelative brightness of Type I supernovae, which because they always happenthe same way always have the same luminosity. The speed of the galaxies canbe reconstructed from the measured redshift of the galaxy. In comparing thistype of data for a large number of galaxies, Hubble found that they were relatedlinearly. In particular,

v = Hd

54 CHAPTER 2. THE UNIVERSE

Figure 2.14: Redshift in the spectrum from a supercluster of distant galaxies(BAS11). For reference, the solar spectrum is provided on the left. The sameseries of absorption lines are present in both spectra, as would be expected sincestarts have similar compositions. The spectrum from the galaxy cluster is veryclearly shifted to the red, indicating that the cluster is moving away from us.(Image courtesy of Georg Wiora, licensed under the GNU Free DocumentationLicense)

where v represents the speed of the galaxy relative to Earth (in km/sec), dis the distance between the galaxy and the Earth (in megaparsec, where onemegaparsec is 1.91735281 × 1019 miles), and H is a constant known as theHubble constant, which has a value somewhere between 45-90 km/s/Mpc. Thisrelationship is known as Hubble’s law.

So, the moral of the story is that, within the context of relativity (which wehave very good reason to believe is correct), every galaxy outside of our localcluster is moving away from us, and the further away a galaxy is, the faster itis moving. How are we to interpret this data? The current theory that explainsthese observations is the big bang theory, which suggests that some 13.7

Big bang theory:A theory which says that theuniverse started very smalland very dense, and has beenexpanding for the last severalbillion years.

billion years ago the universe was very small, hot, and energetic, and that ithas been expanding and cooling ever since then. While redshift is an importantpiece of evidence supporting this idea, the big bang theory can also account forseveral other observations that either conflicted with or were beyond the scopeof previous cosmological models. Some of these other observations are listedbelow.

Cosmic Microwave Background

In 1963 Arno Penzias and Robert Wilson were working on a microwave commu-nications system when they noticed a persistent background signal, no matterhow their receiver was oriented. Background signals are not uncommon in sci-ence. But since we want to get the best possible data, we try diligently toreduce the background. Penzias and Wilson did the same (there are even sto-ries about them scrubbing bird droppings off the antenna), but no matter whatthey tried, they could not get rid of that persistent fuzzy signal. Eventually,it was determined that the signal was real, and the microwaves they were ob-serving came from space. This microwave signal is what we call the cosmicmicrowave background, or CMB (see figure 2.15).

Cosmic microwave back-ground:A persistent signal of mi-crowaves coming at us fromall directions in space.

The CMB is surprisingly uniform, with a wavelength that corresponds toa temperature of 2.7250 K. The deviations from this wavelength are no largerthan about 0.0005 K, or one part in 5000. What is the source of this mysteriouscosmic signal? Well, in the context of the big bang model, we would expect tosee a signal from the early days when the universe was very hot. As the spacebetween the Earth and that hot dense past has expanded, the radiation has

2.4. THE BIG BANG MODEL: PART I 55

also been stretched, effectively reducing its frequency.

Figure 2.15: The Wilkinson Microwave Anisotropy Probe (WMAP) image of thecosmic microwave background. The different colors represent small variations(anisotropies) in the microwave signal. (Image is in the public domain.)

The Composition of the Universe

As explained earlier, based on the spectrum of light emitted from stars anddistant galaxies we can deduce their composition. Our measurements indicatethat the visible universe is roughly 74.5% hydrogen and 24% helium. All otherelements account for only about 1.5% of the matter visible to us. Hydrogen is,of course, the lightest element. Helium is the second lightest. Is there somereason why the universe is composed almost exclusively of these light elements?Is there any type of significance to the ratio of helium to hydrogen?

In the context of the big bang theory, the answer is yes. Very early in itshistory, the universe was so hot that matter could simply not stay together. Weare talking about conditions far more extreme than those required to break thebonds between atoms, or even to strip electrons off of atoms. The universe wasso hot that the nuclei of atoms could not form! Big bang nucleosynthesis

The matter history of the universe, again in the context of the big bangtheory, is as follows. In the first few moments of the big bang, matter wasonly found in the form of quarks and leptons. These are all fundamental sub-atomic particles. As the universe cooled, eventually the quarks were able tocoalesce into heavier particles, including protons and neutrons (the other so-called baryons are unstable, and would have rapidly decayed). As the uni-verse cooled even further, some of the protons and neutrons were able to bindtogether as well. However, nuclear fusion (the binding together of neutrons,protons, and/or clumps of neutrons and protons) requires relatively high tem-peratures, and it did not take long before the universe cooled to where fusioncould no longer happen. The majority of the protons remained unbound, andthe clumps that did form were fairly small (such as two protons with one or twoneutrons). Finally, as the temperature dropped even further the protons andother atomic nuclei (we call the clumps of protons and neutrons nuclei) couldbegin to capture the stable leptons (which we know as electrons).

Of necessity most of the details have been omitted in this discussion, butsuffice it to say that the big bang theory predicts that the universe should bepredominantly composed of the smallest atoms: about 75% being hydrogen andthe majority of what’s left helium.

56 CHAPTER 2. THE UNIVERSE

Galactic Evolution

Another interesting observation is that when we look at distant galaxies, whichyou will recall from our class discussion last week is equivalent to looking backin time, we note that they are not as well developed as our own Milky Waygalaxy. The more distant the galaxy, the less developed. Taking into accountthe amount of time it would take the light from the distant galaxy to reach us,it appears that pretty much all galaxies were formed at the same time and aredeveloping at the same rate. How can we explain this observation?Galactic evolution

Going back to the paradigm of the big bang, as the universe continued tocool it was left with a lot of hydrogen and helium gas. In accordance with grav-itational theories, those gas atoms, even though they are very light, exertedgravitational forces on each other and began to coalesce. As the gasses fell in-ward, the temperatures increased - creating once again the necessary conditionsfor atomic nuclei to fuse. Thus, we have the formation of the first stars.

Stars in turn are massive objects and exert gravitational forces on eachother. So, as the stars were forming, they in turn were pulled into large groupsof stars, which we call galaxies. As the stars continued to rotate in these earlygalaxies, the galaxies themselves began to take on the characteristic shapes andorder that we see in the Milky Way and its nearest neighbors.

Is the Big Bang the Final Word?

In short, probably not. The big bang theory is the best model that we havecome up with to explain the empirical observations we have made with respectto the universe. It may be in the future that we make additional observa-tions that conflict with the big bang theory. When such observations are made(remember, in order to count in the realm of science these must be empiricalobservations) and verified then the model will be revised.

REVIEW QUESTIONS

1. Explain what redshift is.

2. What evidences suggest to us that the universe is expanding?

3. What other possible explanations are there for the observations listedin this section?

4. Do these other explanations fall within the scope of scientific inquiry?If so, how could you distinguish between these other ideas and thebig bang theory? If not, explain why.

5. Does the big bang theory prove, as some have suggested, that Goddoes not exist and that life has no purpose? On the other hand, doesthe revealed word of God exclude the big bang theory as a processwhereby the universe may have come into existence?

2.5. THE BIG BANG MODEL: PART II 57

2.5 The Big Bang Model: Part II

OVERVIEW

Summary: Recent research involving the connections between particlephysics and cosmology have given us new insights into the nature of ouruniverse.

Learning Objectives:

• Identify the limitations of the Big Bang model.

• Identify how much of our universe is composed of dark matter anddark energy.

• Identify several questions that remain unanswered in the context ofthe big bang model.

Vocabulary:

• Quark soup

• Inflation

• Antimatter

• Multiverse

For today’s reading, we would like to direct your attention to an article enti-tled “The Origin of the Universe”, published in the journal Scientific Americanin 2009. BYU-Idaho has an online subscription for this journal, and if you areon the campus network you should be able to access the article at

http://www.nature.com/scientificamerican/journal/v301/n3/full/scientificamerican0909-36.html

or, if you are using the electronic version of this text, you can access it by click-ing here. On the right side of the screen, you should see a link to a PDF copyof the article, which may be easier to read. If you encounter any difficultiesaccessing this article, please notify your instructor.

REVIEW QUESTIONS

1. (From last class period’s discussion) What are the evidences thatled cosmologists to propose the existance of dark matter and darkenergy?

2. Who is the author of this article? Is he a credible source of informa-tion?

3. Explain the significance and connection between Hubble’s law andthe big bang theory. In your explanation, address the significance ofthe italicized words.

4. Identify the questions cosmologists are trying to answer today.

5. How complete is our understanding of the universe?

58 CHAPTER 2. THE UNIVERSE

Chapter 3

Atoms

Figure 3.1: High-resolution transmission electron microscopy (HRTEM) image of a gallium arsenide crystallattice. The distance between each white dot is the inter-atomic distance. The blur on the left is amorphousGaAs, and the region on the right was recrystallized by in-situ annealing. This image essentially provides usa picture of the atoms in the crystal lattice. Amazingly enough, scientists knew quite a bit about atoms longbefore such images were possible. (Image courtesy of Steve Jurvetson, licensed under the Creative CommonsAttribution 2.0 license)

Atoms, as most people are aware, are the building blocks of all matter. But atoms are so incredibly tiny,that they are undetectable by the best optical microscopes in the world, let alone the unaided human eye.Nevertheless, we were aware of the existence of atoms before electron microscopes (which can discern individualatoms) were developed.

In this chapter, we will address the question “How do we know there are atoms?” We will chart the historyof observations and ideas that has led us to our current understanding of what atoms are and how they work.This discussion will include the topics of radioactivity, the periodic table, and bonding.

59

60 CHAPTER 3. ATOMS

3.1 Where Do Atoms Come From?

OVERVIEW

Summary: All of the matter you see around you is made of atoms. Theseatoms were synthesized during the big bang and in stars.

Learning Objectives:

• Identify which particles are present in atoms, and where they arelocated.

• Explain what isotopes are.

• Recognize and use the correct notation for describing the composi-tion of atoms.

• Identify the missing component of an incomplete equation for a nu-clear reaction.

• Identify where, how, and when the various elements were formed.

• Distinguish between s-process and r-process nuclear reactions.

Vocabulary:

• Elements

• Nucleus

• Electron

• Proton

• Neutron

• Atomic number

• Mass number

• Isotopes

• Nuclear reactions

• Transmutation

• Big bang nucleosynthesis

• Stellar nucleosynthesis

Atoms, Element, Isotopes

In this reading you will learn the basics of atomic structure and then aboutwhere atoms come from.

All matter is made of atoms, which are the smallest pieces of a particularelement, that have all of the characteristics of that particular element. There

Element:A fundamental type of mat-ter. are two major parts of an atom: the nucleus and the electrons (Figure 3.2). The

nucleus, is a small region that resides at the center of the atom and containsNucleus:The small region at the cen-ter of an atom which con-tains most of the atom’smass.

the vast majority of the atom’s mass. The electrons are found in a region

Electron:A fundamental subatomicparticle with negative chargeand very little mass.

called the electron cloud that surrounds the nucleus. The size of the electroncloud defines the size of an atom. The diameter of the electron cloud is about100,000 times bigger than the diameter of the atom’s nucleus.

The nucleus of an atom is made of subatomic particles called nucleons.There are two kinds of nucleons: protons and neutrons. Protons and

Proton:A positively charged sub-atomic particle found in thenucleus of an atom.Neutron:An electrically neutral sub-atomic particle found in thenucleus of atoms.

neutrons have equal masses, but a proton carries a positive charge while aneutron does not have an electric charge.

For most atoms the number of electrons is the same as the number of pro-tons. Since electrons and protons have equal but opposite electric charges, thismeans that most atoms have a net electric charge of zero. Atoms that are notelectrically neutral, i.e., they have a net positive or negative charge, are calledions.

Atoms are characterized by the number of protons and neutrons found intheir nuclei. The number of protons in the nucleus is referred to as the atomic

3.1. WHERE DO ATOMS COME FROM? 61

Figure 3.2: A depiction of an atom. The protons and neutrons (red and gray)are bound together in the nucleus at the center, and the electrons (blue) arefound in a region called the electron cloud. Image is not drawn to scale. (Imagecourtesy of Brigham Young University - Idaho)

number. Since most atoms are electrically neutral, the atomic number also Atomic number:The number of protons in thenucleus of an atom.

tells you how many electrons the atom will have, and thus determines thechemical properties of the atom. All of the atoms of a particular element havethe same chemical behavior, and thus all have the same atomic number: allatoms with an atomic number of one are hydrogen, all atoms with an atomicnumber of two are helium, and so on. All of the known elements and theiratomic numbers are shown in Table 3.1 (the atomic numbers of the elements inthis table are indicated by the letter Z).

The number of neutrons in the nucleus determines whether or not the nu-cleus is stable. The total number of protons and neutrons (nucleons) in anatom is the mass number of that atom. Atoms with the same number of

Mass number:The total number of nucleonsin the nucleus of an atom.protons (atomic number), but differing numbers of neutrons (mass number) are

called isotopes. We specify a particular isotope by referring to the atomic Isotopes:Atoms with the same num-ber of protons but differentnumbers of neutrons.

number (or element) and mass number. For example, isotopes of the elementcarbon include carbon-12 that has six protons and six neutrons, carbon-13 thathas six protons and seven neutrons, and carbon-14 that has six protons andeight neutrons. A useful notation for an isotope consists of the chemical sym-bol for the particular element, preceded by a superscripted number indicatingthe mass number. For example, the isotope 14

6 C refers to an atom of carbonthat has six protons (like all carbon atoms) and eight neutrons, making its massnumber 14. Specifying the element name automatically tells how many protonsan atom has, so including the atomic number is somewhat redundant. For thisreason isotopes are often written with only the mass number, such as 14C.

To get an idea of how many isotopes have been discovered or produced in alaboratory, see http://www-nds.iaea.org/livechart.

Nuclear Reactions and Conservation Laws

When the nucleus collides with some other projectile, e.g., a neutron, proton,another nucleus, etc., many things can happen. The projectile can be sim-ply absorbed by the nucleus, or perhaps it will knock out additional nucleons.The probability of each result occurring can be determined through quantum

62 CHAPTER 3. ATOMS

Z Element Symbol Z Element Symbol

1 Hydrogen H 61 Promethium Pm2 Helium He 62 Samarium Sm3 Lithium Li 63 Europium Eu4 Beryllium Be 64 Gadolinium Gd5 Boron B 65 Terbium Tb6 Carbon C 66 Dysprosium Dy7 Nitrogen N 67 Holmium Ho8 Oxygen O 68 Erbium Er9 Fluorine F 69 Thulium Tm10 Neon Ne 70 Ytterbium Yb11 Sodium Na 71 Lutetium Lu12 Magnesium Mg 72 Hafnium Hf13 Aluminum Al 73 Tantalum Ta14 Silicon Si 74 Tungsten W15 Phosphorus P 75 Rhenium Re16 Sulfur S 76 Osmium Os17 Chlorine Cl 77 Iridium Ir18 Argon Ar 78 Platinum Pt19 Potassium K 79 Gold Au20 Calcium Ca 80 Mercury Hg21 Scandium Sc 81 Thallium Tl22 Titanium Ti 82 Lead Pb23 Vanadium V 83 Bismuth Bi24 Chromium Cr 84 Polonium Po25 Manganese Mn 85 Astatine At26 Iron Fe 86 Radon Rn27 Cobalt Co 87 Francium Fr28 Nickel Ni 88 Radium Ra29 Copper Cu 89 Actinium Ac30 Zinc Zn 90 Thorium Th31 Gallium Ga 91 Protactinium Pa32 Germanium Ge 92 Uranium U33 Arsenic As 93 Neptunium Np34 Selenium Se 94 Plutonium Pu35 Bromine Br 95 Americium Am36 Krypton Kr 96 Curium Cm37 Rubidium Rb 97 Berkelium Bk38 Strontium Sr 98 Californium Cf39 Yttrium Y 99 Einsteinium Es40 Zirconium Zr 100 Fermium Fm41 Niobium Nb 101 Mendelevium Md42 Molybdenum Mo 102 Nobelium No43 Technetium Tc 103 Lawrencium Lr44 Ruthenium Ru 104 Rutherfordium Rf45 Rhodium Rh 105 Dubnium Db46 Palladium Pd 106 Seaborgium Sg47 Silver Ag 107 Bohrium Bh48 Cadmium Cd 108 Hassium Hs49 Indium In 109 Meitnerium Mt50 Tin Sn 110 Darmstadtium Ds51 Antimony Sb 111 Roentgenium Rg52 Tellurium Te 112 Ununbium Uub53 Iodine I 113 Ununtrium Uut54 Xenon Xe 114 Ununquadium Uuq55 Caesium Cs 115 Ununpentium Uup56 Barium Ba 116 Ununhexium Uuh57 Lanthanum La 117 Ununseptium Uus58 Cerium Ce 118 Ununoctium Uuo59 Praseodymium Pr60 Neodymium Nd

Table 3.1: Elements and their corresponding atomic numbers.

3.1. WHERE DO ATOMS COME FROM? 63

mechanics, though the details are beyond the scope of this class.In any of these events, which we refer to as nuclear reactions, there are

Nuclear reaction:Any type of interaction in-volving the nucleus of anatom.

several quantities that remain constant, i.e., quantities which are conserved. Inparticular, the total atomic number (or equivalently the total electric charge)of the reactants will always equal the total atomic number (charge) of theproducts. Second, the total mass number of the reactants will always equal thetotal mass number of the products. For an example of how these conservationlaws apply to a nuclear reaction, consider what happens when a neutron (10n)collides with an iron nucleus (5626Fe) and gets absorbed, but in the process knocksout a proton. You can represent this reaction as follows:

5626Fe + 1

0n −→ 11p + ?

What should replace the question mark in the equation? Remember that thesum of the mass numbers and the sum of the atomic numbers on the left sideof the arrow must always equal the sum of the mass numbers and the sum ofatomic numbers on the right side of the arrow, because they are both conserved.Since this is the case, the nucleus that belongs where the question mark is musthave 56 for its mass number and 25 for its atomic number. If you look at Table3.1 for the element that has an atomic number of 25 you will find that element25 is manganese, so the complete equation is:

5626Fe + 1

0n −→ 11p + 56

25Mn

Another example could involve an atom of carbon (126 C) colliding with an alphaparticle (42α, or simply α), which, by the way, is the same as the nucleus of ahelium atom). This reaction would proceed as:

126 C + 4

2α −→ 168 O

Did you notice what happened in both of these examples? You startedwith the nucleus from one kind of element as a reactant and ended up withthe nucleus of a different kind of element as the product! This changing ofthe nucleus of one kind of element into the nucleus of another kind of elementthrough nuclear reactions is called transmutation.

Transmutation:Changing one type of ele-ment into another throughnuclear reactions.

We will now consider how transmutation happened on a large scale amongthe atoms of stars, galaxies, and the universe to produce nuclei of all of theelements. As nuclear reactions take place, the makeup of nuclei that are in-volved change. We refer to this type of large-scale transmutation as chemicalevolution. . Chemical evolution took place during the earliest stages of the

Chemical evolution:Transmutation on a largescale over a long period oftime.

formation of the universe and continues today in the cores of stars.

Big Bang Nucleosynthesis

The term nucleosynthesis refers to the synthesis of atomic nuclei. Once

Nucleosynthesis:The synthesis of atoms (i.e.nuclei) through nuclear reac-tions.

nuclei are formed they can become complete atoms with a nucleus and anelectron cloud.

Every atom in every bit of matter in the universe had to be synthesized(made or assembled) at some point, and it all started during early stages of thebig bang. In the first 10−12 seconds after the beginning of the big bang theuniverse was extremely hot (more than 1015 degrees). Under these conditionsany particles in existence could not stay bound together. By 10−7 seconds intothe life of the universe the temperature cooled to about 1014 degrees, and thesynthesis of protons and neutrons from quarks that were already in existencewas in full swing, but it was still too hot for protons and neutrons to coalesceto form deuterium nuclei (one proton and one neutron stuck together), or anyother nucleus made up of more than one nucleon.

In order for protons and neutrons to fuse into nuclei, they have to be movingquickly when they collide with each other; otherwise the repulsive electrostaticforces between positively charged protons keep them from getting close enough

64 CHAPTER 3. ATOMS

to bond. In practical terms, this means that the temperature needs to be highenough to cause the very rapid movement of protons and neutrons, yet coolenough to allow any nuclei that form to remain bound. Larger nuclei typicallyrequire higher temperatures to form. After about 100 seconds after the birthof the universe the temperature cooled to between 109 and 107 degrees - coolenough for protons and neutrons to form deuterium nuclei (one proton and oneneutron) and helium nuclei (two protons and two neutrons), as well as traceamounts of nuclei of other light elements like lithium.

Once the temperature cooled to around 107 degrees, the universe was toocold for nucleons and nuclei to fuse together. Even so, it took about another300,000 years for the universe to cool sufficiently to allow the nuclei formedduring big bang nucleosynthesis to capture and retain electrons and thus becomecomplete atoms. By the way, electrons were readily available since they wereformed within the first 0.0001 seconds of the big bang. Atoms then began toclump together under the influence of gravity. As clouds of mainly hydrogenand helium atoms collapsed in on each other, the temperature within these gasclouds increased. Eventually the core of these gas clouds became places whereit was dense and hot enough for nucleosynthesis to begin again, and the nowdense and spherical clouds of hydrogen and helium began to shine as stars.

Stellar Nucleosynthesis

Nuclear fusion that takes place in the core of stars is called stellar nucleosynthe-sis. Practically all of the atoms of hydrogen in the universe today were formedduring big bang nucleosynthesis. The vast majority of heavier atoms, how-ever, were made in the cores of stars. These heavier atoms include the carbon,oxygen, nitrogen, and other atoms that make up your body.

Stars synthesize heavier elements out of hydrogen and helium. There aretwo ways in which this happens: the stellar fuel cycle, and neutron captureprocesses.

Stellar Fuel Cycles

The PP-I cycle

The first stage of the stellar fuel cycle involves the fusion of hydrogen nucleito form helium nuclei. Conditions in the core of stars are such that nucleicannot retain their electrons; all stellar nuclear reactions therefore involve onlynuclei and not electrons. Although most of a star’s volume consists of hydrogen,stellar nucleosynthesis reactions occur only in the relatively small cores of starswhere temperatures and pressures are high enough to drive those reactions.The synthesis of helium nuclei from hydrogen can occur in multiple ways, butthe most common pattern, the PP-I cycle, is shown in Figure 3.3. During thisprocess two hydrogen nuclei (i.e., two protons) fuse together, though in theprocess one of those protons becomes a neutron. This deuterium atom thenfuses with another proton to form a 3

2He nucleus. Two 32He nuclei then fuse

together, resulting in two free protons and a complete 42He (helium) nucleus.

During these reactions positrons (01e, positively charged electrons), gamma rays(γ-rays, high energy light), and neutrinos (ν, extremely light neutral particles)are released. The nuclear reaction equations for these processes are as follows:

11H +1

1 H −→21 H +0

1 e + ν21H +1

1 H −→32 He + γ

32He +3

2 He −→42 He + 21

1H

Note that for each reaction, the total atomic number and total mass numberare conserved.

3.1. WHERE DO ATOMS COME FROM? 65

Figure 3.3: The PP-I cycle, wherein four protons are fused together to forma helium-4 nucleus. (Image courtesy of “Borb”, licensed under the GNU FreeDocumentation License)

There are two other stellar nucleosynthesis processes (PP-II and PP-IIIcycles) whereby protons can be synthesized into helium nuclei. While these re-actions differ in some respects when compared to the PP-I cycle, they all startwith protons and eventually produce helium nuclei. The PP-I cycle is by farthe most dominant of the three processes, and so we will forgo any additionaldiscussion of the other two.

The Triple Alpha Reaction

As the hydrogen fuel is used up, the star begins to collapse under the influ-ence of gravity. If the star is massive enough this collapse causes the temper-ature and pressure in the star’s core to rise to the point where helium nuclei(also called alpha particles or α particles) can fuse together. When two al-pha particles fuse they make beryllium (8Be), which is an unstable isotopethat spontaneously decays back into two alpha particles in an extremely shortamount of time. However, if there are enough alpha particles around it is possi-ble for a third alpha particle to be absorbed by 8Be before it can decay. Whenthis happens the nucleus of a carbon atom (12C) is synthesized. This stableisotope of carbon comprises the majority of carbon atoms found in your body.The stellar nucleosynthesis reaction that produces 12C is shown in Figure 3.4.Note that gamma rays are emitted during this reaction.

The CNO cycle

When carbon is present in the core of a star, whether that carbon was madeduring helium fusion or more likely was simply there already when the star wasformed (i.e., from an old star that has since passed away), and additional fuel

66 CHAPTER 3. ATOMS

Figure 3.4: The triple alpha reaction. (Image courtesy of “Borb”, licensedunder the GNU Free Documentation License)

cycle called the CNO cycle (carbon-nitrogen-oxygen), will continually processhydrogen into helium nuclei using carbon as a catalyst. This series of reactionsis illustrated in Figure 3.5. If you look carefully you should see that four protonsare used during the reaction to produce one helium nucleus. Nuclear reactionscause two protons to be changed into neutrons during this process.

Figure 3.5: The CNO cycle, where a carbon nucleus is used as a catalyst tosynthesize helium from hydrogen. (Image courtesy of “Borb”, licensed underthe GNU Free Documentation License)

As helium nuclei (alpha particles) are used up the star’s core begins to col-lapse and heat up once again. Depending on how massive the star is, otherstellar nucleosynthesis reactions take place as carbon nuclei capture alpha par-ticles, as shown below:

3.1. WHERE DO ATOMS COME FROM? 67

126 C +4

2 He −→168 O + γ

168 O +4

2 He −→2010 Ne + γ

2010Ne +4

2 He −→2412 Mg + γ

Each successive alpha capture reaction requires higher and higher tempera-tures that can be achieved only in stars that are increasingly massive, and themass of a star is determined by the amount of mass that was in them when theywere formed. In smaller stars stellar nucleosynthesis may stop with the produc-tion of carbon. If, however, it is hot enough and the pressure in a star’s core isgreat enough carbon nuclei can begin to fuse, and at even higher temperaturesoxygen may fuse.

In very large stars temperatures and pressures can be great enough to allowfusion reactions to continue and produce elements up to iron. For elementsheavier than iron more energy is required to get the nuclei to fuse than isreleased by the reaction, which essentially marks the end of the life of the star.

Capture Processes

The cores of stars are loaded with extra neutrons and protons. It is possible forthese neutrons and protons (as well as alpha particles) to be captured by otherlarger nuclei, which also causes transmutation. For our purposes we will focuson neutron capture processes.

If the number of neutrons available is relatively small, a stable nucleus cancapture a neutron (increasing its mass number by one) and then become an un-stable isotope. That unstable nucleus decays by a process called beta emission,which we will cover in more detail in a later section. For now, you only needto know that this process converts one neutron into a proton, thus increasingthe atomic number of the atom. This newly formed isotope can then captureanother neutron, and so on. This slow neutron capture process, also known asthe s-process, can go on for some time producing successively larger nuclei.

s-process:Nucleosynthesis due to thecapture of neutrons (whenthere aren’t many neutronsaround).

For example, if you start with a nucleus of silver (109Ag) you can synthesizeisotopes of cadmium, indium, tin, and antimony, as shown in Figure 3.6. Thes-process can produce nuclei up through lead (though it usually only gets up toabout tin), but it cannot produce the largest naturally occurring nuclei, suchas Uranium.

Figure 3.6: s-process nucleosynthesis, starting from silver and ending up at theelement antimony. (Image courtesy of “Rursus”, licensed under the GNU FreeDocumentation License)

68 CHAPTER 3. ATOMS

When Stars Die

Eventually a star will use up all of its nuclear fuel and fusion reactions stop.When this happens the star “dies.” There are basically two things that canhappen when stars die, and the fate of a star depends on its mass.

For low mass stars like our sun, as the hydrogen fuel is used up the star willbegin to collapse under the influence of gravity. This causes the temperatureof the core to increase. The outer layers of the star swell and cool during thisprocess. Eventually helium fusion begins, and the star is called a red giant.Once the helium fuel is used up gravity increases the pressure further in thecore, but the core conditions are not sufficiently hot to initiate carbon fusion.At this point the outer gas layers of the star evaporate away, leaving a core ofsolid carbon about the same size as Earth. The core is still quite hot and glowswith a white light. We call this leftover core a white dwarf. Over time the corecools, its luminosity decreases and it fades slowly from view. (This cool, darkchunk of carbon that is left over should not be confused with black holes!)

Things are far more exciting in high mass stars. The processes of fusionreactions and alpha particle captures continue until iron (Fe) nuclei are syn-thesized, and an iron core is produced. As stated above, iron fusion no longerreleases energy, so as the high mass star makes iron its nuclear fuel runs out forgood. The iron core of the star then collapses from about the size of the Earthto about the size of a large city, all in just a fraction of a second. When thishappens the weak force (a force we will not discuss in this class) starts turn-ing all of the free protons in the iron core into neutrons. A flood of neutrinosis released, and the neutrons in the core then begin to snap into a crystallinestructure. When this happens a massive shock wave is sent rippling through therest of the star, blowing it apart in a massive explosion called a supernova, ormore specifically a type II supernova. Supernovae release an incredible amountof energy, and can often be as bright as an entire galaxy of stars!

Interesting things happen in supernovae with respect to stellar nucleosyn-thesis. The outer layers of neutrons are carried through the outer layers of thestar due to the shock wave, resulting in huge numbers of neutrons available tobe captured by larger nuclei at a rapid rate. This rapid capture of neutronsis called the r-process. Because the rate of neutron capture is so rapid, the

r-process:Nucleosynthesis involvingthe rapid capture of manyneutrons. resulting unstable isotopes do not have time to decay before they capture more

neutrons. As a result increasingly large nuclei are formed, and it is by thisprocess that the largest nuclei (up to uranium the largest naturally occurringelement) are synthesized.

Once the outer gas layers of the former star have dispersed what is leftbehind is that core of crystallized neutrons, which we call a neutron star. Neu-tron stars are incredibly dense; a teaspoonful would weigh as much as the entireEarth, and would have extraordinarily large magnetic fields. Charged particlesget caught in these magnetic fields and emit X-rays, which we can see usingspecial telescopes. The more massive the parent star is, the more massive theneutron star it will produce. The larger the neutron star is, the greater the pullof gravity at its surface. If the pull of gravity is large enough not even light canescape, and we call this leftover product a black hole.

In summary, practically all of the elements heavier than helium are synthe-sized in stars, whether during the regular fusion cycle, or by capture processes.This includes every atom of those heavier elements that you find in the worldaround you - including the carbon and oxygen in your own body!

3.1. WHERE DO ATOMS COME FROM? 69

REVIEW QUESTIONS

1. What is meant by the terms “element” and “isotope”?

2. What do the mass number and atomic number of an atom tell you?

3. Which elements were synthesized during the big bang, and why didthe process stop?

4. Which elements are synthesized in the fusion cycles of stars?

5. Which elements are synthesized during supernovae?

70 CHAPTER 3. ATOMS

3.2 Atomic Models

OVERVIEW

Summary: The development of atomic models is an interesting case studyin the process of science. The simple model that originated with the Greekswas revised as new data was obtained, ultimately leading to our presentday quantum mechanical model.

Learning Objectives:

• Describe the experimental observations that gave information aboutthe properties of the electron.

• Explain how the observations from Ernest Rutherford’s gold foil ex-periment differed from what the atomic model at the time predicted,and how those observations could be explained.

• Describe the size of the nucleus relative to the size of the atom.

• Compare and contrast the features of the following atomic mod-els: the hard sphere model, the plum pudding model, the planetarymodel, the Bohr model, and the quantum mechanical model.

• Identify the observations that led to the revision of each of thesemodels.

Vocabulary:

• Hard sphere model

• Plum pudding model

• Planetary model

• Bohr model

• Quantum mechanical model

Our present understanding the atom - what all matter is made of - is oneof the greatest advances science has made over the last century. As we havelearned more about what matter is made of, we have been able to harness thatknowledge to improve the quality of human life.

The story of how mankind has learned about matter provides a fascinatinginsight into how science works.

Earliest Models

Empedocles (c. 491-430B.C.): Greek philosopherwho proposed matter wasmade of four elements.

Early man was able to distinguish between different types of materials. Herecognized that stone was hard and good for making tools. He understood thatwood was useful for various things, including fuel for fire. He recognized thatother materials were good for eating. But did early man ever stop to reflect onwhat those things were really made of?

The Greek Philosopher Empedocles (492-432 B.C.) suggested that all mat-ter was ultimately composed of four things: air, earth, fire, and water. Theproperties of specific objects depended on how much of these four elementswere present in the object. For example, a material containing a large amountof water would be wet. A material that composed primarily of fire and earthshould be hard and dry. This model of matter is the first that we have recordof, and represents a huge leap in mankind’s understanding of the world aroundhim - the idea that all materials are made up a few simple constituent parts.

3.2. ATOMIC MODELS 71

The Hard Sphere Model

There was a glaring problem with Empedocles’ model: it suggested that ifyou break something down far enough, you should eventually see pieces thatresemble the four basic elements. There was not, however, any observation tosupport this expectation.

Democritus (c. 460-370B.C.): Greek philosopherwho proposed that matterwas made of atoms (paintingby Antoine Coypel).

Empedocles was not the only Greek philosopher to ponder the nature ofmatter. Democritus (460-370 B.C.) reasoned that if you take a piece of ordinarymatter, say a stone, you can divide that stone over and over again, and eachtime you will end up with pieces that have the same properties as the originalstone. If you continue to divide each piece, he reasoned, you will eventuallyend up with a piece of stone that can no longer be divided. Democritus calledthese fundamental pieces atomos, which in Greek means indivisible. This wouldmean that there were many more than four basic elements, for stone wouldeventually break down into indivisible pieces of stone, hair would break downinto indivisible pieces of hair, and meat would break down into indivisible piecesof meat. This model is sometimes referred to as the hard sphere model, since

Hard sphere model:An early atomic model thatsuggested all matter wasmade of indivisible piecescalled “atomos”.

the (assumed spherical) particles were indivisible.With our hindsight, we can see that Democritus’ model was closer to the

truth than that of Empedocles. However, some of the more influential Greekphilosophers, namely Plato and Aristotle, accepted the ideas of Empedocles.Due to their great influence, Democritus’ model was shelved for nearly 2,000years. It was not until the 17th century, when Evangelista Torricelli (who,incidentally, was a student of Galileo) observed that air had weight, that the“indivisible particle” model was brought back into the scientific arena. DanielBernoulli was able to explain the weight of air by hypothesizing that air iscomposed of tiny hard spheres that are very loosely packed. These particleswould have to be in constant motion - speeding around in all directions andbouncing off of hard surfaces and each other - in order to not settle to theground like dust.

Additional experiments showed that substances could combine together toform new substances with new properties, and that those combinations alwaysoccurred in definite proportions. These experiments suggested again that mat-ter is ultimately made of small indivisible pieces - as Democritus’ model sug-gested. Further experiments allowed scientists to begin to measure the massesof various atoms relative to each other, and they were able to determine thathydrogen was the least massive of all the elements.

Electrons and the Plum Pudding Model

A key idea of the hard sphere model was that the spheres, which we now beginreferring to as atoms, were indivisible. They were not made of other things.They simply existed.

J.J. Thomson (1856-1940 ): Discoverer of theelectron, for which he wasawarded the Nobel prize inphysics in 1906.

In 1897, J.J. Thomson was conducting experiments with cathode rays. Bythis time, the laws which govern electric and magnetic phenomena were verywell understood. One of the things scientists knew at this point in time wasthat it was possible to create an electrical discharge through a chamber contain-ing small amounts of gas in what otherwise would be a vacuum. One simplyhad to put an electrode at either end of the chamber and apply a very largevoltage. It was apparent from the discharge that something was traveling fromthe negatively charged electrode (the cathode) toward the positively chargedelectrode (the anode). These mysterious beams were called cathode rays (seeFigure 3.7).

Thomson subjected cathode rays to magnetic fields, and discovered that themagnetic fields would deflect the cathode rays. This indicated that whatevermade the cathode ray must be electrically charged, and in particular mustcarry a negative charge. Thomson concluded that cathode rays really consistof negatively charged particles. By measuring the deflection of the cathode

72 CHAPTER 3. ATOMS

Figure 3.7: A cathode ray being deflected by a magnetic field. By measuringthe radius of the circular path and knowing the speed of the particles in thecathode ray, one can determine the charge-to-mass ratio of the particles, whichturn out to be electrons (Image courtesy of Marcin Bialek, licensed under theGNU Free Documentation License)

rays, he was able to determine the charge-to-mass ratio of these particles. Hebelieved that these particles were not atoms, but rather parts of an atom. Hecalled the particles electrons.

Thomson’s beliefs were quickly confirmed by additional experiments thatindirectly measured the charge of electrons. Between knowing the charge andthe charge-to-mass ratio, one can easily determine the mass of the electron.The startling result was that, indeed, the mass of the electron is about 2,000times smaller than the mass of the lightest known atoms!

From these experiments, Thomson and his contemporaries devised a newmodel for the atom. Knowing that atoms were electrically neutral, they pro-posed that the atom consisted of relatively soft positively charged mass through-out which the electrons were embedded. A representation of this model (Figure3.8 would closely resemble a bowl of pudding with plums scattered throughout,and so became known as the plum pudding model.

Plum pudding model:An atomic model whereinthe electrons are embeddedin a soft positively chargedmass, much like plums orraisins in a pudding.

Figure 3.8: The plum pudding model of the atom. (Image is in the publicdomain.)

The Rutherford Experiment and the Planetary Model

Consider, in the context of the plum pudding model, what would happen to athin sheet of heavy atoms, such as gold, when such a beam of alpha particlescollided with it (alpha particles are a type of radiation which are positivelycharged). Since the atoms are electrically neutral, they should exert no elec-trostatic force on the incoming alpha particles. Therefore, the alpha particles

3.2. ATOMIC MODELS 73

should easily rip right through the malleable positively charged substance withinthe atom.

In 1911 Ernest Rutherford performed an experiment where a beam of alphaparticles was directed onto a gold foil. The results of the experiment conflicted Rutherford gold foil experi-

mentwith the plum pudding model: while many of the alpha particles zipped throughhis gold foil with little or no deflection, every so often an alpha particle wouldbounce back from the gold foil. Rutherford remarked that it “was [as] if youfired a 15-inch [artillery] shell at a piece of tissue paper and it came back andhit you.” This stark conflict between what was predicted by the plum puddingmodel and what was actually observed caused a crisis in our understanding ofatoms - a clear indication that the atomic model needed to be revised!

Figure 3.9: The Rutherford gold foil experiment. The image to the left showsthe general layout of the experiment, where a radioactive source emitting alphaparticles is placed in a container. The alpha particles stream out of the opening,creating a beam of alpha particles. The gold foil in turn is surrounded by aphosphorescent screen or photographic plates. The alpha particles, representedby the red beam, sometimes backscatter from the gold foil. The figure tothe right shows how alpha particles would be expected to interact in the plumpudding model (top) and the planetary model (bottom). (Image is in the publicdomain.)

Rutherford hypothesized that a massive positive charge must exist withineach atom (see Figure 3.9). By analyzing the scattering pattern of the alphaparticles, Rutherford was able to determine that these positive charges mustcontain 99.9% of the mass of the atom, and be about 100,000 times smallerthan the atom itself!

Certainly, this observation conflicted with the plum pudding model, whichsuggested the positively charged substance should be uniformly distributedthroughout the atom. To revise the model, Rutherford hypothesized that all ofthe positive charge is localized in a very tiny region at the center of the atom,which he called the nucleus, and that the electrons were orbiting around on theoutside. Because of the similarities between this model and the solar system,it became known as the planetary model (Figure 3.10).

Planetary model:An atomic model where theelectrons orbit around a posi-tively charged nucleus, muchlike planets orbiting a star.Emission Spectra and the Bohr Model

In the last chapter, we described the phenomena of discrete emission spectra:low density gasses can only emit or absorb light with certain particular wave-lengths, and the set of wavelengths that a given element can emit constitutesan atomic fingerprint for that element.

A great limitation of the planetary model was that, within the context ofthe model, an electron could orbit the nucleus with any arbitrary speed - itwould just have to move closer to or further away from the nucleus. If this

74 CHAPTER 3. ATOMS

Figure 3.10: The planetary model of the atom. (Image courtesy of Colin M.L.Burnett, licensed under the GNU Free Documentation License)

were indeed the case, an electron could absorb or emit any arbitrary amount ofenergy, and therefore could emit light at any arbitrary frequency. This would

Planetary model conflictswith the observed emissionspectra. predict that atoms emit the entire visible spectrum of light (all of the colors

from red to blue), which is clearly not the case - indicating that we had onceagain arrived at a crisis in our understanding of atoms.

Furthermore, it was well known that when charged particles accelerate(change speed or direction), they emit electromagnetic radiation and lose en-ergy. An electron in a circular orbit would therefore continuously lose energyand quickly fall into the nucleus, destroying the atom!

Niels Bohr (1885-1962 ):Assumed that electronscould only occupy quan-tized angular momentumstates, leading to the firstsemi-quantum model of theatom.

Only a short time after the development of the planetary model, Niels Bohrsuggested that the electrons could only have certain particular energies, andtherefore would only be able to absorb and emit certain wavelengths of light.He started with an ad hoc hypothesis that the angular momentum of a givenelectron (a quantity related to how fast the electron is orbiting and at whatdistance from the nucleus) must be some whole number multiple of a constantcalled h (h-bar), a number which had been used to explain other quantummechanical phenomena which are beyond the scope of our present discussion.In other words, Bohr assumed that the angular momentum was quantized. Fromthis model, which today we call the Bohr model, was able to calculate what

Bohr model:An atomic model where theelectrons are only allowed tohave discrete set amounts ofangular momentum or en-ergy.

the allowed electron energies would be for hydrogen. Knowing these energies,he was in turn able to calculate which wavelengths of light hydrogen should beable to emit. The startling result: Bohr’s calculation agreed with the observedhydrogen emission spectrum!

Figure 3.11: The Bohr model of the atom. (Image courtesy of “Brighterorange”,licensed under the GNU Free Documentation License)

Despite it’s success with the hydrogen atom, Bohr’s model failed miserablyin predicting the emission spectrum for heavier elements. Furthermore, it did

3.2. ATOMIC MODELS 75

not address the issue of the electron losing energy in a circular orbit.

The Quantum Mechanical Atom

During the 1920’s the theory of quantum mechanics was formalized. In short,this theory described electrons not as small indivisible particles, but ratheras localized waves. This description came about when several phenomena wereobserved that suggested electrons and other small particles have wave-like prop-erties.

To apply this theory of quantum mechanics to atoms, one no longer thinksof the electron as occupying a particular point in space at a given time, butinstead envisions “clouds of probability” where the electron wavefunction hasthe largest amplitude and which tell you where you are most likely to findthe electron. In other words, it envisions a particular volume of space wherethe electron is most likely to be found (see Figure 3.12). The shape of thisprobability cloud depends on the so-called quantum state of the electron. Afew probability clouds for hydrogen are shown in Figure 3.12.

Figure 3.12: Select electron probability clouds for various electrons. (Image isin the public domain.)

This new quantum mechanical picture of the atom replicated the successesof the Bohr model (i.e. it explained the hydrogen emission spectrum). Inaddition, the emission spectra of the other elements can be predicted, and thewave-like nature of the electron eliminates the conundrum of the electron losingenergy. This model can also explain why atoms bond together in the way thatthey do.

The quantum mechanical model serves as the basis of our current under-standing of atoms. A few minor revisions have been made (which are beyondthe scope of our present discussion), and this model has been successful in ex-plaining the atomic observations we have made to date.

REVIEW QUESTIONS

1. What was the basic premise of the Greek hard sphere model?

2. What experiments demonstrated the existence of electrons andatomic nuclei?

3. Explain why the plum pudding model was inconsistent with the re-sults of the Rutherford gold foil experiment.

4. What observations were inconsistent with the atomic model knownas the planetary model?

5. How is the scientific process evident in the development of our cur-rent atomic model?

76 CHAPTER 3. ATOMS

3.3 Bonding and the Periodic Table

OVERVIEW

Summary: Experimental evidence demonstrates that elements combineto form compounds in specific discrete ratios, and that in regular chem-ical reactions the total mass does not change. These observations canbe explained by the existence of atoms. Once scientists began to isolatethe different elements, they noticed similarities in the way some elementsreacted. Several models were developed that attempted to organize andclassify the elements. These models were upgraded and refined as addi-tional discoveries were made, culminating in our modern periodic table.

Learning Objectives:

• Describe the observations from the experiments conducted byPriestly, Lavoisier, and Proust, and explain how those observationscontributed to Dalton’s atomic theory.

• List the four parts of Dalton’s atomic theory.

• Identify the unique features of the periodic table formulated byDmitri Mendeleyev compared to earlier attempts at organizing theelements.

• Explain the challenge that the discovery of the noble gasses presentedto Mendeleyev’s periodic table, and how those challenges were sur-mounted.

• Identify the difference between an ionic, covalent, and hydrogenbond.

• Understand how bonding patterns can influence the physical andchemical properties of a molecule.

Vocabulary:

• Compound

• Law of definite proportions

• Periodic table

• Noble gasses

Formalization of the Atomic Theory

In previous readings we learned what atoms are and about the observationsdemonstrating their existence and properties. However, most of the substanceswe find around us are not composed of just one kind of atom, as Democritusproposed. Rather, they are specific combinations of different types of atoms,and it is to this combining process that we now turn our attention.Foundational experiments in

chemistry Starting in the late 18th century, experiments performed by Joseph Priest-ley, Antione Lavoisier, and Joseph Proust demonstrated that fundamental sub-stances, what we would today call elements, always combine in certain definedsmall number ratios (such as one to two, or two to three) and that there isno change in total mass when they combine1. In particular, they made thefollowing discoveries:

1. Joseph Priestley’s experiments involved the heating of a mineral known1If you would like to see some of the details of their experi-

ments, you might consider working through the tutorial found athttp://web.visionlearning.com/dalton playhouse/ad loader.html. Your instructor mayuse this tutorial as a homework assignment.

3.3. BONDING AND THE PERIODIC TABLE 77

as mercury calx. When heated, the calx would produce a gas whichPriestley called “dephlogisticated air” - what we now call oxygen. Themass and volume of the gas produced was proportional to the amountof mercury calx that was heated. These experiments demonstrated thatsome substances, such as the calx, are actually combinations of otherelements.

2. In Antoine Lavoisier’s experiments, the a gas called phlogiston (hydro-gen) was combined with oxygen via combustion. Lavoisier found that thephlogiston was consumed more quickly than the oxygen, at a two-to-oneratio. Also, the mass of the product of combustion (which turned outto be water) was the same as the amount of mass lost by the phlogis-ton and oxygen combined. In other words, mass is conserved in chemicalreactions.

3. In further experiments, Lavoisier burned diamond and charcoal by meansof a lens collecting sunlight. In these experiments, the amount of oxygenconsumed was proportional to the amount of substance burned. Theamount of gas formed in the burning process was the same for equalmasses of diamond and charcoal. Last of all, once again the mass of thegas formed was equal to the mass of the reactants that were consumed.

4. Joseph Proust conducted numerous experiments combining other sub-stances with oxygen. He found that in all cases, elements would alwayscombine with oxygen in the same way. This led Proust to propose the lawof definite proportions, which states that reacting elements combine

Law of definite propor-tions:When elements react to formcompounds, they react in de-fined whole number ratios.

in defined whole number ratios.

John Dalton (1766-1844 ):Formalized an atomic the-ory which explained how ele-ments form compounds.

In 1808 John Dalton formalized the results of all these experiments in hisatomic theory, which consisted of four parts:

1. All matter is composed of indivisible particles called atoms.

2. All atoms of a given element are identical, and atoms of different elementshave different properties.

3. Chemical reactions involve the combination of atoms. When one atomchemically combines with a different atom, they form a compound.

Compound:A chemical combination ofatoms of two or more ele-ments.

4. When elements react to form compounds, they react in defined wholenumber ratios.

Despite its shortcomings (we know now that atoms are not indivisible, andthat atoms of the same element may have a different number of neutrons),Dalton’s atomic theory formed the foundation for modern chemistry.

Organizing the Elements - The Periodic Table

Following the articulation of atomic theory by Dalton, there was a remarkableage of discovery where many of the elements of nature were isolated and charac-terized. (See Table 3.2, which organizes the elements chronologically accordingto their year of discovery.) With the discovery of these elements, some scientistsbegan to recognize interesting patterns in the weights assigned different atoms.One pattern was described in 1829 by Johann Dobereiner, who saw that therewere groups of three elements that behaved similarly under certain reactionconditions and shared a numerical relationship among their atomic weights.One group of elements that exhibits the pattern described by Dobereiner is cal-cium, strontium, and barium. In addition to having very similar properties, theatomic weight of strontium is halfway between the atomic weights of calciumand barium. Two other groups also follow the same pattern: chlorine, bromine,

78 CHAPTER 3. ATOMS

Element Z Year Element Z Year Element Z Year

Ancient History Palladium 46 1803 Neon 10 1898Copper 29 9000BC Cerium 58 1803 Xenon 54 1898Lead 82 7000BC Osmium 76 1803 Polonium 84 1898Gold 79 6000BC Iridium 77 1803 Radium 88 1898Silver 47 5000BC Rhodium 45 1804 Radon 86 1898Iron 26 5000BC Potassium 19 1807 Actinium 89 1899Carbon 6 3750BC Sodium 11 1807 1900sTin 50 3500BC Ruthenium 44 1807 Lutetium 71 1906Sulfur 16 2000BC Calcium 20 1808 Rhenium 75 1908Mercury 80 2000BC Boron 5 1808 Hafnium 72 1911Zinc 30 1000BC Iodine 53 1811 Protactinium 91 1913

Medieval Age Lithium 3 1817 Technetium 43 1937Arsenic 33 800 Cadmium 48 1817 Francium 87 1939Antimony 51 800 Selenium 34 1817 Astatine 85 1940Bismuth 83 800 Silicon 14 1824 Neptunium 93 1940Age of Englightenment Aluminium 13 1825 Promethium 61 1942

Phosphorus 15 1669 Bromine 35 1825 Americium 95 1944Cobalt 27 1732 Thorium 90 1829 Curium 96 1944Platinum 78 1735 Lanthanum 57 1838 Berkelium 97 1949Nickel 28 1751 Erbium 68 1842 Californium 98 1950Magnesium 12 1755 Terbium 65 1842 Einsteinium 99 1952Hydrogen 1 1766 Caesium 55 1860 Fermium 100 1952Manganese 25 1770 Rubidium 37 1861 Mendelevium 101 1955Oxygen 8 1771 Thallium 81 1861 Nobelium 102 1958Nitrogen 7 1772 Indium 49 1863 Lawrencium 103 1961Barium 56 1772 Helium 2 1868 Rutherfordium 104 1968Chlorine 17 1774 Gallium 31 1875 Dubnium 105 1970Molybdenum 42 1778 Ytterbium 70 1878 Seaborgium 106 1974Tungsten 74 1781 Holmium 67 1878 Bohrium 107 1981Tellurium 52 1782 Thulium 69 1879 Meitnerium 109 1982Strontium 38 1787 Scandium 21 1879 Hassium 108 1984Zirconium 40 1789 Samarium 62 1879 Darmstadtium 110 1994Uranium 92 1789 Gadolinium 64 1880 Roentgenium 111 1994Titanium 22 1791 Praseodymium 59 1885 Copernicium 112 1996Yttrium 39 1794 Neodymium 60 1885 Ununquadium 114 1999Chromium 24 1797 Dysprosium 66 1886 After 2000Beryllium 4 1798 Germanium 32 1886 Ununhexium 116 2000

1800s Fluorine 9 1886 Ununoctium 118 2002Vanadium 23 1801 Argon 18 1894 Ununtrium 113 2003Niobium 41 1801 Europium 63 1896 Ununpentium 115 2003Tantalum 73 1802 Krypton 36 1898 Ununseptium 117 2010

Table 3.2: The known chemical elements, listed by year of discovery.

3.3. BONDING AND THE PERIODIC TABLE 79

iodine, and sulfur, selenium, tellurium. While the pattern is interesting, it couldnot account for all fifty-five elements known to exist at the time.

The next significant advancement didn’t come until 1864 when John New-lands recognized that when placed in order of increasing atomic weight, proper-ties and patterns of chemical reactivity repeated among the elements. Newlandsorganized the elements into a table with eight columns and seven rows. It is sig-nificant to note that the triads of elements first recognized by Dobereiner couldbe found in the rows of Newland’s table. While the rows and columns listedelements with similar properties, there were still additional misfit elements thatdid not share similar properties with other elements in the rows and columns.

Figure 3.13: John Newland’s octaves (Image is in the public domain.)

In 1869 Dmitri Medeleyev published a new periodic table. Mendeleyev hadbeen hard at work for many months on developing a table of elements to includein the publication of a two-volume textbook on chemistry he began writing in1868. He was convinced (as were many other scientists) that there existed asystematic pattern for both the weight of the different elements and how theyreact with other elements. Mendeleyev stated the principle in these words:“The elements, arranged according to the magnitudes of their atomic weights,exhibit a clear periodicity in their properties.” Mendeleyev enjoyed playing thecard game solitaire, in which cards are turned over in sets of three and organizedaccording to suit (hearts, diamonds, clubs, spades) and rank (2, 3, 4, ..., J, Q,K, A). Mendeleyev had made his own special deck of cards with the symbolof each chemical element and a list of its fundamental properties analogous tothe suit and rank of playing cards. He then set about playing his own game ofchemical solitaire in which he made arrangements of the elements based upontheir atomic weights and chemical properties.

The extensive effort Mendeleyev put into the development of a systematictable of the elements is evident in the many drafts and notes he accumulatedthrough this process. One early draft of his table of elements shows the elementsorganized into columns in order of decreasing atomic weight; in this table theelement listed just below the element at the top of the column has a smalleratomic weight and the next element down the column has an atomic weight thatis less than the element above it and so on. With this arrangement however, therepeating pattern of chemical properties was not readily apparent by looking atthe table. One account of Mendeleyev’s work relates that during one episodeof working on finding a suitable arrangement for the elements in his table, herested at his desk, fell asleep, and had a dream. Mendeleyev’s own words recall, Mendeleyev’s dream“I saw in a dream a table where all the elements fell into place as required.Awakening, I immediately wrote it down on a piece of paper. Only in one placedid a correction later seem necessary.” Many authors of scientific history havepresented this account as though it were a miraculous manifestation in whichthe periodic table was given as a near-perfect vision. It is unfortunate that thesedescriptions do not accurately portray the significant amount of time and effortthat had been poured into the development of the periodic table. Careful studyof the entire story surrounding the development of the periodic table suggeststhat the last bit of inspiration Mendeleyev needed was to invert the order of

80 CHAPTER 3. ATOMS

the elements in the columns so the atomic weights were arranged in order ofincreasing atomic weight. In this new arrangement, the periodic trend becamemore readily apparent.

In addition to changing the order of elements based on atomic weight,Mendeleyev recognized the almost certain possibility that there were still undis-covered elements that would eventually have their proper place in his table. Thisinsight caused Mendeleyev to leave gaps (represented as question marks in Fig-ure 3.14) in his initial formulation of the table, but he didn’t leave the problemthere: he went so far as to predict the chemical and physical properties of severalundiscovered elements! It took nearly fifteen years before Mendeleyev would bevindicated in his astounding predictions by the discoveries of gallium (1875),scandium (1879), and germanium (1886), thus adding significant credibility tohis periodic table.

Figure 3.14: Dmitri Mendeleyev’s periodic table. Note the gaps left for undis-covered elements (represented with question marks). (Image is in the publicdomain.)

Dmitri Mendeleyev(1834-1907 ): Developed thefirst version of the periodictable of the elements.

The most significant challenge to Mendeleyev’s work came with the discoveryof a group of gases that was completely unknown when the periodic table wasfirst published in 1869. If the arrangement proposed by Mendeleyev did reflecta true pattern in the properties of the elements, then it would have to makeroom for these new elements. In the late 1890’s, William Ramsay repeateda series of experiments originally carried out by Henry Cavendish nearly onehundred years earlier that involved the reaction of the nitrogen present in airwith oxygen. Rather than analyzing the products of the reaction as Cavendishdid, Ramsay turned his attention to the small bubble of gas left after all thenitrogen had reacted. This gas proved to be completely inert: it did not reactwith anything. Ramsay called the gas argon.

Recall that Mendeleyev had constructed his periodic table based on pat-ters of reactivity observed when different elements reacted with hydrogen oroxygen. An element that reacted with one equivalent amount of hydrogen wasdescribed as having a valence of one, if the element combined with two equiva-

3.3. BONDING AND THE PERIODIC TABLE 81

lent amounts of hydrogen, it was assigned a valence of two, etc. Mendeleyev hadgiven valence greater significance than atomic weight in the formulation of histable. Because of its inability to react with anything, the new gas discovered byRamsay must be assigned a valence of zero and with an atomic weight betweenchlorine and calcium, would have to be placed somewhere near those elements.Looking at Mendeleyev’s table, this could be accomplished by inserting thisnew gas between chlorine and calcium, but doing so would create a new rowof undiscovered elements! (See Figure 3.15.) Ramsay persisted in his efforts todiscover the missing group of elements with characteristics similar to argon andfound that the inert gas isolated from the atmosphere was actually a mixtureof the inert gases helium, neon, argon, krypton, and xenon - what we call thenoble gasses.

Noble gasses:A group of elemental gassesthat are chemically inert.

Figure 3.15: Dmitri Mendeleyev’s periodic table, including a new row to acco-modate the noble gasses. (Image is in the public domain.)

After Ramsay’s experiments, there were many discoveries made with re-gard to the structure and composition of individual atoms. Many of theseexperiments were carried out on hydrogen and helium and it was discoveredthat atoms are made of tiny, negatively charged particles called electrons, moremassive positively charged particles called protons, and equally massive butneutrally charged particles called neutrons, with the latter two particles lo-cated in a small dense nucleus at the center of the atom. Electrons are foundin clouds that surround the nucleus of the atom.

The next most significant advancement in the development of the periodictable came in 1914 when Henry Moseley conducted a series of experiments inwhich a sample of each known element was exposed to X-rays. Moseley noticedthat the sample of the element gave off X-rays, but the X-rays given off werelower in energy than the original X-rays. Interestingly, there were regular differ-ences in the energies of the X-rays given off by the different elements. Moseleyproposed that the lower energy X-rays produced from the elements was theresult of the interaction between the X-rays and the positively charged nucleus,

82 CHAPTER 3. ATOMS

and that the regular changes in the X-rays given off by different elements wasdue to regular changes in the charge in the nucleus. Based on these experi-ments, Moseley introduced the idea of atomic number, a number that indicatesthe amount of positive charge, or number of protons, in the nucleus. Moseley’sexperiments provided an experimental method for organizing elements in theperiodic table.

The modern periodic table (Figure 3.16) ranks elements based on the num-ber of protons in the nucleus and organizes the elements based on the occupancyof electrons in specific clouds surrounding the nucleus. The rows are called pe-riods and the columns are called groups or families. The groups or familiescontain elements that share similar chemical properties.

Molecular Bonding

With the understanding we gain from the periodic table, coupled with addi-tional information gathered from atomic and molecular research, we now havea more complete picture of how bonding between atoms occurs.

Suppose you have two atoms and are able to slowly bring the two atomscloser together. What part of the atoms will contact first? What type ofinteraction will this cause? As you can imagine, the negative charge of theelectrons from both atoms causes them to push against each other; however,if the two atoms are able to continue past the point of initial repulsion, thenthe electrons from one atom will encounter the positive charge centered in thenucleus of the other atom. What type of interaction will that cause? As youmay have guessed, the electrons from one atom are attracted to the positively-charged nucleus of the other atom. The degree of attraction felt by the electronsof one atom to the nucleus of another atom depends upon the identity of thetwo atoms in contact. In some cases, there is only a modest attraction thatis just strong enough to hold the atoms together (nonpolar covalent bond). Inother cases, the attraction is strong enough that the electrons from one atomspend more time surrounding the nucleus of the other atom, which creates anunequal distribution of charge between the two atoms (polar covalent bond).It’s also possible for there to be a very strong attraction between the electronsTypes of chemical bondsof one atom and the nucleus of the other atom; in fact, sometimes one or moreelectrons completely abandon their original atom and leave to take up residencein the electron cloud of the other atom (ionic bond). This transfer of electronscreates ions with either a positive charge (the atom that lost electrons) or anegative charge (the atom that gained the electrons).

These three scenarios describe the basic principles of chemical bonding. Youmay be familiar with the phrase “covalent bonding is the sharing of electrons”.This scenario is described in the first and second examples, but as you cansee, not all covalent bonds are equivalent. The third scenario describes ionicbonding. So how can we determine how atoms of different elements will bondwith each other? The relative position of the elements in the periodic table cangive us good clues. For elements from groups close together in the periodic table,the more likely it is that they will simply share electrons, but for elements fromgroups far apart in the periodic table, it’s more likely that electrons will transferfrom one atom of an element to the atom of the other element. The attractionan atom has for the electrons it donates to a bond is called electronegativity.An electronegativity value is assigned to each element and by calculating thedifference in electronegativity between two elements, we can get a good idea ofthe nature of the interaction that exists between those elements. It’s importantto recognize that the different types of interactions between individual atomshave a significant impact on the larger molecules formed from those atoms.

Since we’ve dealt with the kind of interaction or bond that can form betweenatoms of different elements, we must also consider how many bonds a given atomcan accommodate. Again, the periodic table can give us clues about how atoms

3.3. BONDING AND THE PERIODIC TABLE 83

Figure 3.16: The periodic table of the elements. The elements are ordered byatomic number. Elements in a column (called a group or family) have similarchemical characteristics. (Image is in the public domain.)

84 CHAPTER 3. ATOMS

of different elements will interact with each other. Essentially, the position onthe periodic table, particularly the column the element resides in, tells us howmany bonds a particular atom can form.

A set of atoms that has bonded together is called a molecule. The propertiesof a molecule depend largely on what types of atoms it is made out of andthe types of bonds between those atoms. Polar covalent bonds can result insituations where electric charge (either positive or negative) tends to group up atcertain places on the molecule. These partial charges can result in electrostaticforces between polar molecules, or may affect the shape of the molecule itself.

A comprehensive discussion about the types of molecules found in natureand their various properties would require volumes of written text. To helpyou understand and appreciate how molecular bonding relates to other areasof interest, we would like to focus on several classes of biologically importantmolecules2.

The first class of biologically important molecules are carbohydrates, whichare composed of carbon, oxygen, and hydrogen. Carbohydrate molecules in

Carbohydrates:Molecules composed of car-bon, oxygen, and hydrogenwhich have a distinct rigidring-like structure.

general have a ring formed of carbon atoms. This ring provides rigidity to themolecule.

The second class of biologically important molecules are lipids. LipidsLipids:Molecules composed of car-bon, oxygen, and hydrogenwith a characteristic rod-likestructure.

are also composed of carbon, oxygen, and hydrogen, but the structure of a lipidis very different from that of carbohydrates. In particular, the ring of carbonatoms is gone. Lipid molecules look like a straight (or sometimes bent) rod.Because of their structure, they cannot provide the rigidity of carbohydrates.Furthermore, where carbohydrates typically have good bonding sites at eachcorner of the carbon ring, lipids tend to only form bonds at one end.

The third class of biologically important molecules are the amino acids.

Amino acids:Molecules composed of car-bon, oxygen, hydrogen, andnitrogen which serve as thebasic building blocks for life.

They are composed of the same three elements as the previous classes, but alsoincorporate nitrogen. Amino acids are known as the basic building blocks oflife. Most of the structures in your body are built from amino acids. Aminoacids are the building blocks of proteins. Individual amino acids react with eachother to form long chains called polypeptides. The long polypeptide chains foldto form proteins. Folding gives the protein a distinct shape.

Proteins:Long, folded chains of aminoacids. The fourth class of biologically important molecules that we will discuss are

nucleotides. Nucleotides are molecules that serve as the building blocks forNucleotides:Molecules consisting of a car-bohydrate (sugar), a groupof phosphates, and a nitroge-nous base which serve as thebuilding blocks for DNA.

RNA and DNA. They consist of a carbohydrate (sugar), a group of phosphates(structures which include phosphorus atoms), and a third structure called anitrogenous base (see Figure 3.17). There are four nitrogenous bases whichare incorporated into nucleotides: adenine (A), thymine (T), cytosine (C), andguanine (G). Nucleotides are often identified by these four letters. Nucleotidescombine together to form molecules of ribonucleic acid (RNA) and deoxyri-bonucleic acid (DNA).

DNA:(Deoxyribonucleic acid) amolecule made of nucleotideswhich carries the geneticcode.

We will return to the discussion of these molecules in chapter 5.

2For a better picture (literally) of these molecules, we direct your attention to the follow-ing website: http://biomodel.uah.es/en/model3/index.htm. The “Instructions” link at thebottom in the pane on the right of the browser window performs the obvious function. Inthis applet, the colors of the atoms represent their atomic number (or equivalently tell youwhat element they are).

3.3. BONDING AND THE PERIODIC TABLE 85

Figure 3.17: The molecular structure of a nucleotide. Note that there are threebasic parts: a phosphate group (highlighted in orange), a sugar (deoxyribose,highlighted in green), and a nitrogenous base (highlighted in pink). (Image isin the public domain.)

86 CHAPTER 3. ATOMS

REVIEW QUESTIONS

1. What lessons were learned from the experiments conducted byJoseph Priestley, Antoine Lavoisier, and Joseph Proust?

2. Summarize the main points of John Dalton’s atomic theory.

3. What does the development of the periodic table teach us aboutscientific models?

4. In what way was Dmitri Mendeleyev’s period table unique whencompared to earlier attempts at organizing the elements?

5. Why was William Ramsay’s discovery a challenge to Mendeleyev’speriodic table? How was the model revised?

6. Describe the following types of bonds: nonpolar covalent, polar co-valent, ionic.

7. Describe the following classes of biologically important molecules:carbohydrates, lipids, nucleic acids.

3.4. RADIOACTIVITY 87

3.4 Radioactivity

Wilhelm Roentgen (1845-1923 ): Discovered X-rays(1895).

Henri Becquerel (1852-1908 ): Discovered naturalradioactivity (1896).

OVERVIEW

Summary: Radiation was discovered late in the 19th century. Thereare five primary types of radioactive decay. Radioactivity diminishes withtime in a very specific way, and we can use radioactive decay as a type ofclock.

Learning Objectives:

• Identify the observations that led to the discovery of radiation.

• List the properties of alpha, beta, and gamma radiation.

• Identify the type of radiation and/or daughter nucleus produced inan incomplete equation for nuclear decay.

• Predict whether an isotope is most likely to undergo alpha decay,beta plus decay, or beta minus decay.

• Understand the random nature and condition-independent proper-ties of radioactive half life.

• Perform mathematical calculations relating to the half-life of a ra-dioactive isotope.

Vocabulary:

• Ionizing radiation

• Parent

• Daughter

• Alpha decay

• Beta decay

• Gamma decay

• Decay series

• Half life

While working with cathode ray equipment in 1895, Wilhelm Roentgennoticed that the equipment was emitting some type of ray that was able topenetrate materials opaque to light, subsequently detected by fluorescence 3.Roentgen dubbed these mysterious rays “X-rays”, and their discovery markedthe beginning of our understanding of ionizing radiation, or radiation which

Ionizing radiation:Radiation which carries suf-ficient energy to knock elec-trons from atoms.

carries sufficient energy to knock electrons from atoms.One year later Henri Becquerel was working with uranium salts in his lab-

oratory when he accidentally discovered natural radioactivity. A sample of thesalts had been placed near or on a wrapped photographic plate, and the pres-ence of the sample caused the plates to darken, even though the covering wasimpenetrable by light. Unlike the production of X-rays, the emission of ... well,whatever it was ... from the uranium salts happened spontaneously. Furtherinvestigations revealed that these mysterious rays were not affected by chemicalchanges, temperature, or pressure.

Marie Curie (1867-1934 ):Discovered the radioactiveelements Polonium and Ra-dium (1898) and developedtechniques for isolating ra-dioactive isotopes (1902)

Just a few years later in 1898, a Polish scientist named Marie Curie coinedthe term “radioactivity”. Additionally, she and her husband Pierre discoveredtwo radioactive elements - polonium and radium. She also refined the techniquesfor isolating radioactive isotopes from larger samples.

Also in 1898, Ernest Rutherford and others began investigating the proper-ties of this natural radiation. By the year 1903 Rutherford was able to demon-strate that the source of this radiation was the spontaneous disintegration of

3Fluorescence is a phenomena that occurs when an electron in an excited state in an atomdecays by emitting visible light. For those who want the gory details, Roentgen’s experimentsutilized a barium platinocyanide surface as a fluorescing material.

88 CHAPTER 3. ATOMS

atomic nuclei. His investigations revealed that radioactivity decreased in a reg-ular way over time, leading to the concept of radioactive half life.

Rutherford recognized that there were two distinct types of radiation. Shortlythereafter Paul Villiard discovered a third type. These three classifications werereferred to using the first three letters of the Greek alphabet: α (alpha), β(beta), and γ (gamma). Each type of radiation was found to have a distinctelectric charge. These three types of radiation are discussed below.

Ernest Rutherford (1871-1937 ): Demonstrated thatradioactivity is the spon-taneous disintegration ofatoms, discovered the expo-nential nature of radioactivedecay (half-life), and rec-ognized there were threedistinct types of naturalradioactivity.

Type I: Alpha decay

In a spontaneous decay process, the original radioactive nucleus is referred toas the parent nucleus. The nucleus that remains after the decay is called the

Parent:The original radioactive nu-cleus in a spontaneous decayprocess.

daughter nucleus.

Daughter:The nucleus that remains af-ter a spontaneous decay pro-cess.

The first type of decay we will discuss is alpha (or α) decay. Alpha decayoccurs when a heavy nucleus spontaneously emits a 4

2He nucleus (two protons

Alpha decay:Spontaneous decay by theemission of a 4

2He nucleus, oralpha particle.

and two neutrons strongly bound together). In light of the conservation lawsdiscussed in the previous section, the resulting daughter nucleus must have amass number that is four units lower than the parent and an atomic numberthat is two units lower. (Note: this means that when an atom undergoes alphadecay, it turns into a different element.) For example, consider the decay of22890 Th:

22890 Th −→ 224

88 Ra + 42He

Notice that on both sides of the arrow in the equation above the total massnumber is 228 and the total charge number (atomic number or proton number)is 90. Radioactive decay follows the same conservation laws as other nuclearreactions!

Alpha radiation is the least penetrating of the three major classifications.A sheet of paper, a piece of cloth, or even your skin is enough to stop an alphaparticle. However, because alpha particles are relatively large and energetic,they are able to do more damage to biological tissue than other types of ra-diation. This capacity to damage tissue is quantified by a number called therelative biological effectiveness or RBE4 (see Table 3.4). Alpha particlestypically have an RBE up to about 20.

Figure 3.18: Alpha decay (Image is in the public domain.)

Type II: Beta decay

The class of radioactive processes called beta decay actually consists of threedistinct types of decay. Each of these decays is caused by a fundamental forcecalled the “weak force” which we will explore in a little more depth later in thisreading.

4By definition, the RBE is defined as the amount of absorbed X- or gamma-ray energyrequired to inflict the same damage as one unit of absorbed energy from the specific source.Thus, by definition, gamma- and X-rays have an RBE of 1.

3.4. RADIOACTIVITY 89

Beta minus (β−) decay

The first subclass of beta decay, called beta minus decay, involves a neu-tron spontaneously turning into a proton and electron, with the electron sub-sequently being ejected from the nucleus. At its most basic level, this processis represented by

10n −→ 1

1p + 0−1e + ν

Notice again that the total mass number and charge number is unchanged bythis process. Incidentally, the decay also results in the emission of a neutrino,which is a very small particle that rarely interacts with normal matter.

Beta minus decay:Spontaneous decay where aneutron is transformed intoa proton and electron, andthe electron is subsequentlyejected from the nucleus.

An example of beta minus decay occurs in the decay of 60Fe:

6026Fe −→ 60

27Co + 0−1e + ν

As with alpha decay, the daughter nucleus is of a different element than theparent.

Figure 3.19: Beta minus decay (Image is in the public domain.)

Beta plus decay

Protons can also spontaneously turn into neutrons and positrons, with thepositron being ejected from the nucleus. Positrons are subatomic particles thatare in every way identical to electrons, except they are positively charged. Thisprocess is called beta plus decay, and is represented by

11p −→ 1

0n + 01e

+ + ν

Beta plus decay:Spontaneous decay where aproton is transformed into aneutron and positron, andthe positron is subsequentlyejected from the nucleus.

This decay process is illustrated in Figure 3.20. An example is the spontaneousdecay of 22

11Na:

2211Na −→22

10 Ne +01 e+ + ν

Beta particles (whether electrons or positrons) can generally be stopped bya metal foil, a few millimeters of plastic, or a few millimeters of skin or othersoft tissue. Because of their small mass (and smaller charge, when comparedto the alpha particle), they generally do less damage when they are absorbed.In fact, their RBE is typically around one.

Neutrinos, on the other hand, can easily penetrate through several kilome-ters of lead. Not to worry, though, because the neutrinos rarely interact withanything, so they don’t do any damage as they pass through you.Electron capture

Electron capture is essentially the reverse process of beta minus decay.

Electron capture:The spontaneous decay pro-cess wherein an atomic elec-tron and proton combine toform a neutron.

In the quantum mechanical model of the atom (discussed in the next section),an electron has a probability of being inside the nucleus. When this happens,it can combine with a proton to form a neutron:

11p + 0

−1e −→ 10n + ν

90 CHAPTER 3. ATOMS

Figure 3.20: Beta plus decay (Image is in the public domain.)

As with all previously discussed radioactive decay processes, the daughter nu-cleus is a different element than the parent (one less proton). An example ofthis process is the electron capture decay of 58

29Cu:

5829Cu +0

−1 e −→5828 Ni + ν

Electron capture is illustrated in Figure 3.21

Figure 3.21: Electron capture (Image is in the public domain.)

Type III: Gamma decay

When a nucleus has any excess amount of energy (which typically happensafter any other radioactive decay process), it will give off that extra energy inthe form of a gamma ray. Gamma rays are exactly like light rays, except thatthey have a much, much shorter wavelength and carry a lot more energy. Thisprocess is called Gamma decay.

Gamma decay:The spontaneous emission ofa gamma ray by an excitednucleus.

Primarily because they carry no electric charge, gamma rays can penetrateseveral feet into concrete, several inches into lead, or completely through ahuman body.

Figure 3.22: Gamma decay (Image is in the public domain.)

Tables 3.3 and 3.4 summarize these three types of radioactive decay.

Other processes

There are other decay processes involving the spontaneous emission of mag-nesium, carbon, or neon nuclei from heavy parents. There are processes that

3.4. RADIOACTIVITY 91

Decay type Particles emitted

Alpha α (42He nucleus)Beta

Beta minus electron (β−) and neutrinoBeta plus positron (β+) and neutrinoElectron capture neutrino

Gamma gamma ray (γ)

Table 3.3: Particles emitted in radioactive decay

Decay type Stopped by RBE

Alpha sheet of paper, clothing, skin up to 20Beta

Beta minus metal foil, millimeter of plastic ≈1Beta plus metal foil, millimeter of plastic ≈1Electron capture miles of lead N/A

Gamma few inches of lead, few feet of concrete exactly 1

Table 3.4: Penetration and relative biological effectiveness (RBE) of the typesof radioactive decay.

involve the emission of neutrons, either independently or in conjunction witha beta particle. Some nuclei may decay by emitting multiple beta particles.Some nuclei decay by spontaneously splitting into two smaller nuclei. Theseadditional decay processes are extremely rare - rare enough to not be discov-ered until well after the other decay types were classified.

Decay series

Frequently, the daughter of a given radioactive decay process is itself radioac-tive. The daughter will subsequently decay, and the daughter of that processmay also be radioactive. The process continues until a stable isotope is pro-duced. This sequence of radioactive decays is called a decay series.

Decay series:A sequence of radioactive de-cays that ensues when thedaughter nucleus of a decayprocess is itself radioactive.

For example, see Figure 3.23 which illustrates the decay series of 238U.Several different isotopes are produced in the decay process, but eventuallythe entire sample ends up as 206Pb.

What Makes Atoms Radioactive?

So why are some isotopes stable while most are radioactive? The stability ofan atom is primarily decided by the interaction of three different forces: theCoulomb (electrostatic) force, the nuclear strong force, and the nuclear weakforce.

Most people know what happens when you bring two positively chargedobjects together. That’s right, they repel! That repulsive force is called theCoulomb force (or electrostatic force). Now think about what you have in thenucleus of an atom. There’s the neutrons, which don’t have an electric charge,and the positively charged protons. If it were not for the other forces in thenucleus, which we will talk about shortly, the Coulomb force would push all ofthe protons as far away as possible, effectively ripping the nucleus apart!

So what holds it all together? It’s the strong nuclear force. This force is verydifferent from the forces you have experienced first hand. In fact, you have onlyexperienced gravity and electromagnetic forces. You already know that gravitycan act at a distance (i.e. you do not have to be in contact with the Earth inorder to experience its gravity), and you may also know that electromagneticforces can act at long ranges.

In an atom, it is only the neutrons and protons that can experience thestrong force. In order for this to happen they must be really close together.

92 CHAPTER 3. ATOMS

Figure 3.23: Radioactive decay series for 238U. Ultimately, all of the 238U be-comes 206Pb. (Image courtesy of “Tosaka”, licensed under Creative CommonsAttribution)

3.4. RADIOACTIVITY 93

How close? The characteristic distance is 10−15 meters. The nucleus of an atomis about this size, so the protons and neutrons can stick together via the strongforce. In a stable atom, the strong force beats out the Coulomb force, and theatom stays together.

As you pack more and more nucleons into an atom, the nucleus gets largerand larger. If it gets too large, the strong force starts to weaken, and Coulombrepulsion causes a positively charged chunk of the nucleus (an alpha particle)to be ejected. This is why the heavy isotopes tend to decay by alpha particleemission.

Free neutrons, or neutrons that are not found in an atom, are unstable. Leftto themselves they will undergo beta minus decay, usually within about half anhour. The presence of protons in the nucleus, however, stabilizes the neutrons.But if too many neutrons are present, that stabilizing effect weakens, and aneutron will decay. Thus, it’s the isotopes with a greater number of neutronsthat undergo beta minus decay.

If the nucleus has considerably more protons than neutrons, the Coulombforce begins to get the upper hand. In order to settle the score and stabilizethe nucleus, one of the protons will spontaneously decay into a neutron (i.e.beta plus decay). More often, however, the proton rich isotopes will decayby electron capture. The laws of quantum mechanics tell us there is a smallprobability that an electron can be found inside the nucleus of an atom. Whenit’s there, it can combine with a proton to form a neutron. The more protonsthere are, the more likely this is to happen.

Atoms will undergo gamma decay when there is excess energy stored in thenucleus. This extra energy can show up as a result of a previous decay or anuclear reaction. The nucleus would like to have as little energy as possible, soit gets rid of it by sending out a gamma ray.

Decay Rates and Half Lives

For each of the isotopes found in figure 3.23, you will find a time near thebottom of the octagon. This time has a special significance, and is called thehalf life - it is essentially the amount of time it will take half of a sample of

Half life:The amount of time it takesfor half of a sample of ra-dioactive material to decay.that isotope to decay. You will notice that the half lives for different isotopes

can be widely different: 238U has a half life of 4.5 billion years, while lowly214Po has a half life of only 164.3 microseconds (0.0001643 seconds).

There are thee important things you need to know about radioactive halflives:

1. Radioactive decay is a completely random process, and the half life ap-plies only to a large sample where you can statistically say somethingmeaningful. You cannot predict when a single atom will decay. You canonly say, on average, how long it takes atoms of that isotope to decay.Typically a sample will consist of somewhere on the order of 1020-1026

atoms, so the statistical description is pretty good.

2. Statistically speaking, only half of any given sample will decay in a halflife. So if I were to start out with 1,000 atoms of a radioactive isotope,after one half life I would have 500 of the original atoms remaining and500 daughter atoms. If I wait another half life, I will be left with 250radioactive atoms and 750 daughter atoms, and so forth.

3. Physical conditions or chemical process that happen on Earth do notaffect radioactive half lives. Bonding a radioactive atom to another atomchemically does not affect the half life either. In fact, the only place whereradioactive half lives do change is in the stars, and then only because it isso hot that the individual nucleons can be kicked to higher energy levels,and those higher energy levels have a tendency to decay at different rates.

94 CHAPTER 3. ATOMS

In any Earth conditions, including those found in the mantle and core,the rates at which radioactive isotopes decay are constant.

This last important idea establishes radioactive decay as a rate constantprocess. Additionally, whenever a radioactive atom decays, it leaves behind(at least somewhere down the line, if we are thinking decay series) a stabledaughter, so the “number of ticks” is also being recorded. Radioactive decaythus has the necessary qualities to act as a clock.Radioactive decay as a clock

To see how such a radioactive clock works, consider the graph shown inFigure 3.24. The horizontal axis represents time (measured in half lives) andthe vertical axis represents the amount of a particular isotope, expressed as apercentage of an original quantity. The red thick curve represents how muchof the original radioactive material remains, and the blue thin curve representshow much of the daughter nuclei have accumulated.

Figure 3.24: Amount of parent and daughter nuclei as a function of elapsed halflives. (Image is in the public domain.)

Now consider the following: suppose we knew that only 20% of the originalsample remained (the way we would figure this out is by looking at the ratioof the number of daughter nuclei to the number of parent nuclei). We locatethe 20% line on the vertical axis and follow it to the red curve. Once it inter-sects the red curve, we trace down to the horizontal axis and determine thatapproximately 2.3 half lives have elapsed. If the half life of the parent isotopewas 100,000 years, then we could say that 230,000 years had elapsed.

A moment ago we talked about daughter-to-parent ratios. Considering thedecay series shown in Figure 3.23, what we would actually measure is the ratioof 206

82 Pb to 23892 U. Then, assuming that there was no lead present when the

Uranium sample was first formed (the details of this so-called “initial daughterproblem” will be covered in the next chapter), we simply consider what happensto this ratio over time, as shown in Figure 3.25. If the ratio comes in at, say0.5, we see that about 0.4 half lives have passed. For 238U, which has a half lifeof 4.5 billion years, this would correspond to an elapsed time of approximately1.8 billion years.

The precision to which we can infer time from radioactive decay depends onthe precision of the measured half life and isotope ratios. Techniques have beendeveloped to measure these quantities to well within one tenth of a percent ofuncertainty.

3.4. RADIOACTIVITY 95

Figure 3.25: Daughter-to-Parent ratio as a function of elapsed half lives. (Imageis in the public domain.)

REVIEW QUESTIONS

1. Imagine that you had three radioactive sources, one that emittedalpha particles, one that emitted beta particles, and one that emittedgamma rays. Which sample would you prefer to carry in your pocket,provided you had to carry one of them? Which would you prefer toswallow, if required?

2. Why do we find naturally occurring 23892 U, but not any naturally

occurring 23994 Pu, even though both are made in supernovae? (Hint:

look up the half lives for these two isotopes on the Internet. Tryhttp://www-nds.iaea.org/livechart.)

3. Explain what is meant by a “decay series”.

4. The isotope 137Cs (cesium 137) has a half life of 30 years. Supposeyou were to produce a sample of pure 137Cs, which you then lockedaway in a storage vault. How much of the 137Cs will still be aroundafter 30 years? 60 years? 120 years? 300 years?

5. Explain why and how radioactive decay can be used as a clock.

96 CHAPTER 3. ATOMS

Chapter 4

Earth

Figure 4.1: The first image taken by humans of the whole Earth. Photographed by the crew of Apollo 8(probably by Bill Anders) the photo shows the Earth at a distance of about 30,000 km. South is at the top, withSouth America visible at the covering the top half center, with Africa entering into shadow. North America isin the bottom right. (Image courtesy of NASA.)

Earth: it is the only planet that man has ever (or probably will ever) step foot on. We depend on the Earthto provide all of the materials we need to survive. And yet, it was not until the 18th century that mankindbegan to show any interest in the planet aside from its utilitarian value.

In this chapter we will explore how we know what we know about the Earth, in the process answering thequestion “Why do we believe that the Earth is 4.5 billion years old?” We will discuss how rocks form, ways todiscern the order in which rock units formed (relative dating), their ages (absolute dating), and how the Earthis continually changing.

97

98 CHAPTER 4. EARTH

4.1 Uniformitarianism and Relative Dating

OVERVIEW

Summary: As a physical science, geology had a bit of a rocky start (punintended). Traditional and dogmatic notions dominated the public andscientific understanding of Earth’s history. With time, however, scientificprinciples based on indisputable observations prevailed. Some of theseprinciples allow us to determine when various geologic formations weremade relative to each other and provide a relative timeline for the Earth.

Learning Objectives:

• Explain what relative dating is and how it is useful in our under-standing of the age of our earth.

• Explain the relationships of each of the relative dating techniques,including how and when each one can be used.

• Define uniformitarianism and catastrophism, and identify the differ-ences between these two ideas.

• Use relative dating techniques to determine the sequence in whichgeologic features were formed.

Vocabulary:

• Catastrophism

• Uniformitarianism

• Relative dating

• Original horizontality

• Principle of superposition

• Principle of lateral continuity

• Cross-cutting relationships

• Inclusions

• Fossil succession

Man has not always viewed the Earth as we do today. To the modern mind,many ideas about the Earth that seem like “no-brainers” took considerable time,effort, courage, and genius to discover. Because so much is understood today wetend to look back on earlier times and wonder how they believed what they did.How, for example, could they have believed that the Earth was the center ofthe universe? Why didn’t they understand things correctly? Were they stupid?No, their lack of understanding was not the result of diminished intellectualcapacity; rather, it was created by lack of opportunity for innovation, falsetraditions, and insufficient information.

This chapter explores the development of mankind’s ideas about the Earth,helps you further develop your understanding of science, and introduces you tothe fundamental ideas of geology.

Our modern conception of the Earth rests solidly on a foundation composedof four ideas: relative dating, uniformitarianism, radiometric dating, and platetectonics. We will discuss the first two ideas in this section. The other twoideas are in the sections that follow. The development of these ideas tells thestory of how our modern view of the Earth came to be.

The Birth of Geology

Geology is the scientific study of the Earth. Its development occurred in severaldistinct stages: the pre-scientific period, the early scientific period, the age ofinstrumentation, and most recently the modern era.

During the period of pre-scientific development man’s primary interest inPre-scientific geology

4.1. UNIFORMITARIANISM AND RELATIVE DATING 99

the Earth was to find useful materials - like rocks that produce a sharp edgewhen broken or minerals that produce metals when melted. Greece was thefirst western culture to think deeply about the Earth, and the most influentialGreek philosopher to study nature was Aristotle. He was the first prominentfigure to adopt a scientific approach to the study of nature. Some of his ideaswere precociously correct. Others were not. During the Middle Ages Aristotle’sexplanations and opinions were dogmatized because at this time the westernworld determined the truth of a matter by reference to authority (an approachcalled scholasticism) and Aristotle was considered “the” authority on nature.Also dogmatized at this time were interpretations of biblical teachings. Thesedogmatized traditions were a great impediment to the development of trueideas about the Earth as well as other things (and are still obstacles for manyindividuals).

The early scientific stage of geology began in the late eighteenth century Early scientific geologywith the concept of uniformitarianism and ended in the decades surrounding1900 when the use of scientific instrumentation became widespread. Duringthis period geologists developed a basic understanding of the history and sur-face processes of the planet. During the subsequent age of instrumentationknowledge about Earth was increased through measurements of the Earth’sproperties, including its age, structure, and composition. Geology became fullymodern with the development of Plate Tectonic Theory in the 1960s.

Relative Dating and Uniformitarianism

During the Middle Ages, study of the Earth was limited in the west becausepeople believed that nature was base and dark. Think, for example, abouthow nature is portrayed in stories like Hansel & Gretel or Little Red RidingHood. This philosophy is also apparent in the dogmatization associated withthe geocentric model in chapter two - the Earth, being base and corrupt, wasplaced at the center of the universe (with hell in turn being found at the centerof the Earth). It was not until the Romantic movement of the 1800s that thewestern world began to think of nature as ennobling and worthy and not untilthe 1970s that western civilization began to be concerned about protecting theenvironment.

During the Renaissance, Earth began to be studied by wealthy individualsinterested in natural philosophy and by those involved in mining and canalbuilding operations. These early naturalists sought to understand the originand functioning of the natural world. Unfortunately, the scholastic dogmatismof the Middle Ages did not yield easily to the observations and discoveries theymade. Slowly, the light of rational inquiry began to disprove the false dogmaticnotions that had solidified during the Middle Ages. During this period, theprinciples of relative dating and uniformitarianism were developed.

Nicholas Steno (1638-1686 ): Proposed manyof the basic principles ofrelative dating.

Relative dating is a set of simple but powerful principles used to determine

Relative dating:Determining the sequence ofevents in Earth’s historywithout knowing actual ages.

the sequence of events in Earth’s history. The basic principles of relative datingwere developed by Nicholas Steno, a Danish naturalist who became interested inunderstanding layered rocks after dissecting a shark head in 1666 and wonderingabout the relationship between shark teeth and similar fossils found in rocks. Atthis time the word fossil did not mean what it does today: it was used to referto any curiosity found in a rock, what we would call today minerals, fossils,veins, etc. Steno developed the principle of original horizontality, which

Original horizontalitystates that the sediments that make sedimentary rocks are originally depositedin horizontal layers; the principle of superposition, which states that older Superpositionsedimentary rock units in an undisturbed sequence are found below youngerunits; and the principle of lateral continuity, which states that sedimentary Lateral continuityrock layers that have been separated by erosion were once continuous.

During the next century additional principles were discovered: the principleof cross-cutting relationships, which states that a rock unit that is broken Cross-cutting relationships

100 CHAPTER 4. EARTH

formed before the break in it did; and the principle of inclusions, which statesInclusionsthat material wholly included inside a rock existed before the rock that sur-rounds it did. Combining all of these principles allows a person to determinethe relative ages of adjacent rock units, such as those shown in Figure 4.2

Figure 4.2: A geologic cross section from Glacier National Park. The relativeages of the rock units and other features in this diagram can be determinedusing the principles discussed in this section. (Image courtesy of U.S. NationalPark Service)

The last principle of relative dating, the principle of fossil succession, wasFossil successiondiscovered in the late 1700s by William Smith who, through most of his life, wasemployed as a surveyor. While working at a coal mine in southeastern EnglandSmith observed that the rock layers in the mine were always found in the samerelative sequence and that each of the layers could be identified by a unique suiteof fossils. He hypothesized that the fossils found in Earth’s vertical sequence ofsedimentary rocks change throughout that sequence in a unique, reliable orderthat can be correlated over broad geographical distances. Later, he tested thishypothesis while surveying canals and traveling across England. Smith used thisprinciple to identify and correlate rock units throughout England and to create ageologic map of England - the first modern geologic map, which he published in1815. Because he was not an aristocrat Smith struggled to find the acceptancehis work deserved. Not long after publication the map was plagiarized andSmith was undersold. Financially ruined, Smith was incarcerated in debtor’sprison. Upon his release, he spent years as a homeless, itinerant surveyor.Later, due to the efforts of a noble employer he finally received the recognitionhe was due - sixteen years after the publication of ‘the map that changed theworld’.

William Smith (1769-1839 ): Proposed theprinciple of fossil succession.

Smith’s principle of fossil succession was independently developed in Franceby Georges Cuvier - a prominent naturalist of the era and the leading proponentof the then-accepted explanation of Earth’s past, catastrophism: a hypoth-

Catastrophism:A hypothesis stating thatEarth’s geological historycould be accounted for by aseries of catastrophic events.

esis that essentially said Earth’s geologic record could be accounted for by aseries of brief, large-scale catastrophic events. Observations of the Earth thathad accumulated over centuries indicated that its history was complex and vast.Unfortunately, the false traditions of the Middle Ages allowed only for beliefin a very young Earth. During the 17th century Archbishop James Ussher, us-ing the Bible, his knowledge of history, and numerous assumptions determinedthat Earth had been created at nightfall the evening before Sunday October23, 4004 BC. Ussher’s chronology became widely accepted and appeared in themargins of some editions of the King James Bible. Catastrophism was an at-tempt to reconcile Earth’s extensive and complex history with a young Earthby proposing that Earth’s immense changes occurred in a few enormous catas-trophes via no-longer-active processes that accomplished tremendous amountsof change in very short periods of time. Though catastrophism appealed to theyoung-Earth dogmatists of the Middle Ages, it could not explain the geologicrecord. Eventually, it crumbled under the weight of observation.

4.1. UNIFORMITARIANISM AND RELATIVE DATING 101

In the late 1700s, James Hutton, a wealthy Scottish physician who becameinterested in nature, developed the principle of uniformitarianism. Hut-

Uniformitarianism:A hypothesis stating thatEarth’s geological historycan be explained by the sameprocesses occurring today.

ton’s observations of the rock record and of Earth’s surface processes led him toquestion catastrophist assumptions regarding the need to accomplish Earth’shistorical changes in such a brief period of time. Instead, he hypothesized thatEarth’s rocks are a record of natural processes, like those we see working today,operating over vast periods of time. Uniformitarianism states that the prod-ucts (rocks) of past events and processes are best interpreted by matching themwith the products of active processes. This idea is sometimes expressed by thephrase “the present is the key to the past.”

For example, uniformitarianism suggests that a rock unit with character-istics identical to those found in modern beach deposits formed in a beach.Uniformitarianism assumes that the laws governing nature have remained uni-form through time. It opens the window to deep time and the vast historyof our planet. Shortly after its development the word ‘geology’ entered themodern lexicon and geology as a distinct scientific discipline was born. Theintellectual battle between uniformitarianism and catastrophism, which ragedfor several decades, produced a reaction against catastrophism so strong thatfor more than a century explanations of natural phenomenon by catastrophicprocesses - even natural ones - was frowned upon. Hard-line interpretations ofuniformitarianism softened in the later part of the 20th century when conclu-sive evidence for natural catastrophic processes such as asteroid impacts wasdiscovered. Modern uniformitarianism recognizes that rates of processes varyand that natural catastrophes do occur.

James Hutton (1726-1797 ): Proposed the princi-ple of uniformitarianism.

James Hutton was in his late fifties when he first presented his ideas to theRoyal Society of Edinburgh in 1785. In 1788 he described his uniformitarianview of Earth’s deep past in these words, “we find no vestige of a beginning, noprospect of an end.” Later that year he published a several-hundred page bookexpounding his ideas and in 1795 he published a two thousand page version.Though Hutton developed uniformitarianism and is considered the ‘father ofgeology’ his prose was so difficult to wade through that his ideas did not reacha wide audience until John Playfair summarized them in 1802 in Illustrationsof the Huttonian Theory of the Earth. Charles Lyell, a Scottish lawyer-turned-geologist, was introduced to uniformitarianism through Playfair’s summary.Lyell became uniformitarianism’s champion and the foremost geologist of hisday. Lyell was a lucid, effective writer. In 1830-1833 he published the firstmodern geology textbook, Principles of Geology - a three-volume uniformitarianexplanation of Earth processes and history. This book, which was in its twelfthedition in 1875 when Lyell died, is likely the most influential geology textbookever written.

Charles Lyell (1797-1875 ):Champion of uniformitari-anism; author of one ofthe most influential geologytexts.

Once developed, geologists began using the principles of relative dating anduniformitarianism to determine the sequence of events in Earth’s history. Overthe next century this historical sequence was formulated into the Geologic TimeScale. The Geologic Time Scale is divided into units of time: eons, eras, periods,and finer scale divisions (Figure 4.3). Each unit of time in the scale representsa group of rock units characterized by a shared, unique assemblage of fossils.Today, the time scale continues to be refined as new discoveries are made.

102 CHAPTER 4. EARTH

Figure 4.3: The geologic timescale, as determined by relative dating. Noticethat relative dating only tells us about the relative order of the various periods,and not when they occurred. (Image courtesy of BYU-Idaho)

REVIEW QUESTIONS

1. What barriers inhibited the development of key ideas in geology?Are any of these barriers present today?

2. Explain the difference between the two hypotheses of catastrophismand uniformitarianism.

3. Who were the important people in the development of the principlesof relative dating and uniformitarianism?

4. What is meant by “relative dating”?

5. Define each of the following terms:

(a) Original horizontality

(b) Principle of superposition

(c) Principle of lateral continuity

(d) Cross-cutting relationships

(e) Inclusions

(f) Fossil succession

6. Why are the principles discussed in this section unable to tell us howold the Earth is?

4.2. ABSOLUTE DATING AND THE AGE OF THE EARTH 103

4.2 Absolute Dating and the Age of the Earth

OVERVIEW

Summary: Science has discovered several natural “clocks” that can beused to determine the actual age of organic and mineral samples. Thisallows us to associate a range of years with each of the periods found inthe geologic time scale.

Learning Objectives:

• Identify the necessary characteristics of natural clocks, and give sev-eral examples.

• Explain the basics of several methods of absolute dating (includingradiometric dating), identifying their underlying assumptions andlimitations, and evaluating their reliability.

• Explain the processes involved in Carbon-14 and Uranium-Lead dat-ing, and identify when these methods can be used.

• Identify the scientifically accepted age of the Earth, and explain howscientists arrived at this number.

Vocabulary:

• Absolute dating

• Macroscopic dating methods

• Radiometric dating

• Initial daughter problem

You may recall from our discussions earlier this semester that any usefulclock must have two things: 1) a rate constant process, and 2) a record of howmany “ticks” have passed since the clock started. Several “clocks” with thesecharacteristics exist in nature. Scientists learned how these clocks function,and that those clocks can be used in absolute dating, which is the process ofdetermining the actual age of a specimen or geologic formation. Before modernmethods of absolute dating were developed and refined, however, there wereseveral early, faulty attempts at assigning dates to things, such as the age ofthe Earth.

Absolute dating:The process of determiningthe actual age of a specimen,as opposed to relative ages ofseveral specimens.

Early Attempts at Absolute Dating

One early attempt at determining the absolute age of the Earth was carriedout by James Ussher (1581-1656), Archbishop of the Church of Ireland. Hischronology of the Earth was a non-scientific estimate based on a literal readingof the Bible. He assumed that the creation of the Earth took place in only six24-hour days, and that the rest of Earth’s history could be pieced together bystudying genealogies and timelines found in the Bible. Using these assumptionsUssher concluded that the Earth came into existence the evening before October23, 4004 BC. Ussher’s estimated age of the Earth gained considerable popularitywhen it was printed in a number of different issues of the King James Version ofthe Bible. Some people still ascribe to this “Young Earth” view, even thoughthere is overwhelming scientific evidence supporting the conclusion that theEarth is much older than this.

Some of the earliest scientific attempts to determine the age of the Earthrested upon assumptions of how certain natural processes occurred. LordKelvin, a scientist famous for his important contributions to our understandingof heat, made one such estimate. Kelvin recognized that the Earth, like anyother object warmer than its environment, would radiate heat into space. If

104 CHAPTER 4. EARTH

the amount of heat radiated into space exceeded the amount of heat absorbedfrom the Sun, Earth’s temperature should cool. Using this model and his bestestimate of the temperature of a newly formed molten planet the size of Earth,he calculated that Earth must be about 20 million years old. Unfortunatelyfor Kelvin, his model was incomplete. When he did his work, no one knewabout one of the most important factors that affect the temperature of theEarth radiation. Radioactive decay processes in the inner layers of the Earthprovide additional heat Kelvin could not have known about, and this led to hiserroneous estimate.

Another early scientific attempt involved measuring the thickness of sedi-ment layers at the bottom of lakes. The deposits themselves constitute a recordof the number of ticks. The rate of deposition of sediments and the thicknessesof these beds do not, however, occur at a constant rate, and therefore cannotserve as an accurate clock. Other scientists tried to make estimates using oceansalinity. This method also suffers from the lack of a rate-constant basis, andcannot therefore produce reliable estimates of absolute ages. Scientists fortu-nately kept working on this problem, and eventually developed reliable methodsfor determining absolute ages of things.

Macroscopic Historic Dating Methods

Macroscopic dating methods use visible characteristics or formations to de-termine the ages of things. For example, when you look at a cut tree stump

Macroscopic dating:Determining the age ofthings based on visible char-acteristics or formations.

or a cross-sectional cut through the trunk of a tree you will usually see treerings (Figure 4.4). Tree rings are produced annually as trees grow. Duringthe summer trees grow faster and produce a lighter colored, less-dense woodthan they do in the winter. So when a sample core from a tree is collectedit is easy to assign an absolute age to the tree by counting its tree rings. Theoldest known individual living tree is a Bristlecone pine named Methuselah thatlives in the White Mountains of Nevada. It has 4723 pairs of tree rings (i.e.,4723 years old). There is also evidence of an even older Bristlecone pine thatwas cut down accidentally in 1964: it was 5000 years old. Some clonal treestands, trees that reproduce by cloning themselves (usually from the root sys-tem), have been determined to be much older. For example, the root system ofa Norway spruce in Darlana, Sweden, has been dated by radiometric techniquesand genetic comparisons to be over 9550 years old, and some researchers sug-gest that some groves of Quaking Aspen, also clonal trees, may be more than80,000 years old. This use of tree rings as a method of absolute dating is calleddendrochronology.

Dead trees can give us additional information about the ages of things.Certain events, such as fires and local climate variations, leave their marks onthe growth rings of trees. These marks allow us to correlate the ages of differenttree specimens, not unlike fossil succession allows us to correlate geologicalfeatures (see Figure 4.5). This process allows us to determine the ages of forests,not just ages of individual trees. Using this approach, European pine and oakshave been dated back to about 11,000 years ago. Volcanic activity can alsoprovide additional information. At Specimen Ridge in Yellowstone NationalPark there are several separate layers of forests buried in volcanic ash. Bycounting the number of rings in those now fossilized trees within each layer,and adding the ages of all the layers together, scientists calculated that thetrees in the bottom layer lived at least 27,500 years ago.

There are other natural systems that are also highly reliable natural clocks.Glacial lake sediments, called varves, contain an extremely accurate record ofthe ages of lakes by the number of sediment layers in them. During the winter,glacial lakes freeze over, limiting the amount of dust and other sediment thatcan accumulate on the bottom, and eliminating the churning effect of the windon the surface. Sediments in the water settle during the winter and form a

4.2. ABSOLUTE DATING AND THE AGE OF THE EARTH 105

Figure 4.4: Seasonal rings of a tree. One pair of light and dark rings is producedeach year by the seasonally varying growth rate of the tree. By counting thenumber of rings, you can determine the age of the tree. (Image is in the publicdomain.)

Figure 4.5: Dendrochronology of overlapping tree rings from wood obtainedfrom different trees can produce a chronology that extends further backin time than that obtained from a single specimen. (Image courtesy ofhttp://dendrodan.files.wordpress.com/2009/09/slide1.jpg)

106 CHAPTER 4. EARTH

Half-life Effective DatingIsotopes of Parent Range of Earth

Parent Daughter (years) Materials (years)

Uranium-238 Lead-206 4.5 billion 10 million - 4.55 billion

Uranium-235 Lead-207 0.713 billion 10 million - 4.55 billion

Rubidium-87 Strontium-87 49 billion 10 million - 4.55 billion

Potassium-40 Argon-40 1.25 billion 50,000 - 4.55 billion

Carbon-14 Nitrogen-14 5,730 100-60,000

Table 4.1: Major isotopes used in radiometric dating.

distinct layer over the previous year’s sediment layer. Counting these seasonalsediment layers has revealed that some glacial lakes have been around for avery long time: Lake Van in Turkey for approximately 14,570 years, and LakeSuigetsu in Japan for 29,100 years, to cite two examples.

The glaciers also produce seasonal layers. During the winter, new snow isdeposited on the surface of a glacier, and during the summer dust is deposited.As seasonal layer upon seasonal layer is buried, the snow compresses and formsice. These seasonal layers are plainly obvious in some glaciers (see Figure 4.6).Layers in the glaciers of Greenland and Antarctica are hundreds of thousandsof years old. The EPICA Dome C glacier in Antarctica has been sampled downto a depth of 3,240 meters, and includes approximately 800,000 seasonal layers.

Figure 4.6: Seasonal layers in the Covell Glacier (Jasper National Park, Alberta,Canada). (Image courtesy of Wing-Chi Poon, licensed under the Creative Com-mons SA-BY license.)

Radiometric Dating

The discovery of radioactivity led to the development of one of the most power-ful methods of absolute dating: radiometric dating. The assignment of ages

Radiometric dating:Using radioactive isotopes todetermine the age of a sam-ple. to specimens by radiometric dating is based on the observation that radioactive

isotopes decay to stable isotopes at a constant rate. The accuracy and relia-bility of radiometric age dates rests on the constancy of decay rates, precisionand accuracy of instrument measurements, and closed-system behavior of thesample. There are dozens of useful radiometric systems (see Table 4.1).

A system’s decay rate is described by its half-life. Half-lives of systems varyfrom very short (much less than a second) to very long (billions of years). Theconstancy of half-lives has been verified experimentally: decay rates are notsignificantly altered by physical conditions or chemical reactions. Radiometric

4.2. ABSOLUTE DATING AND THE AGE OF THE EARTH 107

age dates require scientists to measure the half-life of the radiometric system,the proportion of parent to daughter atoms, and the amount of daughter atomsthat existed when the radiometric clock started ticking.

Accurate half-lives were measured decades ago, and accurate parent-daughteratom ratio measurements are made routinely in labs around the world. But howdo we measure the amount of daughter atoms that were there when the clockstarted ticking? There are three solutions to this question, also known as ini-tial daughter problem. For systems with a short half-life, the problem is

Initial daughter problem:The quandary in radiometricdating associated with notalways knowing whether andhow much of a daughter iso-tope was in existence when amaterial formed.

circumvented by directly measuring the activity of the sample, i.e., the num-ber of decay events in a period of time. For systems in which the parent anddaughter atoms have vastly different chemical behavior, the problem is avoidedby dating only minerals that do not allow any daughter atoms into their crystalstructure when the mineral forms. The third solution, the isochron method, isequally effective, but requires more space to explain than is available in thisessay.

Useful radiometric dates require that the specimen being dated has notlost or gained parent or daughter atoms since it formed, i.e., it has remained“closed”. For this reason, minerals that crystallize from magma are the mostuncomplicated to date, because determining whether or not the crystals haveremained closed is straightforward. Where needed, several radiometric systemsare used simultaneously to demonstrate the validity of a date, and some systemshave internal checks for closed system behavior.

Radiometric dating is a robust method for determining the age of eventsin Earth’s past, as well as the age of the Earth itself. Radiometric datingallowed scientists to add specific dates to the Geologic Time Scale (Figure 4.7).Radiometric dating indicates that the earliest time period - the Hadean era -began somewhere around 4.6 billion years ago and is based on the Uranium-Lead methodology shown, along with a few other specific methods, below.

Figure 4.7: The geologic timescale, with times assigned by radiometric dating.(Image courtesy of BYU-Idaho)

108 CHAPTER 4. EARTH

Radiocarbon (Carbon-14) Dating

Carbon is an essential component of biologically important molecules such ascarbohydrates, lipids, proteins, and nucleic acids. Living things therefore con-stantly take in, use, and release huge numbers of carbon atoms.

Carbon exists in three naturally occurring isotopes: 12C, 13C, and 14C. 12Cand 13C are both stable and are much more abundant than the radioactiveisotope 14C. However, because living things contain vast numbers of carbonatoms the existence of even trace amounts of 14C in the ’environment and intissues of living things make it possible for us to use radiocarbon dating tomeasure absolute ages of specimens that were once alive.

The half-life of 14C is 5,730 years. This relatively short half-life means thatsignificant amounts of 14C are found only in the atmosphere, in living things,and in things that died less than about 60,000 years ago. A specimen older thanthis has been around so long that most of the 14C in it has decayed and the fewremaining 14C atoms are not sufficient to provide reliable age determinations.

So, if 14C undergoes such rapid decay, where does 14C come from? Radio-carbon (another name for 14C) is made when cosmic radiation and solar windbombard Earth’s atmosphere. These particles are shot into space at such highvelocities that electrons associated with them are stripped away and the re-maining subatomic particles strike our atmosphere. Some of these particles aresingle neutrons. Nearly 80% of our atmosphere is made of Nitrogen-14 (14N),so it is not surprising that just about every neutron that strikes our atmosphereeventually collides with the nucleus of an atom of 14N. The high-impact colli-sion between one of these neutrons and the nucleus of a 14N results in a protonbeing knocked out of the nucleus in following reaction:

10n +14

7 N −→146 C +1

1 p

About 10,000 trillion (1019) atoms of 14C are formed in the atmosphere everysecond. These newly formed 14C atoms can react with oxygen in the atmo-sphere to form 14CO2. 14CO2 is used along with non-radioactive CO2 by plantsduring photosynthesis to make sugars. Then when animals ingest plant materi-als made from these sugars, 14C enters their bodies and is assimilated into theirtissues. All living things therefore contain enormous numbers of 14C atoms.How many? Well, there is one atom of 14C for about every one trillion atoms ofnon-radioactive carbon in the environment. This same ratio of non-radioactivecarbon to 14C is found in the tissues of living things. By carrying out a series ofcalculations we find that there is an average of 9 billion atoms of 14C in everygram of tissue in our bodies. So, there is plenty of 14C per gram of living tissueto do radiometric dating.

The intensity of cosmic radiation and solar wind is not constant, and theamount of 14C that is formed in the atmosphere varies accordingly. Fortunately,scientists have worked out a way to calibrate the varying amounts of 14C inthe environment: by comparing 14C dates from radiometric analysis with astandardized curve of 14C formation obtained from samples of known ages.One way to produce a standardized curve is through dendrochronology. Theages of tree samples can be determined by counting the rings, and then thesame samples are radiocarbon dated. When the two sets of data are graphedagainst each other, it looks like Figure 4.8. Scientists also developed a carbonhistory by examining the remains of leaves, twigs, and insect remains trapped inannual sediment layers (called varves) of glacial lakes. As mentioned earlier inthis reading, sediments in glacial lakes are deposited only in the summer whenglaciers partially melt. Varve-based timelines extend back nearly 50,000 years.Whenever a scientist does 14C dating of a specimen of unknown age they usethe calibration curve to check and, if needed, adjust the calculated ages of theirspecimen. In this way scientists compensate for fluctuations in 14C formationassociated with variations in the intensities of cosmic radiation and solar wind.

4.2. ABSOLUTE DATING AND THE AGE OF THE EARTH 109

When 14C decays it undergoes beta decay during which a neutron in the nucleusbecomes a proton and 14C decays into stable 14N.

Figure 4.8: Radiocarbon calibration curve. The straight line represents theinferred age without calibration. “BP” is an abbreviation for “before present”,where the “present” is defined as 1950 AD. Data is taken from [Stuiver98].(Image courtesy of BYU-Idaho)

As long as an organism is alive the ratio of 14C atoms to nonradioactivecarbon atoms in its tissues remains the same as the ratio of those atoms in theenvironment. When an organism dies, however, the number of 14C atoms in itstissues declines due to radioactive decay. The 14C clock therefore starts tickingas soon as an organism dies, so 14C dating tells you how long ago your specimendied. Because the decay rate of 14C is constant we can calculate the age of aspecimen by comparing the number of 14C decay events in a fresh sample tothe number of decay events in the sample of interest.

When a sample comes in for radiocarbon analysis, it is placed in a machinethat counts the number of 14C decay events. The machine then counts the sam-ple for 24 hours, after which the number of emissions detected by the machineis recorded. That number is then compared to the number of 14C decay eventsfrom a fresh sample and the age of the specimen is calculated1:

t = t1/2 [ln (C/C0) / ln(2)]

where t is the age of the sample in years (what we want to know), t1/2 is the half-life of the radioisotope (5,730 years), C0 is the number of 14C decays/day/gramof carbon in a fresh sample, and C is the number of emissions/day/gram fromthe sample to be dated.

Once the methods of radiocarbon dating were worked out people were inter-ested in knowing the ages of a wide variety of objects. One object of considerablepopular interest is a religious artifact called the “Shroud of Turin.” This pieceof cloth bears the visible full-length image of a man. Tradition holds that thebody of Jesus Christ was wrapped in this cloth when he was placed in the Gar-den Tomb. Until the development of radiocarbon dating, however, there wasno way to test the validity of this claim.

The Catholic Church approved a research project in which small samplesof the shroud would be dated independently by three different labs: one in

1A fresh sample undergoes fourteen 14C decay events every minute for every gram ofcarbon in the sample, so that sample will produce 20,160 14C decay events per gram ofcarbon in 24 hrs.

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Arizona, one in Switzerland, and one in England. Three other labs were givencontrol samples of cloth that were not from the Shroud of Turin, but none of thesix labs knew which had received an actual sample of the shroud and which hadreceived control samples. During 1988 the samples were analyzed, and in 1989the results were published. Data from all three labs that analyzed Shroud ofTurin samples produced similar results: the number of decays/day/gram fromthe sample (C) was somewhere around 18,500, whereas a fresh sample wouldproduce around 20,160 decays/day/gram. When put into the formula above,these two count rates yield an age of approximately 711 years. If you check thecalibration curve (Figure 4.8), you will find that this age needs no adjustment.

The results of radiocarbon dating of samples from the Shroud of Turinshow that the cloth was made from materials that died about 711 years (±30years, the standard range of error for this method) before 1988. Therefore, thecalibrated calendar date of the samples suggest that the Shroud of Turin sampleis from plant material that died between AD 1260 1390, or during medievaltimes.

Uranium-Lead (U-Pb) Dating

Radiocarbon (14C) dating is a powerful tool for measuring the age of organicmaterials younger than about 60,000 years old, but other methods are neededto determine ages of older samples. The Uranium-Lead (U-Pb) method is themost widely used method for determining the age of very old specimens.

The fundamental principle behind the U-Pb dating method is the discoverythat two naturally occurring forms of Uranium (238U and 235U) have differenthalf-lives and produce different forms of lead as the end products of their decayseries. 238U which makes up 99.28% of all naturally occurring uranium has ahalf-life of 4.47 billion years and its decay series produces 206Pb, while 235Uwhich makes up 0.71% of naturally occurring uranium has a half-life of 704million years and its decays series produces 207Pb.

Of course humans don’t have life spans that are long enough to actuallyobserve the entire radioactive decay of a sample of uranium to lead, but wehave observed directly the rate of decay in pure samples uranium. Because wehave done this we know the half-lives of both isotopes of uranium, and we cantherefore use the ratio of the amount of 238U to 206Pb and of 235U to207Pb ina sample to determine the absolute age of the sample.

This method is most accurate when you can use materials that include ura-nium and exclude lead when they are formed (i.e. you know exactly how manyinitial daughters there were). Fortunately the mineral zircon preferentially in-cludes uranium in its crystal matrix, and lead is excluded. So when a zirconcrystallizes its radiometric clock is set. This also means that the only source oflead atoms found inside a zircon crystal is the product of uranium decay. Zirconcrystals are extremely durable (they are chemically inert and are not subject tophysical breakdown) and have an extremely high melting point (900◦C). There-fore, even if a rock in which a zircon is embedded goes through partial meltingthe zircons in the rock maintain their integrity, and their radiometric clocks arenot reset.

The age of a zircon crystal is determined by isolating it from the rock inwhich it is embedded and then analyzing its composition using an instrumentcalled a mass spectrometer. The mass spectrometer displays the amounts of206Pb, 238U, 207Pb and 235U (and all other isotopes) in the zircon crystal. Thesevalues are then compared to a standard curve called a concordia that indicatesthe age of the zircon being dated (see Figure 4.9).

Because the decay rates of uranium isotopes are so slow, the uranium-leaddating method is most useful for dating specimens that are 10 million years oldor older, and the margin of error for this method is usually about ±1 millionyears.

4.2. ABSOLUTE DATING AND THE AGE OF THE EARTH 111

Figure 4.9: Concordia curve of 206Pb/238U and 207Pb/235U used in Uranium-Lead dating. The isotope two isotope ratios define a point on the curve, andthe ages of various points (in billions of years or Gy) are given. (Image courtesyof BYU-Idaho)

Conclusion

Shortly after radioactivity was discovered, scientists realized that the constantdecay rate of radioisotopes, coupled with the fact that each decay left behinda daughter nucleus, could be used to measure when geologic events had oc-curred. They immediately set about making measurements. As with all newtechnologies in the early days of the field of radiometric dating there were somefalse starts, including errors in data collection associated with crude instru-mentation, contaminated specimens, and incomplete calibration curves thatyielded inaccurate and imprecise results by today’s standards. Further devel-opments and tireless work by the early champion of radiometric dating, ArthurHolmes, finally led to the general acceptance of radiometric dating as a crediblemethod in 1931. By the 1950s radiometric dating was accurate enough to beused reliably. Today, the precision of radiometric age measurements is so high(generally above 99.7%) that accuracy is more significantly affected by geologicfactors than by instrumentation.

You should know, however, that there have been and certainly will continueto be skeptics of radiometric dating. Many skeptics insist on referring back toearly, tentative efforts of scientists that sometimes produced erroneous conclu-sions: conclusions that have been corrected during the intervening years. Yet,some skeptics are unwilling to accept the fact that the margins of error asso-ciated with radiometric dating have shrunk considerably, mainly because theresults of radiometric dating are in contradiction to their preconceived notions,ideas, or opinions about the world and their interpretation of old they believethings should be.

We can view science in one of two ways. We can view science as an assaulton beliefs, faith, and our own preconceived notions of the way we think thingsshould be, or we can view science as a useful tool that helps us search for truthand a deeper understanding of the way things in the physical world really are.As we continue to strive to search for truth and understanding about how thephysical world works we should also simultaneously strive to be the kinds of

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people we know we should be.

REVIEW QUESTIONS

1. Explain why some of the early methods of absolute dating were in-accurate.

2. Why do we consider radiometric dating to be a highly reliablemethod of determining the ages of things?

3. Where does 14C come from, and how does it get into the bodies ofall living things?

4. Why is there a need to use calibration curves when doing radiocarbondating?

5. Which method of radiometric dating is most useful for determiningthe ages of really old things, i.e., things billions of years old? Why?

6. Explain why macroscopic dating methods, such as tree rings andglacial layers, are not sufficient for determining the age of the Earth.

4.3. PLATE TECTONICS 113

4.3 Plate Tectonics

OVERVIEW

Summary: The Earth is an active planet that is constantly changing.One of the ways Earth changes is through the mechanisms of plate tec-tonics: the surface of the Earth is composed of relatively low density crustfloating on a sea of hot fluid rock that is constantly in motion.

Learning Objectives:

• Describe the characteristics of an active planet and how this relatesto the transfer of heat.

• Identify the general sequence of layers of material found in the Earth.

• Explain how the density and temperature of Earth materials driveconvection.

• Identify the observations that can be explained with earlier scientificmodels (continental drift and sea floor spreading) and how thesemodels evolved into the modern theory of plate tectonics.

• Identify the three basic types of plate boundaries and how theirrelative motion can explain many of the geographic features andnatural events that occur near these regions.

Vocabulary:

• Active planets

• Tectonics

• Convection

• Convergent boundary

• Divergent boundary

• Transform boundary

Craters are formed when space debris impacts a planet. The Moon andMercury remain heavily cratered from bombardment that the planets receivedearly in the evolution of the solar system, and yet there are almost no craterstoday on Earth. Why is that? Every year billions of tons of sediment are Earth is different from other

planetscarried into the ocean basins by rivers, glaciers, and wind, and yet the oceansare not full of sediment. How can that be? Each year there are several millionearthquakes. What causes them? The answers to these questions rest on thesingle most important characteristic of our planet: Earth’s surface is constantlychanging. In other words, Earth is an active planet.

Active planet:A planet whose surface issubject to change due toconvective processes beneaththe surface.

The energy provided by the Sun causes some activity on planets with at-mospheres; however, the effects of solar-induced processes are largely confinedto the surface of the planet. The temperature in the core of our planet is es-timated to be about 7,000◦C (about 12,500◦F), or about 25 times hotter thanthe maximum temperature of your home oven. The temperature of space isabout -270◦C (about -450◦F). Most of the heat in the Earth originates fromits formation and from the decay of radioactive atoms. Earth’s heat leaves the Earth’s heat comes from ra-

dioactive decayplanet slowly because rocks are good insulators - like the blankets you use toslow the release of your body heat while sleeping. Imagine being insulated byhundreds of miles of rock (or blankets) and you start to understand why heatleaves our planet slowly. The heat leaves the surface of the planet by means ofradiation: the energy is released into space in the form of infrared light waves.

Convection:Heat transfer that occurswhen hot materials rise.Heat moves from the core to the surface of an active planet through conduc-

tion, or heat transfer by physical contact between materials, and convection,which is the slow flow and overturning of a planet’s mantle as deep hot rockrises to the surface, cools, and sinks back toward the planet’s interior. The

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effect of convection on the planetary surface is called tectonics. Planets withTectonics:The movement of Earth’scrust on the surface, drivenby mantle convection.

insufficient heat to drive planetary convection are “dead” planets. Earth’s ac-tive processes, like seismic and volcanic activity and running water, are causedby mantle convection and the heat absorbed from the Sun. These processesmodify the planet’s surface by replacing or changing previously produced land-forms with new surface features. Because the Moon and Mercury are small,they had only enough heat to cause planetary convection early in their histo-ries and have been “dead” for a very, very long time. Mars is larger and wastherefore active longer, but it too has been dead for a long time2. Earth and itssister planet, Venus, are sufficiently large to retain the heat necessary to be anactive planet today. Because the Earth is active the craters that formed earlyin its history have been removed by tectonic activity, weathering, and erosionand replaced by other landforms. The sediments deposited in the ocean basinsare recycled by adding them to continents and by burying them inside Earth.Tectonic movements also force blocks of rock to slide past each other, causingearthquakes and volcanoes. These processes, while operating in the past, werethe driving force in producing the oceans and atmospheric conditions we havetoday - consequently setting up the conditions to which life needed to adapt.

Most of the changes on the Earth occur on the scale of deep time. Thegeologic changes that accumulate during one human lifetime are almost imper-ceptible. For this reason, the changes are not always obvious.

Plate Tectonics: History

The history of the development of the theory of plate tectonics is a great ex-ample of the scientific method at work. The early development of the theorystarted back in the early 1600’s and 1800’s. For example, many individuals,including Sir Francis Bacon and Benjamin Franklin, suggested that the coastsof Africa and South America appeared to parallel each other and that theymight have been connected. Benjamin Franklin even went so far as to suggestthat our planet might be much like a cracked shell which is floating on a densefluid. However, the older scientific methodology and technology was not able totest the ideas of these early scientists. Therefore, these individuals were unableto fully validate their hypotheses.

Alfred Wegener (1880-1930 ): Alfred Wegener.This picture was taken whileon an expedition in Green-land in 1930. He died soonafter this picture was takenas he was traveling back tohis base camp.

Alfred Wegener (a German Meteorologist) also noticed this parallelism andwas intrigued by the idea that these continents may have once been together.However, Wegener additionally realized that if Africa and South America wereconnected in the past there would be similarities in the rocks of the two conti-nents. Wegener tested this idea and found that the rocks on the correspondingsides of the two continents were almost identical in structure and composition.Wegener also further supported the budding theory by determining that fossilsof organisms (plants and animals) that lived on the potentially merged continentcould only be found on corresponding sides of the two continents. Addition-ally, the paleoclimatic evidence within the rocks, such as glacial striations, onlymade sense if the continents were once together. Wegener continued to makethese comparisons with other continents as well. For example, he noticed thatthe rocks in the Appalachian Mountains of the eastern United States are almostidentical in topography, structure, and composition to the northern mountainsof Ireland and Scandinavia.

Based on these observations, Wegener proposed a new theory, called conti-nental drift. Continental drift suggested that the continents were once all partof a supercontinent that Wegener called Pangaea, and that all of the continentswere mobile blocks or plates of less dense crust that were capable of movingthousands of kilometers through the denser oceanic crust.

Harry Hess (1906-1969 ):Harry Hess during Word WarII as captain of the transportUSS Cape Johnson.

Even though the theory had substantial supporting evidence, it did not gainpopularity amongst geologists and other scientists. Why not? Well first of all

2The largest known volcano in the solar system is on Mars.

4.3. PLATE TECTONICS 115

Figure 4.10: Distribution of fossil record across ocean basins. (Image courtesyof USGS)

the theory was a radical new idea that went against the current paradigm ofthe time (most earth scientists believed that the earth’s crust did not change).Moreover, Wegener was a meteorologist and most geologists were insulted atthe idea that Wegener would be suggesting such outlandish ideas about topicsoutside of his specialty. However, the biggest problem with Wegener’s theorywas a lack of any mechanism causing the plates to move. Physicists and otherprominent geologist quickly showed that the dense oceanic crust was much toorigid for continents to be shoved through it by any known force. Therefore,without an acceptable mechanism most geologists and other scientists wouldnot support Wegener’s theory of continental drift.

In the 1960’s it became technologically possible to investigate the topogra-phy of the ocean floor. During World War II, Harold Hess was a captain of atransport ship that had a new technology: sonar. Hess was also a geologist, andduring his trips across the Pacific he would direct their sonar equipment at theocean floor. After the war Hess continued his investigation of the sea floor andfound many interesting topographical features. Some of the most interesting ofthese features included: very deep oceanic trenches off the coast of many vol-canically active regions, large mountain ranges or ridges that were in the centersof the individual basins, and long linear chains of islands and seamounts (under-water mountains). In the Atlantic ocean this mid-oceanic ridge paralleled thesame patterns of the edges of continents. A more detailed view of these ridgesalso showed that a central rift valley existed all along their lengths. Duringthis same timeframe, radiometric dating was becoming a much more precisemethodology and other independent researchers were working on determiningthe age of the ocean floors. These independent researchers determined that theocean floor had a maximum age of ∼180 millions years, and that the age ofthe ocean floor actually increased as you moved away from these mid-oceanicridges.

With these observations regarding the seafloor, Hess proposed a new theorywhich he called sea floor spreading. Hess hypothesized that these central riftvalleys were actually tensional features within the earth’s crust where the sea

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floor was spreading apart due to convection currents within the subsurface ofour planet. He postulated that new oceanic crust was being formed at thesecentral rift valleys as hot material was rising from our earth’s interior. Thisnew crust would ultimately force the ocean crust apart and ultimately movethe crust toward the oceanic trenches where the ocean floor could now sink backinto our earth’s subsurface to be recycled. Therefore, because new sea floor wasconstantly being created at the ridges and destroyed at the trenches the oceanfloor always stayed relatively young.

With the discovery of a potential driving mechanism for the movement ofWegener’s continents a new scientific model was developed that combined theobservational data of Hess’s “seafloor spreading” and Wegener’s “continentaldrift” to explain the motion of the earth’s surface. Sadly, Wegener never sawthe acceptance of this theory within the scientific community since he froze todeath during an arctic expedition in Greenland in 1930. We call this synthesisof data and ideas the theory of plate tectonics. This theory describes how theEarth’s oceanic and continental crust is divided into a number of plates thatmove due to convectional processes in the mantle and interact with one anotherat their boundaries. Additional data, such as the distribution of earthquakes,volcanoes, and mountain ranges around the world, the patterns of paleomag-netic reversals in the seafloor, the direct observations of plate movement fromGPS measurements over time, and the progression of the ages of the seafloorand hot spot tracks (i.e. Hawaii and Yellowstone), have further validated thistheory of plate tectonics.

Plate Tectonics: How it Works

Convection drives planetary change. You have experience with convection,though you may not have thought much about it. Convection is what causes ahot air or helium balloon to rise. When you heat soup or oatmeal, if you havebeen observant, you’ve noticed convection occurring: hot material rises to thesurface, cools as it moves along the surface, and then descends back into theliquid. Convection is based on the principles that materials tend to expand andbecome less dense as they are heated, and that less dense matter rises as higherdensity matter sinks.

On Earth, rocks near the surface that are exposed to the low temperaturesof the atmosphere are cool enough to be brittle. These rocks break when theyare stressed. Deeper in the Earth, where temperatures and pressures are larger,rocks are ductile and plastic. These rocks flow when they are stressed. Thatrocks flow surprises most people. Thinking about ice may help you extend yourintuition. If you drop an ice cube on cement it will break, but if you put iceunder pressure, like that inside a glacier caused by the weight of overlying ice,it will flow. Under the right conditions all “solid” materials flow, includingEarth’s deep rocks.

Convergent boundary:A region where two tectonicplates collide.Divergent boundary:A region where two tectonicplates are separating.Transform boundary:A region where two tectonicplates slide past one another.

The Earth contains three layers of material, each made of a different kindof rock: the crust which is made of relatively low density rock, the mantlemade of more dense rock, and the dense iron core (Figure 4.11). There are twokinds of crust: oceanic and continental. Oceanic crust is somewhat more densethan continental crust. This is why continents “ride higher” on the mantlethan oceanic crust. The Earth’s crust is broken into slabs called tectonic plates(Figure 4.12), which are moved on Earth’s surface by the convective flow ofthe underlying mantle (Figure 4.11). Tectonic plates move around quite slowly:about as fast as your fingernails grow (up to about 10 cm per year). Themovement and interactions of these plates and the convective movements of theunderlying mantle is called plate tectonics.

Plates interact in one of three ways: 1) they collide at convergent bound-aries (i.e. the Himalayas), 2) they move away from each other at divergentboundaries (i.e. Iceland), or 3) they slide past one another at transform

4.3. PLATE TECTONICS 117

Figure 4.11: A simplified representation of the interior of the Earth. (Imagecourtesy of BYU-Idaho)

Figure 4.12: Earth’s major tectonic plates. (Image courtesy of U.S. GeologicalSurvey)

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boundaries (i.e. the San Andreas fault). These boundaries are illustrated inFigure 4.13. Most of the tectonic change on Earth occurs near plate bound-aries, which is why most earthquakes and volcanoes are located within about ahundred kilometers of a plate boundary (see Figure 4.14).

Figure 4.13: An illustration of the types of tectonic boundaries. (Image courtesyof U.S. Geological Survey)

Figure 4.14: Epicenter locations of world earthquakes, 1963-1998. Note thatmost of the earthquakes occur near plate boundaries, as seen in Figure 4.12.(Image courtesy of NASA)

Oceanic crust is created at divergent boundaries and is recycled into Earth’sinterior at convergent boundaries. Continental crust is not dense enough to berecycled: once created, it stays on the surface. When two plates collide, amountain belt can form on the overriding plate as the crust thickens. Newcontinental crust is generated in mountain belts by the welding on of oceansediments and crust and by the intrusion of low-density magma into the crust.Over time, erosion wears down mountain belts and the crust thins. In the end,flat areas like our continent’s interior remain. Every place on every continentwas once part of a mountain belt, but only the most recently formed mountainshave not yet been eroded away!

When two continents reach each other at a convergent boundary, they be-come welded together, as is happening in India and Eurasia today. Sometimesa new divergent margin will form inside a continent and split it into severalpieces, as is happening in East Africa today. Periodically, continental massescollide and a supercontinent is formed. When a supercontinent breaks up andits constituent continents move away from each other, new ocean basins are cre-

4.3. PLATE TECTONICS 119

ated and the formation of the next supercontinent is initiated - for, on Earth’ssphere, when continents move away from each other, they are also moving to-wards each other. The process of breaking up and reassembling a supercontinenttakes several hundred million years.

So why does any of this matter? Is it even important for us to under-stand these processes if they happen so slowly relative to a human lifetime?It turns out that understanding how the Earth changes does provide benefits.Understanding how Earth works enables society to locate and extract the nat-ural resources (e.g., fuel and metals) necessary for the functioning of modern Benefits of understanding

tectonicscivilization, minimize the negative effects of Earth’s dynamic processes (e.g.,earthquakes and volcanic eruptions) on mankind, minimize mankind’s effectson Earth (e.g., pollution), and satisfy man’s thirst for knowledge.

REVIEW QUESTIONS

1. List three ways in which the Earth has changed over time.

2. Are the processes responsible for these changes still active today?Explain.

3. Explain what convection is and how it works.

4. What are the “layers” of the Earth, and what are they made of?

5. Explain how the theory of plate tectonics describes many of theobservations we make relative to the Earth.

6. Why did it take so long for mankind to discover plate tectonics?

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4.4 Earth Changes!

OVERVIEW

Summary: Over its history, the Earth has seen a wide range of changesrelative to its geography, climate, sea level, and biodiversity.

Learning Objectives:

• Understand that our earth is constantly changing, and identify howhistorical conditions (climatic, geographic, and atmospheric) differfrom current conditions.

• Identify the data and observations used to discern Earth conditionsin the past.

• Explain the difference between climate and weather and identify howvarious factors (volcanic activity, atmospheric composition, etc.) canalter climate.

• Identify the natural processes that can alter global sea level.

• Explain the trends in the fossil record over geologic time.

• Identify the basic types of life that lived during the four geologic erasthat span our earths existence.

• Explain what mass extinctions are and how they are related to theperiods of the geologic time scale.

Vocabulary:

• Climate

• Precambrian era

• Paleozoic era

• Mesozoic era

• Cenozoic era

Earth Change

It is often difficult to understand how truly active Earth is, because the Earthchanges slowly, and changes that accumulate during one human lifetime areusually imperceptible. However, as we view the Earth in “Deep Time” wecan see how the accumulation of small-scale changes can produce large, visibleeffects, and cause global-level changes in our planet’s climate, sea level, life,

Climate:The conditions found in agiven region, consisting ofmany factors such as atmo-spheric composition, temper-ature, tectonic activity, solarintensity, etc....

and geology.It surprises most people, for example, to discover that there were long pe-

riods in Earth’s past when environmental conditions were extremely differentthan they are today. During Earth’s early existence there were no continentsor oceans, and there was no free oxygen in the atmosphere. In fact, the earlyatmosphere was so different from the atmosphere of today that from space theplanet did not appear blue, and the surface of the planet could not be seen.As for life, bacteria dominated the majority of the history of life on earth,multicellular life appeared about 1.5 billion years ago, and animal life is foundin the fossil record starting only 600 million years ago. Global temperaturesalso changed greatly throughout Earth’s history, shifting from periods of warmtemperatures to ice ages and back again, and sea levels rose and fell as globaltemperatures swung back and forth. Geologically, there were times when all ofthe continents were connected to each other in large supercontinents, and othertimes when individual continents were largely isolated from each other by seas

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Factor Explanation of Influence

Solar and magnetic variations Variation in Sun’s heat and shielding of thisheat (affects amount of heat Earth receives).

Albedo Determines the amount of solar heat reflectedor retained.

Orbital eccentricity Variations in the circularity of Earth’s orbit(affects distance from the Sun).

Earth’s tilt Variations in the magnitude of the tilt(affects the severity of Earth’s seasons).

Earth’s wobble (precession) Variations in the orientation of Earth’s tilt(affects the severity of Earth’s seasons).

Water vapor Most abundant greenhouse gas. Affects clouds,albedo (reflectivity), precipitation, andvegetation.

Carbon dioxide Captures infrared heat radiated from Earth’ssurface.

Methane Captures infrared heat radiated from Earth’ssurface.

Ocean currents Transports and distributes heat around theglobe.

Continental distribution Changes ocean currents and atmosphericcirculation.

Elevation of continents Promotes glaciation and affects atmosphericcirculation.

Volcanism Source of carbon dioxide, sulfate particles,and short term ash.

Plate tectonics Affect volcanism, carbon dioxide, sulfates,mountain building, and continentaldistribution.

Chemical weathering Affects carbon dioxide removal.Meteor impacts Causes short term ejecta in the atmosphere.

Table 4.2: Factors affecting global climate.

and oceans. Early in Earth’s history large and small meteorites travelling fasterthan the fastest missiles regularly slammed into our planet, producing local andglobal effects. This list of Earth changes could go on and on, but suffice it saythat ours is an ever changing, dynamic planet!

All forms of change on Earth are interrelated. For example, the buildingof a coastal mountain range will cause other effects. The climate becomeswetter upwind of the mountain range and dryer downwind, the old shoreline israised and, therefore, local sea level drops. In addition, formerly interbreedingpopulations of organisms that cannot traverse mountain ranges become isolatedfrom each other, and these isolated populations which are now subjected to newenvironmental conditions evolve separately and may give rise to new species.

Climate

Climate systems are regulated by a complex set of factors, some of which arelisted in Table 4.2. Understanding each of these processes, the magnitude oftheir effects on climate, their interactions and feedbacks, and the timescalesover which they operate is an enormous scientific undertaking. Fortunately,much effort is being invested globally in furthering our understanding of climatesystems (e.g., NOAA, EPA, IPCC). To help you develop your intuition abouthow these processes affect climate, we will briefly explore two of these processes:1) atmospheric composition, and 2) volcanic activity.

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Atmospheric Composition

The abundance of atmospheric greenhouse gases like water vapor and carbondioxide determine the rate at which solar energy that is converted to heatradiates back out into space. Greenhouse gases act like the windows in yourGreenhouse effectcar: light passes easily through the glass, but once the light is absorbed bysurfaces and materials in the car and is then released as heat it cannot passback through the glass as easily as it entered. As a result the inside of your carheats up. Increasing the amount of greenhouse gases in the atmosphere similarlyslows the release of heat back into space, and the average temperature of theEarth increases. That the Earth is warming is now a confirmed scientific fact,but the topic of what we should do about global climate change is the subjectof much political debate. We can only hope that governments and individualswill realize that no matter what opinion they may have on this topic, there isan overwhelming body of scientific observations that show that the release ofgreenhouse gases, mainly from burning fossil fuels, is having a significant effecton global climate as was discussed in reading 1.5.

Volcanic Activity

Large volcanic eruptions affect climate in two ways. Initially an eruption pro-duces a cooling effect, and then, later, produces a warming effect. Sulfur,volcanic ash, and carbon dioxide are emitted into the upper atmosphere duringan eruption and drive these effects. Cooling is caused when solar radiation isreflected off of sulfuric acid droplets and volcanic ash in the upper atmosphereand goes directly back into space without being transformed into heat. Afterseveral years, natural processes remove the sulfuric acid and ash from the at-mosphere and the greenhouse effect of carbon dioxide from the eruption causeswarming. Clearly, the larger a volcanic eruption is, the bigger its effect will beon climate. For example, the eruption of Mt. St. Helens in 1980 had no notice-able effect on the global climate. However, the major eruption of Mt. Tambora(Indonesia) in 1815, the largest volcanic eruption in recorded human history,caused the global climate to cool significantly due to the reflection of the so-lar heat off of emitted sulfur compounds and ash. Incidentally, the restorationof the gospel and subsequent establishment of the Church of Jesus Christ ofLatter-Day Saints was significantly affected by the eruption of Tambora. 1815is referred to as the “year without a summer”, which caused the crops to fail onJoseph Smith Sr.’s rented farm in Vermont. With no hope of financial recovery,the Smiths were compelled to settle accounts and relocate to Palmyra, NewYork.

Sea Level and Changing Environments

Oceans currently cover about 70% of the Earth’s surface, and these oceans havean average depth of 3,700 meters (12,000 ft). Our perception and intuition tellsus that the oceans are huge, but when we consider the combined size and volumeof the oceans in comparison to the size and volume of the Earth, they are quitesmall. For example, if the Earth were the size of a basketball it would takeonly 8.5 mL (milliliters) of water to fill all ocean basins. That’s less than twoteaspoons of water (1 tsp = 5 ml)! Imagine how thin that layer of water wouldbe if you spread that small amount of water over 70% of the surface of thebasketball!

The amount of water on the planet is essentially constant, but, interestingly,sea level is not. Sea level rises or falls when either the size of ocean basins changeor the amount of water in those basins changes. The size of Earth’s ocean basinsis determined by activity at divergent boundaries. When seafloor spreading isfast, the average temperature at mid-oceanic ridges is higher than average, andthe temperature of the crust increases. Hot rocks are less dense, and therefore

4.4. EARTH CHANGES! 123

ride higher on the mantle. When this happens mid-oceanic ridges get largerand displace the water above them, causing oceans to flood continents (i.e. sealevel rise). Sea levels can also rise or fall in a particular area depending onwhether local tectonic events are causing shorelines to be uplifted or lowered.

The amount of water in ocean basins is also affected by global tempera-ture. During periods of global cooling ice ages occur, and water is bound up incontinental glaciers. When this happens the amount of water in ocean basinsdecreases and sea levels drop.

Changes in sea level, tectonic activity, and global climate cause local envi-ronmental changes. Imagine a coastal area with a sandy beach and a swampyarea just inland from the coast. When sea levels rise or tectonic activity causesthe area to subside, sand and materials from the beach and swamp will beswept inland and remaining beach and swamp materials will be covered by sea-water, and will eventually be completely covered by ocean sediments. Volcanicactivity in the area could then cause these existing environments to be coveredby volcanic materials. When global climate change causes the area to receiveless precipitation, the swamp would dry out and become coastal grassland, andthe beach dunes become a dune field. In this way, one environment replacesanother. We know this because rock layers record the history of environments Information about the envi-

ronment is recorded in rocksthat existed in a particular area, and collectively, the history of the Earth.Interestingly, these same rock layers also record the history of life on Earth.

Life

Life has changed markedly over the history of the Earth. The history of lifeis recorded as the fossil record found in sedimentary rocks. Organisms thatare buried before they decay completely can form fossils. Because these or-ganisms are covered and preserved only by sediment, fossils are found only insedimentary rocks.

The oldest sedimentary rocks do not contain fossils. The oldest fossil-containing rocks bear fossils of single-celled organisms (bacteria). Slightlyyounger rocks contain fossils of more anatomically complex single-celled organ-isms (eukaryotes cells with a nucleus), and even younger rocks contain fossilsof animals and plants. All fossil series show a consistent pattern of change fromfossils in older rock layers that look less like living species to fossils in recentrock layers that look more like living species. Any idea that correctly explainsthe history of life on Earth must also explain this set of observations. Thetheory of evolution elegantly explains this and many other observations. Sincelife change (evolution) is the main topic of the next chapter, it is not discussedin greater detail here.

Geologic History of North America

The history you are about to read is a brief version of the events that formedNorth America. You need to be aware that this brief history of North Americais to the geologic history of the Earth as one raindrop is to the Pacific Ocean.Still, it is worth knowing, because it is a case study that shows how continentsare formed and how the Earth works. It will also build your intuition about thechanges Earth has experienced. Earth’s past is recorded in the rocks aroundand underneath us. For those with a willingness to learn, and eyes to see, rockstell stories: they record the events and processes that led to their formation.

This narrative is organized using the Precambrian, Paleozoic, Mesozoic, andCenozoic eras of the geologic timescale. To help you better understand the scaleof “Deep Time”, this reading includes an analogy in which the 4.55 billion yearold age of the Earth is compared to one hour, also known as a “geo-hour”(see Figure 4.15). In this “geo-hour” analogy, one minute equals about 76million years, and the average life span of a human (76 years) is equivalent to

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60 microseconds. For example, the large meteorite impact that contributed tothe extinction of dinosaurs 65 million years ago (Ma) occurred 59 min and 9sec into the “geo-hour” history of Earth, or just 51 “geo-hour” seconds ago.

The history of Earth begins about 4.55 billion years ago (00:00 on our clock),which is by definition the beginning of the Precambrian era. Hereafter, the“times” given in parentheses will refer to the “geo-hour”.

Figure 4.15: The “geo-hour”: 4.55 billion years condensed into one hour. (Imagecourtesy of BYU-Idaho)

Precambrian

Over 87% of our planet’s history takes place in the Precambrian era. ThePrecambrian era:The age of early Earth. Precambrian began with the formation of the planet and ended with the ap-

pearance of fossils of organisms that produced hard parts, such as shells andexoskeletons. We often refer to the Precambrian as the “age of bacteria” sincebacteria were the only organisms in existence for the majority of this era.

The Precambrian is a strange period in Earth’s history compared to con-ditions on Earth today. The Earth came into existence when the solar systemwas formed, and is made of material that was produced during the big bangand during stellar nucleosynthesis of many generations of stars that had gonesupernova. When Earth was newly formed it was made of molten rock and didnot have continents or oceans. Eventually the lowest density materials rose tothe Earth’s surface, cooled, and formed rocks of the crust. At the same time,rocks of intermediate density formed the mantle, and the densest materials,mainly iron, formed the planetary core. Gases released from molten magmathat covered the Earth’s surface and released by volcanic activity produced theearly atmosphere, which was a combination of noxious gases and did not containany free oxygen. About 4,527 Ma (00:18.2 into our “geo-hour”) a Mars-sizedplanet collided with Earth and ejected molten material into space that becameour moon.

By 3,800 Ma (09:53.4) the Earth cooled sufficiently for water to condenseand an ancient ocean surrounded small continental masses of crust. There isconclusive evidence showing that some rocks formed on the shores of these earlycontinents were formed by bacteria. The oldest of these fossils are found in rocks3,800 to 3,500 Ma (09:53.4 to 13:50.8). At least some of these bacteria producedoxygen as a waste product of their biological processes. Oxygen, a highly re-active gas, was poisonous to most of the early organisms that evolved underthe conditions of an oxygen-free atmosphere, yet increasing concentrations ofoxygen in the early atmosphere eventually played a role in the evolution of or-ganisms that require oxygen to live. Anyway, some of these early continentalmasses formed the core of the modern North American continent.

4.4. EARTH CHANGES! 125

Figure 4.16: An artist’s depiction of the Earth’s surface during the early pre-cambrian era. (Image courtesy of NASA)

North America’s ancient core formed by the collision of several small con-tinental fragments between 2,100 and 1,800 Ma (between 32:18.5 and 36:15.8).By this time oxygen was present in the atmosphere in small quantities. Mostof the southern part of North America was added between 1,800 and 1,600 Ma(between 36:15.8 and 38:54.1) as island arcs collided with the North Ameri-can ancient core. These collisions formed the ancient Yavapai and Mazatzalmountain belts. About this same time the oldest fossils of eukaroytic organ-isms (organisms made of cells that contain a nucleus) begin to appear in rocksbetween 1,900 and 1,500 Ma old (between 34:23.7 and 40:54.1).

North America became part of Rodinia, the oldest known supercontinentbetween 1,200 and 1,000 Ma (between 44:10.5 and 46:48.8). The collision of thecontinents that made up Rodinia created the massive Grenville mountain beltand added material to the continent. Rodinia broke up about 780 Ma (49:42.9).Just before Rodinia broke up Earth entered the first global ice age for whichwe have solid evidence. This ice age was Earth’s largest and longest, and someevidence suggests that the entire planet may have been covered by ice duringportions of this ice age: a phenomenon sometimes referred to as “SnowballEarth.”

The first fossils of complex multi-cellular animals appear in rocks 610 Maold (51:57.4), which was shortly after the end of this large ice age. Fossils ofthese early animals are not abundant because they were composed only of softtissues; however, fossils of other animals that produced hard parts, such asshells and exoskeletons, appear in rocks starting about 542 Ma ago (52:51.2),signaling the end of the Precambrian and the beginning of the Paleozoic era.

The Paleozoic Era

The Paleozoic, or age of past life, began about 542 Ma and ended 251 Paleozoic era:The age of ancient lifeMa (52:51.2 and 56:41.4). The early part of this era is sometimes called the

“age of invertebrates”, and when other kinds of animals appeared the “age ofinvertebrates” was replaced by the “age of fishes”, then the “age of amphibians”,and finally the “age of reptiles.” The Paleozoic era ended with the largest knownmass extinction to occur on Earth.

There were four major sea-level cycles during the Paleozoic. During thePaleozoic most of North America was relatively level except for a small higherregion that existed in the center of the continent. Due to these level regions,periods of continental flooding caused extensive marine sediments, (i.e. sand-stone, shale, and limestone) to be deposited over most of the continent. One

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of the causes of these sea-level cycles during the Paleozoic was significant fluc-tuation in global climate. During this time, the climate shifted from a warmperiod to an ice age, to another warm period to another ice age, and endedwarm.

Throughout the Paleozoic, North America “laid on its side” with its currentwest coast directed north. During this era, multiple small plates and island arcswere added onto North America’s southern, eastern, and western coasts. To-day’s Appalachian mountain belt resulted from three separate mountain build-ing events that deformed and added material to the east coast. The first twosets of ancient mountains, the Taconic and Acadian mountain belts, were builtand eroded during the early and middle Paleozoic. The third mountain belt, theancestral Appalachians, was much taller and wider than the modern Appalachi-ans, extended along North America’s east and south coasts, and resulted fromcollisions between North America, Europe, Africa, and South America whenPangaea was formed.

Erosion and mountain building events occurred along North America’s westcoast during the late Paleozoic. These events formed three ancient mountainranges (Antler, Sonoman, and the Ancestral Rocky Mountains) and basins,which extended to Nevada and Idaho and through Oklahoma.

In a geologically short time, fossils of most of the major animal groups ap-peared and became diversified during the early Paleozoic. This rapid appear-ance and radiation of animal life is identified as “The Cambrian Explosion”.The majority of the fossils in the Paleozoic are of marine origin; however, fos-sils of land plants and animals (amphibians) appeared by the middle Paleozoic,460 Ma and 380 Ma, respectively (53:56 and 55:22).

A distinctive suite of fossils characterizes each of the periods of the Paleozoicera. For example, there was the age of trilobites (the ancestors of today’s horse-shoe crabs) and the age of sea scorpions (eurypterids). In one of the periods inthe Paleozoic, we find fossils of very large insects (e.g., foot-long cockroachesand dragonflies with two-foot wingspans). The first fossils of reptiles are found315 Ma, near the end of the Paleozoic (55:42), and the extensive coal depositsof the East were formed about 300 Ma. There were three mass extinctions thattook place during the Paleozoic, and the last one, the Permian extinction, wasthe largest extinction Earth has ever experienced. The fossil record shows thatduring the Permian extinction 95% of all fossil-forming marine species and 70%of all terrestrial vertebrate species died off. This cataclysmic event closed outthe Paleozoic era and ushered in the Mesozoic era.

The Mesozoic Era

The Mesozoic, or age of middle life, began 251 Ma and ended 65 Ma (56:41.4Mesozoic era:The age of middle life. and 59:08.6). This era began with the Permian extinction and ended with

another extinction event that saw the end of the dinosaurs, except for one smallgroup of specialized dinosaurs: birds3. Due to the predominance of dinosaursduring the Mesozoic, this era referred to as the “age of the dinosaurs”.

There was one major sea level cycle during the Mesozoic. Sea level reachedan all-time high during the middle of this era, creating a shallow sea extendingall the way across the middle of North American from the Arctic Ocean to theGulf of Mexico. Many of the sedimentary rocks of this period were deposited intidal flats, river systems, Sahara-like deserts, alluvial fans, coastal swamplands,and inland seas. The western coal and oil fields also formed during this era. TheMesozoic climates were warm, save for a brief period of cooling about halfwaythrough the era, but there were no ice ages.

Pangea began to break up at the beginning of the Mesozoic. As this hap-pened North America moved from its former location near the equator to itspresent position and orientation. Rift basins formed between North America,

3This means that we still share the planet with dinosaurs. Cool, huh!?

4.4. EARTH CHANGES! 127

Europe, and Africa and formation of new oceanic crust pushed these continentsapart, forming the Atlantic Ocean and the Gulf of Mexico. The west coast ofNorth America was also tectonically active during this time. As North Amer-ica moved west and north it began to override the oceanic crust of the PacificOcean basin. The convergent boundary between the Pacific Ocean basin plateand the continental plate of North America created the Sierra Nevada and Idahobatholiths - the igneous rock that contributed to the mountains in these loca-tions. At about this same time micro-continents from the Pacific plate collidedwith western North America, adding the Cordillerian portion of North America,and completing the formation of the western part of this continent. By the endof the Mesozoic, North America contained essentially all of the material is hastoday.

The fossil record indicates that dinosaurs appeared early in the Mesozoic andbecame increasingly abundant and diverse throughout this era. The first fossilsof flowering plants, mammals, and early, feathered, bird-like dinosaurs are foundin the middle of this era, between 220 and 155 Ma (between 57:05.9 and 57:57.4).There were two mass extinctions during the Mesozoic: one near the beginningand one near the end of the era. The end-Mesozoic extinction is famous for thedemise of dinosaurs (other than birds)4. This end-mass extinction was causedat least in part by a collision with Earth of a 10 km diameter asteroid. Thisasteroid’s impact site is 200 km in diameter and is located on the Yucatanpeninsula, which was at the time covered by a shallow sea. The impact eventwas so intense that large impact debris landed as far away as Utah and smallerejecta (i.e. dust) covered the globe! This mass extinction marked the end ofthe Mesozoic era and the beginning of the Cenozoic era.

Figure 4.17: A depiction of Mesozoic life, specifically from the Jurassic period,by Samuel Wendell Williston. (Image is in the public domain.)

The Cenozoic Era

The Cenozoic era, or age of recent life, started 65 Ma (59:09). Even though Cenozoic era:The age of recent life.this is the age of recent life, the early Cenozoic world still looked significantly

different than the world of today. In fact, you wouldn’t start to feel at home4Results of research published in mid-2010 states that birds with modern features didn’t

evolve from a line of dinosaurs that survived the extinction event at the end of the Mesozoic,as we once thought. Rather, birds with modern anatomies actually co-existed with dinosaursfor the last 5-10 million years of the Mesozoic, and these birds are almost certainly the onlydinosaurs that survived to modern times.

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until after the last ice age, about 12,000 to 10,000 years ago (9.5 7.9 millisecondsleft of the “geo-hour”)!

One significant flood and retreat cycle of the sea has occurred so far duringthe Cenozoic. Sedimentary rocks of this era were predominantly deposited innon-marine environments, e.g., in stream, lake, glacial, and alluvial fan systems.In the early and middle Cenozoic, there were large inland lakes in northeasternUtah and southwestern Wyoming. Today, fossil fish, famous the world overfor fantastic preservation, are quarried from the rocks produced in these lakes,such as those now found at Fossil Butte National Monument in southwesternWyoming. The warm climatic conditions that characterized the Mesozoic per-sisted into the early Cenozoic, but by the middle of the era global temperaturesbegan to cool. This cooling continued and Earth entered an ice age around 2.6Ma (59:57.9). Continental glaciers advanced and retreated many times duringthis ice age. Today we live in the most recent interglacial period of that iceage. Present sea level is about 100 meters (about 300 feet) higher than it wasWe are presently in an ice

age during the last glacial period. Even so, today’s sea level is low compared towhat it was throughout much of Earth’s history. The landscape that surroundsus today is largely of glacial origin. One such landform that is important toLatter-Day Saints is the Hill Cumorah, a sedimentary hill that was formed bythe North American ice sheet during glacial periods of the Cenozoic.

During glacial periods, large inland lakes also formed in many localities inthe west. One of the largest of these was Lake Bonneville. It covered most ofwestern Utah and extended into eastern Nevada. The ancient shorelines andother landforms associated with lakes can easily be seen as you drive on In-terstate highway 15 along the Wasatch Front mountain range in Utah today.The campuses of Utah State University, BYU-Provo, and Weber State Univer-sity are all built on deltas that were deposited into Lake Bonneville, and arenow referred to as benches above the valley floor that was once the bed of thisimmense lake.

Early in the Cenozoic a series of upthrust tectonic events formed the modernRocky Mountains, and the Teton Range east of Rexburg was produced byone of the most recent of these events. At this time, the Rocky Mountainswould have looked much like the central Andes Mountains of today, with jaggedpeaks and steep rocky faces that have undergone little erosion. Following thismountain building event, extensive volcanic and magma activity occurred fromthe Sierra Nevada Mountains, California, to Denver, Colorado. Most of themetallic ore deposits of the west formed as a result of this activity. Aboutthis time, the North American plate overran the Pacific plate divergent zoneand formed the San Andreas Fault. This fault connects the spreading ridgesand subduction zones west of Oregon to those running through the Gulf ofCalifornia. The interaction between the ridge system and the continent caused adramatic change: tectonic activity transitioned from compression to extension.This extension caused fault-bounded mountains and their associated valleysto form by accommodating the stretching of western North America. Thisextensional region is called the Basin & Range province and formed the centralvalley of California where cities including Sacramento and Bakersfield are nowlocated. In addition, during this time, many western rivers started to cut downto form the Grand Canyon and much of the topography of southern Utah. Themagmatic activity that formed the Columbia Plateau, Snake River Plain, andYellowstone also began at about this same time.

During the Cenozoic, the fossil record shows that mammals and birds di-versified and became extremely abundant, with mammals filling the majorityof large animal niches left empty by the extinction of the dinosaurs. A few ofthe most famous Cenozoic species are the wooly mammoth, saber tooth cat,giant ground sloth, and Neanderthals. These species all went extinct before ourday. One of the latest additions to the fossil record is the modern human, whosefossils appeared between 130,000 and 190,000 years ago (about 100 milliseconds

4.4. EARTH CHANGES! 129

prior to the end of the “geo-hour”).

Summary

To help you envision the magnitude of change that has occurred just abouteverywhere on Earth, imagine yourself standing outdoors in a place you knowwell. Look around. Take note of the lay of the land. Now think about thedifferent geologic and environmental features that exist on Earth: mountains,plains, shorelines, swamps, lakes, oceans, forests, deserts, glaciers, etc. Nowconsider this: essentially every kind of geologic feature and environmental con-dition has existed at least once where you are standing, no matter where youare. How do we know this? The rocks around and beneath you contain a recordof those environments, and confirm this conclusion.

With that insight in mind, ask yourself this question, “What are normalconditions for the Earth?” The answer to this question surprises most people.The answer is that there is no “normal condition”: the Earth is constantly un-dergoing change, including change in temperature, sea level, biological diversity,environmental conditions, and just about any other way you can imagine. Suchis the nature of our dynamic planet. The Earth of today is the recent versionof our planet, and is the result of a long and complex past, but a past we havebeen able to discover through careful investigation and observation.

REVIEW QUESTIONS

1. What is climate? What sort of processes can affect climate?

2. What processes can affect sea level?

3. Describe what the Earth and/or North America was like during eachof the four eras described in the reading.

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Chapter 5

Life

Figure 5.1: A group of unicellular cyanobacteria from a microbial mat in Guerrero Negro, Baja California,Mexico. Bacteria are the most adaptive, prolific, and populous form of life on the Earth. Cyanobacteria, suchas these, are thought to be the first photosynthesizing organisms on the planet. It was not until these bacteriareleased oxygen into the atmosphere that more complex aerobic organisms, such as ourselves, could develop andsurvive. (Image courtesy of NASA)

In this final chapter, we will discover what science has taught us about life on our planet. Living things areunique in the universe, in that they have the ability to propagate their kind. They manage to do this throughthe replication of a genetic blueprint, DNA. During sexual reproduction, the DNA of organism is mixed in arandom way with the DNA of another organism from the same species. When you throw mutations into the mixas well, you end up with species changing (at least a little) from generation to generation. Understanding howthese changes happen had allowed science to begin addressing several important questions, including “Why islife on Earth so diverse, and yet so similar?” and “How did our species originate?”

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5.1 Observations of Life

OVERVIEW

Summary: The fossil record indicates that life showed up relativelyearly in Earth’s history, and that over the course of billions of years hasevolved and adapted to the ever-changing climatic conditions. In thisreading we will consider many of the traits shared by all forms of life, andlook specifically at a few case studies involving specific species.

Learning Objectives:

• Define what evolution is and what evolution is not.

• List and describe several traits shared by all forms of life.

• Describe at least one set of observations that shows that life hasundergone evolutionary change.

• Explain what biodiversity is.

Vocabulary:

• Biodiversity • Evolution

In the late summer of 1940, just a few months after the German armyinvaded France during WWII, four French teenagers and their dog ventured intocave about 250 miles south of Paris. What they discovered was the astonishingLascaux cave complex where more than 2000 ancient paintings cover the wallsand ceilings (Figure 5.2). These cave paintings include depictions of bison, deer,and a variety of other large animals. After World War II, scientists studiedthese caves and concluded that the paintings are from the Paleolithic timeperiod, meaning that they are over 17,000 years old. This discovery shows thathumans have been keen observers of the diversity of life for a long, long time,and certainly much longer than even these paintings suggest.

In case you are curious, you can click on the link below if you want to see astreaming video tour of the Lascaux cave complex which is now closed to thepublic: http://www.lascaux.culture.fr/index.php?lng=en#/fr/02 00.xml.

Figure 5.2: An example of the Paleolithic paintings from the Lascaux caves,France. (Image courtesy of Wikimedia commons.)

Our interest in the diversity of life remains with us today, as demonstratedby the success of nature programming on television, the vast number of outdoorsmagazines, and ongoing efforts of scientists to identify, describe, and protect

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the diversity of life. Perhaps our real fascination with the diversity of life restsin a few fundamental questions, such as: What does it mean to be alive? Whattraits do all living things share? How many kinds of living things are there?And, where did the diversity of life come from? In this chapter we will delveinto a few of the things we have learned about these questions, focusing mainlyon the last question listed above.

What does it mean to be alive? This can be a tricky question, becausethe only things that scientists can study are evidences of life. Some of themore obvious evidences that you are alive include the observations that youare breathing, your heart is beating, your cells are consuming energy, and thatyou respond to your environment. The essence of life itself, however, remainselusive to scientific investigation. Lets therefore turn our attention to traits ofliving things that we can investigate scientifically. Traits of Life

What traits do all living things share? Some of the most prominent traitsshared by all living things are listed in Table 5.1. In summary, all living thingsare made of the same kinds of organic materials, are composed of cells, andcarry out metabolism and the other processes listed.

Trait DescriptionBiological molecules All living things are made of proteins,

lipids, carbohydrates, and nucleic acids.

Cells Cells are the fundamental anatomical unitof life, and all living things are made ofcells.

Genetic material Living things store their geneticinformation in DNA which they can copyand pass to their offspring.

Universal genetic code All living things use the exact same systemto code for the genetic information intheir DNA and to decode that informationas they make proteins.

Metabolism All living things take in nutrients andenergy from the environment and usebiochemical reactions to transform theminto energy and materials they can use.

Growth All living things grow by increasing insize or by making identical copies ofthemselves.

Reproduction All living things pass their geneticmaterial to the next generation of life.

Interaction with the environment All living things have an ability tocollect information from the environmentand respond to those signals.

Evolution Populations of all living things exhibitgenetic change from one generation tothe next.

Table 5.1: Traits shared by all living things.Number of Species

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How many kinds of living things are there? Biologists have been workingfor centuries to identify and describe as many species as possible. Even thoughmany biologists spend their entire careers doing this, we still do not know forsure how many different species there are. So far about two million specieshave been scientifically described and named, but estimates of the total bio-logical diversity on Earth range from about three million to as many as 100million living species. This range of estimates is extremely wide because wehave systematically sampled only small parts of any ecosystem. No one knows,for example, how many kinds of bacteria, microscopic roundworm species, etc.,live in places we have not studied intensely, such as soil communities or life atthe bottom of the ocean. Regardless of our lack of precision on this question,there are certainly millions of species on the planet.

Scientists also work to arrange the known diversity of life into a frameworkwe can use to keep track of it. One early, largely successful effort to organize allliving things in a way that reflects their degree of relatedness was achieved inthe late 1700’s by Carolus Linnaeus, a Swedish naturalist. Linnaeus inventedthe binomial system of nomenclature to name every kind of living thing. Hismethod of naming things includes giving each kind of organism a genus and aspecies name. He also assigned living things to families, classes, phyla, and soforth (see Figure ??). According to this pattern the scientific name of humansis Homo sapiens. The genus name Homo means “human”, and the speciesname sapiens means “wise”. A scientist named Carl Woese made a more recentcontribution to the effort of organizing species in the 1970’s.

Figure 5.3: The structure of Linnaeus’ taxonomic hierarchy. (Image courtesyof Wikimedia Commons)

Woese assigned all living things to one of three domains of life (Figure 5.4).Two of these domains, the Bacteria and the Archaea, include only organismsthat are prokaryotes. Prokaryotes exist as individual cells or as clusters orchains of cells, and each cell lacks a nucleus and membrane-bound organelles(Figure 5.5). Most of the species people are generally familiar with, however,are included in the other domain, the Eukarya. These organisms have eukary-otic cells with DNA housed in a nuclear membrane, and their cells containmembrane-bound organelles such as mitochondria, chloroplasts, etc. (Figure5.6). The domain Eukarya includes all animals, plants, algae, fungi, proto-zoans, and related groups.

All members of these three domains share all of the traits of life listed inTable 5.1. How can this be, and where did the diversity of life come from? Thecurrent scientific paradigm of the origin of the diversity of life states that alllife on Earth is derived from a common ancestor. Figure 5.7 shows one versionof the evolutionary genealogical history of life on earth.

The bottom line is that all existing species developed from earlier speciesby a variety of well-understood processes collectively referred to as evolution.

Every population of living things undergoes evolution, and this is evidencedEvolution:The change and developmentof species over time. by the fact that every population undergoes small shifts in its genetic make-

up from one generation to the next. Mechanisms causing these population-level genetic changes include the following: (1) pressures from the environmentthat favor one genetic option and select against another; (2) random geneticmutations; and (3) even random meetings of gametes (eggs and sperm) thatresult in greater or lesser than predicted frequencies of certain genes especially

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Figure 5.4: The three domains of life as proposed by Woese. Domains Bacteriaand Archaea contain only species with prokaryotic cells. Domain Eukarya con-tains species whose cells have a nucleus, and includes the kinds of organismspeople are most familiar with. (Image courtesy of Wikimedia Commons)

Figure 5.5: A prokaryotic cell. (Image courtesy of Wikimedia Commons)

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Figure 5.6: A eukaryotic cell. Both prokaryotic and eukaryotic cells have DNAand ribosomes that they use to make proteins, but only eukaryotic cells con-tain a nucleus and membrane-bound organelles. (Image courtesy of WikimediaCommons)

in small populations.It is, frankly, perplexing that many people frame their thoughts about evo-

lution in terms of whether they “believe” or “dont believe” in evolution. Youmay as well ask someone if they “believe” in atoms, if the Earth goes aroundthe Sun, or if the force of gravity is still in effect. That populations evolve isnot a matter of opinion: it’s a matter of observational fact. If you collect theright kinds of data, it’s obvious that populations evolve. Asking someone ifthey understand evolution, however, is a completely different matter. The goalof this section of the course is to provide you with an introduction to what evo-lution is, how science came to understand evolution, and to provide you withthe opportunity to learn about some specific examples of evolution.

In order to complete this reading assignment, study one of the two followingcase studies: evolution of elephants or of whales. You are, of course, welcomeand encouraged to look at both, but you are required to look study one indetail. You should become familiar enough with the case study you chose so thatyou can help someone else understand that particular episode in evolutionaryhistory.

Evolution case study #1: Elephants

The mass extinction event at the end of the Mesozoic Era about 65 millionyears ago saw the demise of the dinosaurs, but mammals, birds, and many otheranimals survived. Mammals, which were mainly small, unspecialized animalsthroughout most of the age of dinosaurs subsequently specialized and filled themajority of niches formerly populated by dinosaurs. One group of mammalsgave rise to modern elephants, the largest living land animals on the planettoday.

In the summer of 2009 researchers announced the discovery of the fossil re-mains of a small mammal from about 60 million years ago that has anatomicaltraits that make it the earliest known member of the evolutionary line thatgave rise to modern elephants. This relatively unspecialized, rabbit-sized ani-mal, Eritherium azzouzorum, has teeth that show evidence of the kind of toothshape found only in elephants and their relations. For this reason Eritheriumis currently considered to be a possible ancestral species of elephants (Figure

5.1. OBSERVATIONS OF LIFE 137

Figure 5.7: An evolutionary genealogical tree of life showing the three domainsof life starting at the left (Bacteria, Archaea, Eukaryotes), and many of theeukaryotic groups as you go the left. Also note that the figure shows majorgeological events and mass extinctions, and suggests a common ancestral linefor all living things. (Image courtesy of Leonard Eisenberg, used by permission.)

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5.8). The closest living relatives of elephants are small animals called the rockhyrax, and manatees and dugongs. This has been confirmed by genetic analysisand shared anatomical traits (see Table 5.2).

Shared with rock hyrax Shared with manatees and dugongsIncisors are small tusks Incisors are tusks in dugongs, manatees

don’t have incisors

Molars on the side of the jaw Molars on the side of jaw

Testes housed inside the abdominal Testes housed inside the abdominalcavity cavity

Odd number of toes (3) on hind Odd number of toenails (3)feet

Toenails flat Toenails flat

Mammary glands near the forelegs Mammary glands near forelimbsOvaries protected by an extra Ovaries protected by an extratissue covering called a vestibule tissue covering called a vestibule

Breathing only via nostrils Breathing only via nostrils

Kidneys with nephrostomes Kidneys with nephrostomes

Nearly hairless bodies

Wrinkled skin, in order to minimizedehydration

Table 5.2: Anatomical characteristics modern elephants share with the rockhyrax, and manatees and dugongs.

The main backbone of the tree that gave rise to modern elephants includesmany intermediate forms. Paleontologists continue to debate exactly whichspecies are direct-line ancestors of modern elephants, but some species that arecontenders for these positions include Moeritherium, Palaeomastodon, Gom-photherium, Primelephas, and mammoths (see Figure 5.8).

As scientists examined the fossil history of the elephant family tree, andelephant evolution, several trends became evident. These are listed in Table 5.3(evolutionary trends of elephants).

Some of these traits are evident, as you look at pictures below of a few of thespecies in the evolutionary tree of elephants. Be sure to refer to the evolutionarytree of modern elephants to see where these species appear on that tree.

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Figure 5.8: An evolutionary history of modern elephants. (Image courtesy ofAlan R. Holyoak)

Anatomical Trait TrendBody size Small (rabbit sized) to large

Limb length Lengthening of fore and hind limbs

Foot shape Feet become relatively short and broad

Skull/neck length Skull and neck shorten

Lower jaw length Lower jaw elongates then shortens again

Nose shape Short nose elongates into a trunk (proboscis)

Incisor shape Small, flat edges teeth for clipping becomeelongated into tusks

Molar shape These shift from being relatively small andunspecialized to being large, broad, andspecialized for grinding plant material

Pattern of tooth replacement Teeth were originally replaced from below (asin humans) to being replaced from the rear ofthe jaw, shoving older, worn teeth forwardwhere they are lost as they crack and fall out

Table 5.3: Evolutionary trends in the ancestral line giving rise to modern ele-phants.

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Figure 5.9: Six extinct species of ancestral elephants. Look for these specieson the evolutionary tree presented earlier in this case study. Starting from theupper left, the species are: Moeritherium (50-37 Ma), Palaeomastodon (36-25Ma), Gomphotherium (24-5 Ma), Deinotherium (20-1 Ma), Primelephas (15-5 Ma), and Steppe Mammoth (4 Ma10,000 yrs ago - the head of an Africanelephant is included for scale). (Image courtesy of Wikimedia Commons)

Evolution case study #2: Whales

The evolutionary history of whales is a fascinating story. For many years noone had any supportable idea about where whales came from. Whales aremammals, and, like all other mammals, whales have hair (at least a little),mammary glands, produce milk, suckle their young, and must breathe air, butunlike most mammals, whales live only in water. Whales have streamlinedbodies, fins and flukes instead of legs and a tail, they lack hind limbs, theyhave a thick layer of blubber for insulation, their nostrils are on the top ofthe head instead of at the tip of the snout, they lack specialized teeth com-monly seen in other mammals, and they even reproduce and give birth inwater. No other mammal has those traits or does those things. So, wheredid whales come from? It wasn’t until the last few decades that key fossilswere discovered that were needed to solve this evolutionary mystery. Beforeyou read further, watch this five-minute video about the evolution of whales:http://www.pbs.org/wgbh/evolution/library/03/4/l 034 05.html

As you already know from earlier readings, about 65 million years ago a me-teorite impact triggered the mass extinction that killed the dinosaurs (exceptbirds) and large marine reptiles such as plesiosaurs and mosasaurs that werethe top marine predators at the time. These extinctions opened up ecologicalspace in the world’s oceans for large-bodied predators, but none appeared forsome time. About ten million years after the dinosaurs went extinct, in what

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is now Pakistan, a group of animals appeared that are the earliest known an-cestors of modern whales. This whale ancestor is named Pakicetus (meaning“Pakistan whale”). Pakicetus, however, didn’t look at all like a whale. It wasa terrestrial, carnivorous mammal about the size of a modern wolf (see Figure5.10). The oldest known fossil of this species is from about 56 million yearsago. Paleontologists were able to make the connection between Pakicetus andwhales via unique bones in the skull that are highlighted in the short videoyou should have watched (see the link above). The only other animal known tohave that unique structure other than Pakicetus were fossil and modern whales.The location and sediments in which Pakicetus were discovered confirm the factthat this animal was not aquatic. It had long legs, small feet, and probablywould have been a very weak swimmer. Its fossils, however, have been foundonly among stream and river sediments where it may have been a predator andscavenger taking advantage of abundant prey that would have been there.

Figure 5.10: Pakicetus (56 Ma). This was a completely terrestrial species.(Image courtesy of ???)

The next known species in the evolutionary history of whales appeared in thefossil record about five million years after Pakicetus, it was called Ambulocetus.This species (whose name means “walking whale”) lived about 50 million yearsago, also in Pakistan and southern India, was about three meters long, and ata glance could have been mistaken for a scale-free crocodile (see Figure 5.11).This was the first aquatic ancestor of whales. It still had four well-developedlegs, and large, sharp teeth, but its limbs were proportionally shorter than itsancestors’, its feet were larger, and may have been webbed. Unlike Pakicetus,Ambulocetus lived mainly in near-shore marine environments, possibly includingestuaries. The composition of its remains show that it probably also spent atleast part of its life in freshwater. These conclusions are supported by the factthat fossils of this species have been found only in near-shore, marine sediments.Most paleontologists conclude that Ambulocetus was amphibious, being able tomove well on land and in the water.

Figure 5.11: Ambulocetus (50 Ma). An amphibious freshwater and marinespecies. (Image courtesy of ???)

The next whale ancestor to appear was a contemporary of Ambulocetus,showing up in the fossil record only a few million years later (48 Ma). This newspecies was Remingtonocetus (meaning “Remingtons whale”). It had a moreelongate body and a longer more slender snout than its ancestors, and probably

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swam by undulating its body up and down rather than side-to-side as fishes do(remember the otter in the video?). Remingtonocetus still had fully functionalhind legs, and, like Ambulocetus, was almost certainly amphibious. Depositsof material in its fossilized teeth show that this species lived only in and alongmarine habitats. It wasn’t long, however, before the next intermediate form,Protocetus, appeared in the fossil record. Evolution was proceeding relativelyrapidly.

Figure 5.12: Remingtonocetus (48 Ma). Amphibious, but lived only in shallowmarine habitats. (Image courtesy of ???)

Protocetus (meaning “first whale”) shows up in the fossil record around 47Ma. It was more streamlined than its ancestors and is an important transitionalform showing distinct connections between its terrestrial ancestor via the shapeof bones in its ankles, and firm connections to its whale descendants in theform of its inner ear bones. Protocetus still had hind limbs, thought its legswere shorter than those of its ancestors. It is unclear whether these animalscould support themselves on land, but in either case this species probably spentthe majority of its time in the water, much like modern seals. Protocetusprobably moved by a combination of paddling with its large feet and by upand down undulations of its body. Fossils of this species are found only incoastal and lagoon sediments. Protocetus fossils have, however, been found incoastal regions in many different locations, suggesting that this was probablythe first widely dispersed whale species, and may have had a global distributionin shallow marine environments.

Figure 5.13: Protocetus (45 Ma). Recent research indicates that Protocetus wasstreamlined, but did not have tail flukes. (Image courtesy of ???)

The next group of whale ancestors appeared about 41 Ma, and is repre-sented by Dorudon (meaning “hard tooth”). The modern whale body shapebegins to take shape in this species (see Table ??). Dorudon was about fivemeters long, had a streamlined, elongate body, and showed several adaptationsto a completely marine lifestyle. It still had tiny hind limbs, but would nothave been able to support itself out of the water. Its skull had an elongatedsnout, and it no longer had fused vertebrae making up the sacral region of itsbackbone. This would have provided its backbone with greater flexibility andan increased ability to undulate its body. This is a characteristic also foundin modern whales. Dorudon was almost certainly a truly aquatic species. Its

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nostrils were located farther up on the head than in any earlier species. Cur-rent interpretations of the fossil record suggest that descendants of Dorudongave rise to both the modern baleen and modern toothed whales around 35Ma. Even so, there was another important group of early whales that helpedunravel this story. They are the basilosaurs.

Figure 5.14: Dorudon (41 Ma). The nostrils on the head are higher than onprevious species. (Image courtesy of ???)

Basilosaurus (“king lizard”) were whales that appeared about 40 Ma. Thesewere not direct line ancestors of whales, but they played an extremely importantrole in unraveling the mystery of whale evolution. Basilosaurus fossils werefirst discovered in the 1800’s, and were erroneously identified as marine reptileswhen they were first described, thus the term “lizard” appears in the name.Anyway, the thing that makes this group special is that a fossil of this type wasdiscovered, and that fossil was complete enough to show that it had a small butcomplete pelvis and small but functional hind limbs complete with toes. Priorto this discovery no whale with hind limbs had been discovered, and this gavescientists clues of what to look for in other whale fossils that were discoveredlater.

Figure 5.15: Basilosaurus (38 Ma). Not a direct line ancestor of modern whales,but this group provided important clues to understanding the evolution ofwhales. (Image courtesy of ???)

The anatomy of modern whales still contains clues of their ancestry. Theyhave, for example, vestigial pelvis and femur bones (Figure 5.17). They alsohave the same structures in their skulls that scientists first used to link them toPakicetus. Trends in the evolution of anatomical characteristics of whales arelisted in Table 5.4.

In summary, there are still questions to be answered about the evolution ofwhales, but we now have solid evidence supporting the conclusion that whalesevolved from an ancient, terrestrial animal that lived over 55 Ma. It makes youwonder what life will look like if it has another 60 million years to evolve?

(Note: Much of the information in this case study was adapted from thefollowing scientific article: Thewissen, J.G.M., and E.M. Williams. 2002. TheEarly Radiations of Cetacea (Mammalia): Evolutionary Pattern and Develop-mental Correlations. Annual Review of Ecology and Systematics 33: 73-90.)

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Figure 5.16: The evolutionary tree of whale species. Note that the represen-tative species included above are found in this tree, as are many other fossilwhale species. These species are highlighted as representatives of the majorancestral groups of modern whales. The Odontocetes coming out of the purpleDorudontidae box are the modern toothed whales, and the Mysticetes are themodern baleen whales. Notice also that basilosaurs are on an evolutionary sidebranch. (Image courtesy of Thewissen and Williams, 2002)

5.1. OBSERVATIONS OF LIFE 145

Anatomical Trait TrendRostrum (snout) length A short snout elongates into a long one

Nostril location Movement of the nostril from the tip of thesnout to the top of the head

Eye location Movement of the eyes from the top of the headto the sides of the head

Teeth Shift from having specialized teeth (e.g.incisors, canines, premolars, molars) tohaving teeth all of the same shape (moderntoothed whales) or no teeth (baleen whales)

Overall body shape Shift form a short, squat body to an elongatedand streamlined body

Forelimb shape Shift from a long slender leg with a small footto a short and broad leg with a flipper/fin

Hind limbs Shift from a long, slender leg, to a smallernon-functional hind leg, finally on tovestigial leg bones or none at all (noexternal hind limb)

Sacral vertebrae (vertebrae Shift from four sacral vertebrae fused togetherof the pelvic region) that make up part of the pelvis, to only one

sacral vertebra, and none of the formal sacralvertebrae are fused to each other (this allowsgreater flexibility of the backbone forswimming

Pelvis size A large pelvis used to support the hind portionof the body on land becomes tiny and vestigialor completely absent

Tail A short, whip-like tail becomes long andbearing flukes (tail fin)

Table 5.4: Major anatomical trends in whale evolution.

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Figure 5.17: The upper drawing is of a modern sperm whale skeleton, and thesmall structure labeled “p” indicates its vestigial pelvis and femur bones. Thelower drawing is of a modern bowhead whale skeleton, and the structure labeled“C” indicates its vestigial pelvis and femur bones. The pelvis and femurs inmodern whales are non-functional, when present, and do not extend beyondthe outer body wall. (Image courtesy of ???)

REVIEW QUESTIONS

1. List and describe several traits characteristic of all forms of life.

2. Identify a few possible hypotheses to account for the observed diver-sity of life on Earth.

3. What specific characteristics might you look for in fossils to deter-mine whether the specimen was related to modern day elephants?

4. What kinds of difficulties might arise when classifying fossil speci-mens?

5. How does the fossil record confirm the idea that species can go extinctand evolve? Explain.

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5.2 Origin of Species: Early Ideas

OVERVIEW

Summary: A review of early ideas that attempted to explain how lifeon Earth could be so diverse and yet so similar, culminating with CharlesDarwin’s theory of evolution by natural selection.

Learning Objectives:

• Give examples of how the accumulation of observations have causedus to develop improved models of the origin of the diversity of lifeon Earth (and is another good example of the self-correcting natureof science).

• Explain why special creation and all of its variations (creation sci-ence, intelligent design, etc.) are not scientific explanations.

• List and describe the Cuvier’s observations that led to the conclusionthat species can go extinct.

• Describe Lamarck’s theory of evolution by the inheritance of ac-quired traits.

• List and explain Darwins four foundational principles of evolutionby natural selection.

• Explain why an understanding of the mechanisms of inheritance isnecessary for any working theory of evolution.

Vocabulary:

• Special creation

• Neo-Platonism

• Inheritance of acquired traits

• Natural selection

There are estimated to be between 5 and 30 million living species on earth[Brooker08]. Where did they all come from, and how did they come to be sowell adapted to their environments? How did they come to be so different, andyet have so many basic traits in common? This section introduces you to ahistory of discoveries and ideas that gave rise to our current understanding ofevolution - a process that provides some answers to these questions.

Idea #1: Special Creation

One of the earliest explanations for origin and diversity of life is referred toas special creation [Brooker08, Freeman01]. This non-scientific explanation

Special creation:A theologically based ideawhich states God created thevarious species the way thatthey are today.

is found in Biblical and other scriptural accounts. Special creation states thatGod created every kind of living thing as immutable species that were theprogenitors of all life on earth. It also states that the diversity of life cameinto being when God created each kind of living thing, and that each kind ofliving thing was formed to be perfectly and uniquely suited to its particularenvironment.

The reason this explanation is non-scientific is that it relies on acts of Godthat cannot be investigated scientifically, and for which empirical evidence can-not be collected. In other words, this explanation cannot be falsified.

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Figure 5.18: “The Creation of Adam” by Michelangelo. This painting, found onthe ceiling of the Sistine Chapel, depicts the notion of special creation. (Imageis in the public domain.)

Idea #2: Inheritance of Acquired Traits and the Conceptof Use and Disuse

The ancient Greeks developed the idea that traits acquired during an individ-ual’s life can be passed to its offspring; a paradigm that lasted for over 2000years! The Greeks also accepted the principle of use and disuse. That is, that acharacteristic or trait that is used and is beneficial is retained by a species, whiletraits that are not used could gradually disappear from a species. The Greeksalso believed that climate and regional differences could be used to explain dif-ferences we see among the races of people [Mayr85]. This idea of use and disuseis how many people today would explain evolution if they were asked, but theconcept of use and disuse is not the basis for evolutionary change, even thoughit was the predominant explanation for over two millennia about how organismschange.

Idea #3: Fossils and the Rebirth of Direct Observation(1400’s-1600’s)

Educators and scholars practiced a method of learning called scholasticismthroughout the Middle Ages and into the early part of the Renaissance pe-riod. Scholasticism is based on the premise that a true scholar should study theteachings and philosophies of the ancients, such as Plato and Aristotle, ratherthan seek out new knowledge. The teachings of the ancients were acceptedon the authority of ancient scholars alone, not on independent confirmation ofwhat they taught. Fortunately, this approach to learning changed around 1600.Early renaissance period scientists therefore spent most of their time collect-ing specimens and pondering them in light of the teachings and conclusionsof ancient philosophers instead of working out their own conclusions based ontheir own observations. This deference to authority caused them to view theworld under a philosophy called Neo-Platonism [Rudwick85] (after the Greekphilosopher Plato). Neo-Platonism includes a combination of Greek mysticism

Neo-Platonism:A philosophy that suggestsall things in nature are basedon perfect ideas, and thusliving organisms and fossilsare just nature’s way of ex-pressing perfection.

and Judeo-Christian beliefs. Neo-Platonism states that all living things have aninherent power within them and an affinity between them. It also states thatthe physical shapes of all things in nature are based on perfect ideas and thatthose ideas may be expressed anywhere: in the heavens, in water, on the earth,or in the earth. Living organisms and fossil specimens were therefore viewedonly as expressions of perfect ideas in stone, and scientists of that time did notrecognize fossils as the remains of once-living things.

A form of Neo-Platonism continues in some ways today, such as when some-one reports that they believe they see an image of an object of worship in a

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Figure 5.19: Fossil trilobites in the Museum of Science, Boston, Massachusetts,USA. Neo-Platonists suggested that such fossils were merely stones in whichnature was attempting to express perfect forms. (Image is in the public do-main.)

non-living thing, e.g., a sighting of an image of the Virgin Mary in a dirtyhospital window.

In the 1600’s education moved away from scholasticism and Neo-Platonism.About this time scientists started to base their conclusions on first-hand obser-vations, or the authority of direct observation. As a result they rejected viewsof Neo-Platonism, and instead proposed that fossils are the remains of thingsthat were once alive [Rudwick85]. This does not mean that they rejected thetheory of special creation, it just meant that they believed that some individualorganisms eventually became fossils. The organic origin of fossils was broadlyaccepted by the late 1600’s, but that did not explain the origin of fossils thatdid not look like any known living thing, or provide a scientific explanation forthe diversity of life.

Idea #4: Extinction Theory and Early Rejection of SpecialCreation as a Scientific Explanation (1796)

In the 1700’s scientists proposed that sedimentary rock layers represent differentperiods of history and that fossils in those rock layers represent a history oflife on earth, as described in chapter 4 of this text. Nevertheless the originof unidentifiable fossils remained elusive until 1796 when the French scientistGeorges Cuvier presented research on the anatomy of living and fossil elephants.

Cuvier was a firm supporter of the theory of Catastrophism, but he was notconvinced that the idea of the immutability of species, or the notion that God’sperfect creations could neither change nor become extinct, was correct. Heinvestigated this question by comparing the skeletal anatomy of living elephantsto the anatomy of fossil elephants. Elephants are excellent subjects for this kindof research because their skeletons are large, and there is no question aboutwhether you are looking at an elephant when you look at these species. Cuvierhypothesized that if significant anatomical differences existed between livingand fossil elephants that such observations would provide evidence that speciesof living things could and had changed.

Cuvier made careful measurements and observations on living and fossilelephants. His data showed that there are significant anatomical differencesbetween living African and Asian elephants. He also discovered that were sig-nificant differences between living and fossil elephants. Some of these differencesare obvious when you compare the jaw and teeth of a fossil mammoth with thejaw and teeth of an Asian elephant (Figure 5.20). These observations showed

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that living and fossil elephants are different, suggesting that they had under-gone change, and that there are species of elephants (e.g., mammoths) thatonce lived on the Earth, but which are now extinct. This set of observationseffectively refuted the dogma of immutability of species and showed that extinc-tion of entire species had occurred. Both of these discoveries were huge stepsforward in our understanding of the history of life on Earth.

Figure 5.20: Georges Cuvier’s sketches of (top) a mammoth jaw and (bottom)an Asian elephant’s jaw. The differences in the two specimens was enough todemonstrate that the mammoth jaw was different from that of Asian or Africanelephants, and therefore belonged to an animal that had gone extinct. (Imageis in the public domain.)

Cuvier’s work on elephants and extinction opened the door to further workin this field. Science currently recognizes about 160 species of elephant-like an-imals, including four living species (Asian, Borneo Pygmy, African Forest, andAfrican Savannah elephants), and the ongoing discovery of new fossils contin-ues to help clarify the evolutionary history of this interesting and anatomicallydistinctive group of animals.

Though extinction is commonly accepted today, Cuvier’s demonstration ofextinction was a testimony-shaking event for some people of his day. It was atestimony-shaker because people at the time believed that a perfect God couldcreate only perfect things and that those perfect beings were perfectly suitedto their environments and would never change or go extinct.

Cuvier’s work did two things. It showed that extinction can occur, and itprovided an explanation for the origin of unidentifiable fossils: they were mem-bers of species that had gone extinct. By the way, modern work on biodiversityshows that there are currently millions of living species and that billions ofspecies have lived on the earth. That means that probably less than 1% of allspecies that ever existed are now alive [Raupp92].

Cuvier also openly rejected the theory of special creation as a scientificexplanation for the diversity of life. He did so because acts of God cannot bestudied via scientific methods, are non-falsifiable, and they therefore fall outsideof the realm of scientific investigation [Rudwick85].

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Idea #5: The Theory of Transmutation and the Conceptof Deep Geologic Time (1800)

Around the same time as Cuvier, other scientists also concluded that speciescan change and that the earth is much older than a few thousand years, aspreviously thought. The concept of transmutation of species (i.e., evolution)was not widely accepted, however, because no one had discovered a method bywhich species could change.

Idea #6: Evolution by the Inheritance of Acquired Traits(1800)

Jean-Baptiste Lamarck, a younger contemporary of Cuvier, was the first scien-tist to publish a theory that explained how species might change. His theoryof inheritance by acquired traits states that traits acquired during an indi-vidual life can be inherited by its offspring [Brooker08]. Lamarck’s explanationwas actually a sort of modernized version of the Greek use and disuse model ofchange.

Inheritance by acquiredtraits:An idea which suggests ani-mal populations evolve whentraits acquired during an in-dividual lifetime are passedon to its offspring.

An example that has been used to explain how Lamarck’s theory works isthis: imagine an ancestral animal that has a relatively short neck. It neverthe-less prefers to eat leaves that grow on trees, so it constantly stretches its neckthroughout its life in order to reach those leaves. As a result it ends up with aneck that is proportionally longer than the neck it would have had if it hadn’tspent its life stretching. When this animal, now with a slightly longer neck,reproduces, Lamarck’s theory states that the animal’s offspring could inheritthe trait of a slightly longer neck, and actually be born with a longer neck.Finally the unusually long necks of giraffes were produced by many generationsof ancestral animals that continued to stretch and feed. Nearly 60 years passedbefore further research and Charles Darwin’s discovery of evolution by natu-ral selection and Mendel’s discovery of the principles of inheritance (which arediscussed in another reading) revealed the errors in Lamarck’s theory.

Idea #7: Charles Darwin and Evolution by Natural Selec-tion (1859)

Charles Darwin was born into a well-to-do, upper middle-class family. Heshowed little aptitude for formal education in his early years, but when hewas 16 he was admitted to the University of Edinburgh (Scotland) where heembarked on the study of medicine. The first operation Charles witnessed hor-rified him. He reportedly bolted from the room and never returned. Darwinthen moved on to study theology at Cambridge University, and began to pre-pare for a career in the clergy. Through all of this, however, Darwin maintainedhis life-long passion for natural history. Darwin completed his theology degreewhen he was 22 years old, but his heart was still with natural history.

Charles Darwin (1809-1882 ): Proposed the theoryof evolution by natural selec-tion.

After graduation Darwin had no desire to enter the clergy. Instead he ac-cepted an offer to serve as a naturalist aboard the HMS Beagle, a ship of theRoyal Navy that had been ordered to carry out a mapping expedition of SouthAmerica.

The voyage of the Beagle lasted from 1831-1836 and included circumnav-igation of the globe. Though Darwin was almost constantly seasick he tookevery opportunity to observe the biodiversity and geology of places the Bea-gle went. During the voyage Darwin collected and shipped over 5,500 naturalhistory specimens (e.g., rocks, fossils, animal and plant specimens, etc.) backto England. Darwin’s collection came to the attention of the English scientificcommunity, and when he returned home he was surprised to find that he hadbecome something of a scientific celebrity [AE98].

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Once back in England Darwin worked with a variety of specialists to identifyand describe the specimens he collected. As he did so, he made many observa-tions that eventually led to the development of his theory of evolution by naturalselection. For example, Darwin compared and marveled at the similarities anddifferences between giant fossil ground sloths and their smaller, living relatives,similarities and differences between geographically isolated populations of liv-ing species, such as rheas (large flightless birds) from southern versus centralSouth America, and the similarities and differences between the unexpecteddiversity of closely-related species of finches and other bird groups he found inthe Galapagos Islands.

Darwin hypothesized that all Galapagos finches must be descendants of asmall group of South American finches that somehow ended up on the GalapagosIslands, 600 miles east of the South American mainland. He also concludedthat all living things must have evolved from preexisting species. At the time,however, Darwin could not explain how, for example, a few South Americanbirds could give rise to the specialized finch species of the Galapagos.

Two books had a profound impact on Darwin’s thinking as he continuedto ponder on this problem of the origin of species. The first book was bythe English geologist Charles Lyell [Lyell30]. Lyell wrote that geologic changeoccurs constantly and gradually through small-scale processes, such as erosion,acting over long periods of time. Lyell also stated that the Earth must be veryold to have allowed enough time for those constant, small, and gradual processesto produce the geologic features that we see today. The second book was bythe English economist Thomas Robert Malthus [Malthus98]. Malthus wrotethat human populations grow and thrive as long as resources exist to supportthem, but when population size exceeds resource availability then competition,disease and war result, and these factors reduce the population to a supportablesize.

Darwin applied the ideas of Lyell and Malthus to his own observations. Dar-win hypothesized that if small-scale geologic processes acting over long periodsof time can produce significant geologic change, then small changes in heritabletraits occurring over many generations could cause major changes in species,including the production of new species. Darwin wondered what force couldcause such species to change. Darwin turned to Malthus’ ideas and concludedthat if competition for resources limits the size and health of human populationsthen non-human populations must also compete when resources are limited.

By the end of 1837, Darwin, now 27 years old, combined the ideas of Lyelland Malthus with his own to develop the foundation of his theory of evolutionby natural selection [Darwin59]. This theory is based on the following four

Natural selection:This theory states that or-ganisms evolve as ecologi-cally favored genetic varia-tions are passed on to off-spring.

principles:

1. There is variation in heritable traits in all populations of living things.

2. Some of that variability is passed from generation to generation.

3. Competition exists because more offspring are produced every generationthan will survive to adulthood.

4. Surviving to adulthood and reproductive success are not random events.

The fourth principle includes the “natural selection” part of Darwin’s theory.Nature selects or favors individuals that already have the best set of traitsfor dealing with local conditions when they are born. Favored individuals (i.e.,those with the best fit to their environment) are therefore more likely to surviveto adulthood, acquire a mate, and produce more offspring than individuals thatlack the best set of traits.

The elegant part of this explanation is that no matter what local conditionsexist, and even if conditions change, selection continues and populations oforganisms continually become better adapted to the environment as favored

5.2. ORIGIN OF SPECIES: EARLY IDEAS 153

Figure 5.21: A depiction of how the processes of natural selection lead to theevolution of a species. In this example, dark coloration is ecologically favored,while light coloration is not. (Image courtesy of Laszlo Szalai, used under theterms of the GNU Free Documentation License.)

individuals survive and reproduce large numbers of offspring, while fewer, less-fit individuals survive, and those that do survive tend to produce fewer offspring.Natural selection can thereby cause populations and species to change over time.Darwin referred to this kind of change as descent with modification [Darwin59].

The difference between Darwin’s theory of evolution by natural selectionand the “use and disuse” theories of the Greeks and Lamarck is that Darwin’stheory is based on the set of inherited traits an individual received from theirparents, while both the Greeks and Lamarck based their ideas on the idea thattraits an individual has can be changed during their lifetimes. Of course, whenan egg and sperm fuse to create a new individual, the genetic make-up of thatnew individual is set and cannot be changed by use or disuse. This is the majorflaw in Lamarck’s theory. At same time, Lamarck had no way of knowing he wasin error, because when he developed his theory no one knew how inheritanceworked.

As Darwin continued to ponder the things he observed, he concluded thatthe history and diversity of life should be viewed as a tree. He hypothesizedthat all species evolved from one or a few ancestral forms found at the base ofthe tree. He stated this conclusion quite elegantly in the last sentence in hisbook, “On the Origin of Species”: “There is grandeur in this view of life, withits several powers, having been originally breathed into a few forms or one; andthat, whilst this planet has gone cycling on according to the fixed law of gravity,from so simple a beginning endless forms most beautiful and most wonderfulhave been, and are being, evolved.” [Darwin59]

Even though Darwin had developed the core of the theory of evolution bynatural selection by 1837, he spent the next 22 years collecting examples thatdemonstrate his theory. Darwin was finally convinced to publish his theory, andit appeared in his book, “On the Origin of Species”, in 1859. Within 15 yearsof its publication Darwin’s theory gained nearly universal acceptance amongthe scientific community, thus making Darwin’s theory evolution by naturalselection the scientific new paradigm for explaining the diversity of life.

Those who know about Darwin’s life also know that he broke his ties withformal religion. Some people have tried to blame that apparent loss of faith onhis work on evolution. What they probably do not know is that most of his lossof faith was directly related to the death of his oldest daughter. When Darwin’s

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daughter died he could not accept that her death was the act of a caring andloving God, and he never again set foot in a church. But if you re-read thequote above it is obvious that Darwin still had faith, and the publication of histheory was never intended to damage anyone’s testimony.

By 1900, Darwin’s theory was in big trouble. Darwin’s theory is based onthe mechanism of the inheritance of variable traits, but the scientific commu-nity did not yet know how inheritance worked or where new genetic variationcomes from. Until these two problems could be answered Darwin’s theory wasin jeopardy. Fortunately someone had already discovered and described theprinciples of inheritance, but we’ll learn about that in the next reading.

REVIEW QUESTIONS

1. Why did people start coming up with these ideas in the first place,i.e. what were the initial observations that led to these ideas?

2. Why does science not embrace the idea of special creation?

3. Why do you think that the notion of inheritance by acquired traitspersisted in scientific thought for such a long time?

4. What did you find interesting or informative about Charles Darwin’slife?

5. Explain how the four principles of Darwin’s theory of natural selec-tion could cause a species to evolve.

5.3. GENETICS AND DNA 155

5.3 Genetics and DNA

OVERVIEW

Summary: In the last section, we saw that Darwin’s theory of natu-ral selection was lacking a mechanism for explaining inheritance and thesource of new genetic variability. In this section we will address theseideas. The genetic code for all living creatures is found in a moleculecalled deoxyribonucleic acid, or DNA. When organisms reproduce, theirDNA is passed on to their offspring.

Learning Objectives:

• List and describe Mendels three laws of inheritance.

• Illustrate the laws of dominance and segregation by constructing andinterpreting a Punnett square.

• Define the terms genome, DNA, chromosome, gene, and allele, anduse these terms correctly as you describe genetics and inheritance.

• Identify the structural components of DNA, including molecules andbonds found in DNA.

• Explain the basic pattern of DNA replication and how it relates tothe principle of inheritance.

Vocabulary:

• Inheritance

• Allele

• Law of segregation

• Dominant allele

• Recessive allele

• Law of independent assort-ment

• Gene

• Chromosomes

• Carbohydrates

• Lipids

• Amino acids

• Proteins

• Nucleotides

• DNA

• Replication

You will recall that the last section ended laying out two major difficultieswith Darwin’s theory: (1) there was no specific mechanism known wherebytraits could be passed from parent to offspring (i.e. the processes by whichinheritance worked were unknown), ; and (2) there was no known mechanism

Inheritance:The mechanism wherebytraits are passed from parentto offspring.to explain how new genetic variations could arise (i.e. the processes whereby

mutations occurred were unknown). These questions were ultimately answeredas early models of genetics were enhanced by the discovery of chromosomes,genes, and DNA.

Idea #8: Mendel’s Principles of Inheritance (1866 & 1900)

In 1866 Gregor Mendel published a research paper with an unremarkable title,“Experiments on Plant Hybrids” [Mendel66], in an obscure German journal.That paper contained the results of a series of experiments which illustratedthe following principles:

1. Traits are inherited as discrete heritable units that we now call genes

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(e.g., in Mendel’s experiments on peas, one gene determined plant heightand another gene determined flower color, etc.).

Gene:A discrete unit containingheritable traits; a segmentof DNA found on a chromo-some.Allele:One of two copies of a par-ticular gene carried by an or-ganism.

2. An individual always carries two copies of a gene. Those copies are calledalleles. Alleles carried by an individual may be identical to each other orthey may be different (e.g., a pea plant could have two “tall” alleles, two“dwarf” alleles, or one “tall” and one “dwarf” allele for the plant heightgene).

3. A parent can pass only one of their two alleles for a particular trait to anoffspring. The offspring receives one allele for a gene from its mother andone allele for the same gene from its father. The allele that is passed froma parent to an offspring is determined at random. Mendel called this thelaw of segregation. The workings of this law are apparent in Punnett

Law of segregation:A scientific law which statesthat the alleles passed ontooffspring are selected ran-domly.

square diagrams, such as Figure 5.22. A Punnett square is one way torepresent how Mendelian inheritance works. In Figure 5.22 the allelesfrom the male parent (top of diagram) are highlighted in blue, while thealleles from the female parent (left side of diagram) are highlighted in red.The four boxes in the table represent possible allele combinations fromthe parents, and you will note that each of these combinations, whichcould be inherited by offspring, include only one allele from each parent.

4. The combination of alleles for a given gene determines which version ofthe trait an individual will produce. Both alleles of a gene are activeand produce whatever they code for, but whenever a dominant alleleis present its product masks the expression of the other allele regardlessof what the other allele may be. An allele that is expressed only when

Dominant allele:An allele whose trait will beexpressed regardless of whichother allele for the same traitis present.

no dominant allele is present is called a recessive allele. In peas the“tall” allele is dominant and the “dwarf” allele is recessive. For example,whenever at least one dominant “tall” allele is present the plant will betall, but a plant must have two “dwarf” alleles to produce a short plant.

Recessive allele:An allele whose trait is ex-pressed only when a domi-nant allele is not present.

The mechanism by which this works is depicted in Figure 5.22, where youwill see that any offspring inheriting a dominant allele from either parentwill possess the dominant trait (purple coloration, in this example). , andis called the law of dominance.

5. Alleles for one trait are passed to an offspring independently of the al-leles for all other traits. Mendel called that the law of independentassortment.

Law of independent as-sortment:A scientific law which statesthat alleles for a given traitare passed to offspring inde-pendently of alleles for othertraits.

The significance of Mendel’s work was not immediately realized. However,these principles of inheritance now form the foundation of the modern field ofgenetics. Mendel’s work also showed how inheritance works and provided theclarification and support Darwin’s theory needed, thereby restoring confidencein the theory of evolution by natural selection, which remains a cornerstone ofbiology today. Mendel did not, however, know what genes are, what they aremade of, how the genetic code is carried, how it is translated into the traits wesee, or where genetic variability comes from.

Idea #9: Genes and Chromosomes (1900-1950)

Scientists built on Mendel’s work on inheritance and discovered that chromo-somes are linear strands of deoxyribonucleic acid, or DNA, and that they

Chromosome:A set of genes; a linearstrand of DNA. carry the genetic code. They also discovered that multiple genes are found

on each chromosome, and that segregation and independent assortment occuramong chromosomes rather than among individual genes [Brooker08]. Whatthose scientists didn’t know was how chromosomes make copies of themselvesor how the genetic code is carried and read. To understand those things youwould have to know the structure of DNA, and by 1950 this was the biggestquestion in biology.

5.3. GENETICS AND DNA 157

Figure 5.22: A Punnett square: a representation of Mendelian inheritance,specifically highlighting the laws of segregation and dominance. The allelesfrom the two parent plants (indicated by the “B” and “b” boxes at the topand side of the square) can combine in one of four different ways. In thiscase, the “B” represents a dominant allele resulting in a purple color, and the“b” represents a recessive allele resulting in a white color. (Image courtesy ofMadeline Price Ball, used under the terms of the GNU Free DocumentationLicense.)

Idea #10: The Structure of DNA (1953)

In 1951 an ambitious young American PhD, James Watson, arrived in Englandwhere he met and formed a friendship with a British graduate student, Fran-cis Crick. They decided to try to discover the structure of DNA by buildingmolecular models. At about the same time Rosalind Franklin, an English x-raycrystallographer, joined the research group of Maurice Wilkins. Franklin andWilkins did not get along well, but they both used x-ray crystallography to at-tempt to discover the structure of complex molecules, such as DNA. Using thismethod they produced and analyzed photographs that showed the scatter pat-terns of x-rays shot through strands of DNA. The teams of Watson/Crick andFranklin/Wilkins both knew that DNA was made of nucleotides. What theydidn’t know was how nucleotides fit together to form a molecule that couldcarry the genetic code and make exact copies of itself [Watson68, Maddox02].

By the spring of 1953, and after a number of personally embarrassing at-tempts at model building, Watson and Crick had built their most recent modelof DNA. Its structure was based on a combination of their own intuition, sugges-tions from colleagues, published research, and the use of Franklin’s unpublisheddata (without her knowledge or consent). [Watson68, Maddox02]. After carefulreview by their colleagues, as well as by Wilkins, Franklin, and others, it be-came clear that Watson and Crick’s model was correct [Watson68, Maddox02,Watson53].

They discovered that the basic shape of a DNA molecule is that of a doublehelix (Figure 5.23). If you could untwist and flatten out the helix it wouldlook much like a ladder (see Figure 5.24). The sides of the ladder are made ofalternating sugar and phosphate groups of nucleotides that are bonded to eachother. The rungs of the ladder are made of nitrogenous bases of nucleotidesthat line up with each other on opposite sides of the ladder, and they are alsobonded to each other. These bonds, however, are weak hydrogen bonds: bondsthat are easily formed, broken, and reformed. Notice that the nitrogenousbases always pair up in a certain way: guanine bonds only with cytosine (G-

158 CHAPTER 5. LIFE

C), and adenine bonds only with thymine (A-T). They bond this way becausethere are always two hydrogen bonds between A and T, and there are alwaysthree hydrogen bonds between C and G. Incidentally, the initial evidence thatthe nitrogenous bases bond this way was discovered by Erwin Chargaff, whoseexperiments showed that in all species the number of A’s in the DNA was equalto the number of T’s, and that the number of C’s was equal to the number ofG’s.

Figure 5.23: The double helix shape characteristic of DNA. The solid, ribbon-like lines represent the backbone of the DNA, which consists of the deoxyri-bose and phosphate group. The nucleic acids occupy the “middle part” ofthe molecule. Note that this figure shows only a small segment of the DNAmolecule. (Image is in the public domain.)

Figure 5.24: The molecular structure of DNA. When untwisted, the moleculelooks much like a ladder. The rungs of the ladder consist of pairs of nitrogenousbases. Note that the base adenine always pairs with thymine, and cytosinealways pairs with guanine. The dotted lines represent hydrogen bonds, whichare easily broken. (Image courtesy of Madeline Price Ball, used under the termsof the GNU Free Documentation License.)

Another important thing to know about DNA is of the strength of bondsthat hold molecules in DNA together (Figure 5.24). You may want to referback to the information on molecular bonding back in Section 3.3, where wefirst encountered the DNA molecule. The bonds between phosphate groups

5.3. GENETICS AND DNA 159

and sugars that make up the outer backbones of DNA are a specific kind ofcovalent bonds, called phosphodiester bonds. Once those bonds are formedthey are extremely difficult to break. The bonds between the nitrogenous bases,however, the hydrogen bonds, are relatively weak (indicated by the dotted linesin Figure 5.24).

One of the mysteries of inheritance, you will recall, was how the genetic codecould make exact copies of itself and then be passed to offspring. Scientists knewthat had to happen, but until we knew the structure of DNA there was no wayto understand that process. Now that we know the structure of DNA we havebeen able to discover how DNA makes exact copies of itself - a process whichis called replication. The text that follows is an introduction to this process.

Replication:The process by which exactcopies of DNA are made.When a cell is ready to divide it has to make a copy of every chromosome

it has. Take another look at Figure 5.24. This figure shows that each nitroge-nous base in the middle of a DNA double helix bonds only to a complementarynitrogenous base, as has already been mentioned: remember that A forms hy-drogen bonds only with T (A-T) and that G forms hydrogen bonds only withC (G-C). When DNA starts to replicate itself a rather complex series of eventsinvolving multiple enzymes becomes activated. We won’t get into the detailsof that here, but suffice it to say that the double helix is put under stressand some of the bonds break. The hydrogen bonds (being the weakest bondspresent) break and the two DNA strands that make up the double helix arepulled apart. This is sometimes referred to as unzipping.

When the double helix is unzipped enzymes move nucleotides that are float-ing around into the open space between the DNA strands and form hydrogenbonds with exposed bases on the original strands of the parent double helix. Inthe meantime enzymes also help the sugars and phosphates of newly attachednucleotides to form phosphodiester bonds with each other. This process contin-ues until each DNA strand of the original double helix has an entirely new setof complementary nucleotides bonded to it. In this way strands of the originalor parent double helix act as a template. Complementary bases are bonded toeach template strand until every base on both templates has a new complemen-tary base bonded to it. This is called the semi-conservative pattern of DNAreplication (Figure 5.25).

There are about three billion base pairs in human DNA. That means thatevery time a human cell prepares to divide, whether it is during growth, thereplacement of a worn out or damaged cell, or during production of gametes,three billion bases are added to each set of DNA strand templates. You wouldthink that somewhere in there some mistakes would happen. Well, they do,but there is an entire group of enzymes that do nothing more than move alongstrands of replicating DNA functioning as “spell checkers.” Whenever theylocate a spelling error they clip out the error and correct it. Even so, errorscan and do happen during DNA replication, and that is one source of geneticmutations (discussed later).

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Figure 5.25: The semi-conservative pattern of DNA replication. The doublehelix is unzipped, and additional nucleotides are paired up to match the twostrands, resulting in two copies of the original strand of DNA. Note that theenzymes facilitating this process are not shown in this diagram for simplicity.(Image courtesy of Madeline Price Ball, used under the terms of the GNU FreeDocumentation License.)

5.3. GENETICS AND DNA 161

REVIEW QUESTIONS

1. Explain how Mendel’s principles of inheritance work.

2. Why are the laws of segregation and independent assortment impor-tant?

3. Explain how alleles, genes, chromosomes, and DNA are related.

4. Describe the three primary parts of a nucleotide.

5. Describe the structure of DNA, or in other words how nucleotidescombine to make DNA.

6. Explain how replication works.

7. How would you feel if someone used your unpublished work withoutyour knowledge and/or permission, and subsequently received con-siderable recognition (i.e. image that you were Rosalind Franklin)?How does this relate to the ethics of science?

162 CHAPTER 5. LIFE

5.4 Molecular Evolution

OVERVIEW

Summary: Once the structure and function of DNA was understood, itbecame apparent that genetic mutations were changes in the genetic code -thus completing the molecular evolutionary mechanism, and showing hownew genetic variation could come into existence. Additional research hasshown that these same mechanisms are in operation constantly in all formsof life, and indeed that every form of life is evolving.

Learning Objectives:

• Describe what the universal genetic code is and how the discoveryof this code provides strong evidence for the theory that all livingthings share a common ancestry.

• Explain how transcription and translation work, and identify thepolypeptide coded for by a particular segment of DNA.

• List the different kinds of point mutations and chromosomal muta-tions, discuss their relative severity, and explain why genetic muta-tions are sources of new genetic variability.

• List the Hardy-Weinberg criteria, and then explain how the violationof any of those criteria can allow evolution to happen.

• Discuss the differences between microevolution and macroevolution.

Vocabulary:

• mRNA

• Transcription

• Ribosome

• Codon

• tRNA

• Translation

• Mutation

• Microevolution

• Macroevolution

• Phyletic gradualism

• Punctuated equilibrium

Charles Darwin’s theory of evolution by natural selection provided a goodmodel for describing how populations of organisms change from generation togeneration, and even how new species can arise from preexisting ones, but it didnot explain how evolutionarily important traits are passed from one generationto the next or where new genetic variability comes from. The answers to thesequestions could not be answered until after the structure of DNA was discoveredin the mid-1950s. Watson and Crick’s model of DNA allowed scientists tounderstand how DNA copies itself and transfers genetic information from onegeneration to the next, but science still didn’t know how DNA carries the geneticcode, how the code is read to produce structures in living things, or how thecode can change to produce new genetic variations. These are the topics of thisreading.

Idea #11: Chromosomes and The Universal Genetic Code

Within 10 years of the description of the structure of DNA, scientists discoveredhow DNA carries the genetic code: it is determined by the specific order ofnucleotide nitrogenous bases (i.e., the As, Ts, Cs, and Gs). The order of thesebases specifies which allele a gene carries, and consequently which protein a gene

5.4. MOLECULAR EVOLUTION 163

produces. Depending on its shape, a protein can be used to make a physicalstructure or it can function as an enzyme to help chemical reactions take placein the body.

Further research showed that genes are carried on chromosomes. A chro-mosome is a linear strand of DNA. Linear chromosomes have been found in allliving things except bacteria (bacterial DNA is a double helix, but it is arrangedin a loop that is attached to the cell membrane, but we won’t look at bacte-rial DNA in more detail than this). Each linear chromosome contains multiplegenes. One gene carries the code to make one protein. One complete set ofhuman chromosomes contains about 3 billion pairs of nitrogenous bases (A-Tand C-G pairings), and so far about 30,000 genes have been identified, but wedo not yet know exactly how many genes there are in the human genome.

How exactly does DNA carry the genetic code? As you already know, DNAis made of four kinds of nucleotides, and their bases are A, T, C, and G. Some-how the order of these nitrogenous bases has to carry the code for the specificorder of the 20 different kinds of amino acids that make proteins. If scientistsassigned only one nucleotide to one amino acid, the code could account for onlyfour of the 20 amino acids, and that clearly won’t work. If they assigned acombination of two nitrogenous bases to one amino acid, then this model couldcode for only 16 amino acids. But, when combinations of three nitrogenousbases were used, a maximum of 64 different triplet-combinations of nitrogenousbases is possible, and this approach provided more than enough combinations tocode for the 20 different kinds of amino acids. Further research confirmed thatthe three-base model was correct. Each three-base triplet is called a codon.

Codon:A group of three nitrogenousbases.How exactly does the order of nitrogenous bases carry the code for a protein,

and how is that code read to make a protein? The process used to translatethe genetic code carried by a gene into a protein is sometimes referred to asthe central dogma of molecular biology. Here’s how this process works. First,a cell receives a signal telling it to make a specific protein. When that messageenters the cell’s nucleus, the region of a chromosome carrying the appropriategene is unzipped and a copy of that gene is made through a process calledtranscription. During transcription the DNA code of the gene is not copied

Transcription:The process wherein the ni-trogenous bases that code fora particular gene is copiedinto a strand of mRNA.

into a new strand of DNA, it is instead copied into a strand of another kind ofnucleic acid called messenger RNA (mRNA). RNA stands for ribonucleic

mRNA:messenger RNA; a singlestranded molecule made ofnucleotides that carries thegenetic message out of thecell nucleus and into thebody of the cell.

acid. RNA is a single-stranded molecule that is also made of nucleotides, butit uses a nucleotide called Uracil (U) in place of Thymine (T). The process oftranscription is depicted in Figure 5.26.

Figure 5.26: DNA-mRNA Transcription. An enzyme called RNA polymeraseunzips the double helix in the region of the target gene, and a mRNA strand isproduced. (Image is in the public domain.)

164 CHAPTER 5. LIFE

After the code is copied into a mRNA molecule the mRNA carries the codeout of the nucleus into the cytoplasm of the cell and to an organelle called aribosome. The ribosome is able to read the genetic code carried by the mRNA

Ribosome:The structure within a cellwhere the genetic code isread and proteins are assem-bled.

in a process called translation. During translation a ribosome attaches itself

Translation:The process in which pro-teins are formed based onthe genetic code carried bymRNA.

to the mRNA and the ribosome then reads the nitrogenous bases one codon ata time. Each codon signals the ribosome to do one of the following things: 1)to start making a protein (the “start” codon), 2) to add to a certain amino acidto the amino acid chain; or 3) to stop adding amino acids to the amino acidchain (the “stop” codon) (Figure 5.27).

Each codon indicates which amino acid is to be added next to the aminoacid chain. Each amino acid is brought into the ribosome by a carrier moleculecalled transfer RNA (tRNA). There are as many kinds of tRNA molecules

tRNA:Transfer RNA; a singlestranded molecule com-posed of nucleotides thatcarries amino acids into theribosome.

as there are codons that code for amino acids, and there is a location on eachtRNA molecule that carries a complementary set of nitrogenous bases for eachcodon on the mRNA. This is how the ribosome ensures that the amino acidsare added in the correct order. For example if a codon on the mRNA strand is“AGG” the complementary set of bases on the tRNA will be “UCC”. In thisway a ribosome produces a string of amino acids, one amino acid at a time.When the ribosome finally encounters the “stop” codon the ribosome detachesitself from the mRNA, and the amino acid chain, also called a polypeptide, isreleased. The polypeptide then folds itself up into its specific three-dimensionalshape. It is this three-dimensional shape that that determines the protein’sfunction.

Figure 5.27: The process of protein synthesis carried out by the ribosome.The ribosome matches specific transfer (tRNA) molecules to the codons onthe mRNA. The amino acids connected to the tRNA molecules are added tothe polypeptide chain, which subsequently detaches and folds into a protein.(Image is in the public domain.)

While that’s interesting, the next discovery was earth shaking!Further research discovered that no matter what the organism you are work-

ing with, from a bacterium to a blue whale, the same codon always codes forthe same amino acid. This means that all living things use the same universalgenetic code [Alberts02]. Figure 5.28 shows this universal code. You probablynoticed that there are U’s instead of T’s in the table. This is because the ta-ble shows mRNA codons instead of DNA codons, and they are shown becausemRNA codons are the ones that are read by ribosomes during translation.

As was mentioned earlier in the reading, there are a total of 64 differentcodons in the table showing the universal genetic code. Wouldn’t only 20 dif-

5.4. MOLECULAR EVOLUTION 165

Figure 5.28: The universal genetic code, showing which amino acids are asso-ciated with each codon. Note that the codon AUG indicates the amino acidmethionine/“start” codon. Three of the codons instruct the ribosome to ter-minate the polypeptide chain, and all other codons code for a particular aminoacid, as listed in the table. (Image is in the public domain.)

ferent codons be enough to meet the needs of protein synthesis, one codon foreach kind of amino acid? What are the extra 44 codons for? If you look atthe table you will see that all but two amino acids have more than one codonassociated with them, plus there are “start” and “stop” codons that are usedto signal the ribosome to start or stop reading the code. The extra codons pro-vide a degree of redundancy in the genetic code. This redundancy is extremelyimportant, since scientists also discovered that it is not uncommon for mistakesor changes called mutations to make their way into the genetic code. So, if a

Mutation:A change in the order of ni-trogenous bases in a gene.mutation is small, perhaps a change of only one nitrogenous base, this redun-

dancy sometimes still allows the same protein to be produced. For example,if you take another look at Table 5.28 you will see that the codons “AGA”and “AGG” both code for the amino acid Arginine. So, if a codon started as“AGA” and mutated to “AGG” the code would still indicate Arginine, and theresulting protein remains unchanged.

Idea #12: Molecular Genetics and Variability

A new field of study called molecular genetics grew out of our improved un-derstanding of how DNA carries the genetic code. Molecular genetics is thestudy of how even small mutations affect a protein produced by a gene, andconsequently how mutations represent evolutionary change.

During DNA replication (the process of making new chromosomes of DNAfrom old ones), transcription, and translation, if everything goes well, there areno mutations of the genetic code. However, if there is a mutation it is possiblethat the mutated code will produce a protein with a three-dimensional shapedifferent than that the one that would have been produced by a non-mutatedgene. Luckily, not every mutation results in a defective protein. If, however, themutation affects the number or order of amino acids enough, then the resultingchange in the three-dimensional shape of the resulting protein gives that proteina different function. Thus, a change in the shape of a protein can completelychange the way the protein behaves.

One graphic example of how a mutation produces a significant change in the

166 CHAPTER 5. LIFE

shape of a protein is the genetic disorder cystic fibrosis. Cystic fibrosis occurswhen a person inherits a recessive allele from both parents. This recessive alleleis mutated in that it has lost one codon. The loss of this single codon causes achange in the shape of a protein that prevents the normal movement of sodiumand chloride ions across cell membranes. In a person with cystic fibrosis, thismutated gene results in the concentration of sodium and chloride ions on oneside of membranes, which in turn causes mucous secretions to become sticky,and to respiratory problems, among other things. Unfortunately, even thoughwe know the genetic basis of cystic fibrosis there is not yet a cure for this disease.

Mutations result from the gain, loss, or replacement of one or many bases.Single-base changes are called point mutations. Mutations that involve thegain, loss, or order reversal of hundreds of bases or more are called chromosomalmutations.

What causes mutations? Mutations can be the result of uncorrected mis-takes that happen during DNA replication or transcription, as well as by theeffects of toxic substances produced by the body, or the introduction of toxicchemicals into the body, as well as radiation, and other factors [Freeman01].

Do all mutations affect heritable traits? No. There are two types of cellsin the body, somatic cells and germ cells. Somatic cells are cells of the bodyand germ cells are cells that produce gametes. In sexually reproducing speciesonly mutations that occur in germ cells can be passed to offspring, but it ismutations that occur in the germ cells that produce new genetic variability intothe population.

Idea #13: The New Synthesis

During the latter part of the 20th century biologists combined our modernunderstanding of molecular genetics, DNA structure, the central dogma ofmolecular biology, principles of ecology, and the theory of evolution by nat-ural selection to produce an updated theory of evolution referred to as the NewSynthesis [Eldredge85]. The new synthesis combines ecological and environ-mental pressures and their effects on the genetic code at the molecular level.This combination of factors was then applied to populations, and this synthesishelped explain how genetic modifications can change existing species and evenproduce new species.

Idea #14: Hardy-Weinberg Criteria

We can know that evolution is taking place in a population whenever allelefrequencies change from one generation to the next. This conclusion was reachedindependently in the early 1900s by two researchers, Godfrey Hardy [Hardy08]and Wilhelm Weinberg [Weinberg08]. It took most of the rest of the 20thcentury to produce the breakthroughs in genetics and molecular biology thatwere needed to test their hypothesis.

Hardy and Weinberg hypothesized that evolution occurs whenever thereis a change in the number of individuals in a population having certain alleleswhen compared to the number of individuals having those same alleles in earliergenerations or in subsequent generations. They further stated that the followingfive criteria must be met in order to prevent a population from undergoingevolution [Freeman01]:

1. There is no selection (the genetic makeup of individuals gives them noadvantage or disadvantage in dealing with challenges they face).

2. There is no mutation.

3. There is no migration (no immigrants or emigrants).

5.4. MOLECULAR EVOLUTION 167

4. There are no chance events, (the population must be large enough so thatrandom meetings between eggs and sperm during fertilization that couldproduce a shift in gene frequencies are neutralized by the huge numberof other fertilization events, thus maintaining the predicted unchangingallele frequencies).

5. All mating is completely random (no mate selection).

What is the likelihood of these five criteria being met at the same time inany natural population? Never! That is the point! The violation of at leastone of these criteria will ensure that a population experiences a change in allelefrequencies. This means that all naturally occurring populations of living thingsare constantly evolving.

Idea #15: Microevolution, Macroevolution, and Punctu-ated Equilibrium

Microevolution:Changes in allele frequenciesthat occur within a popula-tion from generation to gen-eration.

Microevolution is defined as changes in allele frequencies that occur withinpopulations from generation to generation [Freeman01]. Those small-scalechanges can be driven by any violation of the Hardy-Weinberg criteria. Mi-croevolution therefore represents generation-to-generation allele frequency fluc-tuations that may move in one direction for a while and then other directionfor a while.

Macroevolution:Large scale evolutionarychange representing theaccumulation of many smallscale changes.

Macroevolution, also known as speciation, is defined as large-scale evo-lutionary change that represents the accumulation of many microevolutionarychanges, and produces new species from old ones. The accumulation of mi-croevolutionary changes in reproductively isolated populations of a species cancause those populations to diverge genetically from each other to the pointwhere members of the two populations either no longer recognize each other asprospective mates or are no longer genetically compatible with each other andcannot therefore produce viable offspring. When this happens a new species isformed [Freeman01].

It’s interesting to note that some speciation appears to be able to take placeslowly and gradually over many generations. This pattern of change is referredto as phyletic gradualism. On the other hand, there is strong evidence that

Phyletic gradualism:Speciation that occurs grad-ually over large spans oftime.

some speciation happens rapidly by process called punctuated equilibrium[Eldredge72]. Evolution via punctuated equilibrium occurs when a popula-

Punctuated Equilibrium:Processes that result in rapidmacroevolution.

tion undergoes rapid change that causes them to diverge quickly from otherpopulations of the same species. This rapid form of macroevolution appearsto happen most frequently after major extinction events when many species goextinct and increased ecological space becomes available to surviving species.

The patterns and processes of evolution introduced in this document havebeen tested in a wide range of places and on a diverse array of species. Onehundred and fifty years of critical review of the theory of evolution continue toadd confidence to this scientific explanation of how populations of living thingschange and how new species arise.

End Note

Why does it matter that we understand evolution? As you may have heardearlier this semester, physicists have been on a quest for some time to discovera grand unifying theory that explains the way things work at all scales of nature,from subatomic particles to the level of the universe. Such a theory has thus farevaded their efforts. In biology, however, the modern theory of evolution, whichgrew directly out of Charles Darwin’s theory of evolution by natural selection,provides a grand unifying foundation for all of biology. It combines all fieldsof biology, e.g., cell and molecular biology, genetics, anatomy and physiology,paleontology, zoology, botany, ecology, and even medicine, into one complete

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Figure 5.29: Claudograms representing the processes of phyletic gradualism andpunctuated equilibrium. Phyletic gradualism (left) suggests a gradual changein the morphology of a species. Punctuated equilibrium (right) suggests thatspecies remain mostly invariant for long periods of time, followed by relativelybrief periods of rapid evolution. (Image is in the public domain.)

framework of understanding. Evolution explains the unity and diversity we seeamong living things. It also explains how and why entities such as the HIV(AIDS) virus changes so quickly and is so difficult to treat, how all specieschange over time, the patterns and processes of speciation and extinction, andthe mechanisms by which these kinds of changes take place.

All evidence collected over the more than 150 years since the publication ofDarwin’s “On the Origin of Species”, as well as efforts to challenge and some-times refute the theory of evolution, has done nothing but bolster and confirmthe basic premises and strength of evolution as a description of how life works.The significance of evolution to our understanding of life and how it workswas stated clearly in 1973 by the evolutionary biologist and Russian OrthodoxChristian Theodosius Dobzansky who wrote, “Nothing in Biology Makes SenseExcept in the Light of Evolution.”

REVIEW QUESTIONS

1. Explain the process whereby the genetic code embedded in DNA isused to produce the proteins responsible for expressing traits.

2. Explain how a mutation, or change in the genetic code, can modifyor eliminate a trait.

3. Explain how each of the five Hardy-Weinberg criteria would cause apopulation to evolve.

4. Discuss the differences between phyletic gradualism and punctuatedequilibrium.

5. In what way does the quote by Dobzansky accurately portray thesignificance of evolution to our understanding of biology?

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 169

5.5 Human Evolution I: Anatomical Evidence

OVERVIEW

Summary: In this section and the next, we will present several pieces ofevidence supporting the theory that humans and other primates evolvedfrom a common ancestor. In particular, this section focuses on the compar-ative anatomy of humans and other primates and investigates the evidencefound in the fossil record.

Learning Objectives:

• Explain how scientific observations support the conclusion that thehuman physical body is the result of evolutionary change.

• List anatomical characteristics that provide evidence that all pri-mates, including humans, share a common ancestor.

• List anatomical characteristics that show how humans differ fromother primates, thus showing that the human line diverged fromother primate lines.

• Define what an intermediate form is, and provide chronologicallyorganized examples and characteristics of intermediate forms thathave been discovered in the human line.

Vocabulary:

• Comparative anatomy

• Primates

• Phylogenetic tree

• Intermediate species

As you probably know, human evolution has been a topic of considerablecontroversy over the years both within and outside of religious circles. Since thisis the case it is important for you to know that we will focus this reading andour class discussions on the scientific observations that pertain to this topic. Atthe same time, you should be aware that two separate First Presidencies of theChurch of Jesus Christ of Latter-Day Saints have released official statementsthat pertain to the topic of human evolution. These statements, which representthe official position of the Church on this issue, are included at the end of thetext in the appendix in a document titled “The BYU Evolution Packet” whichwas reviewed and approved by the Board of Trustees of the Church Boardof Education in 1992. For your information, the Board of Trustees includesthe entire First Presidency, selected members of the Quorum of the TwelveApostles, the General Young Women’s President, the General Relief SocietyPresident, and additional board members as assigned by the First Presidency.

In this reading we consider the hypothesis that humans and other primateshave a common ancestor, a species that gave rise to more than one daughterspecies. These daughter species are related to each other only because theyshare the same common ancestor. This hypothesis includes the explanationthat the physical body of modern humans is the result of evolutionary change.We will test this hypothesis by examining two lines of evidence: 1) the com-parative anatomy of modern humans and other living species of primates,

Comparative anatomy:Similarities and differencesin the physical structures oftwo species.and 2) fossil evidence of human evolution. Throughout this reading and reading

5.6 it is important that you realize that the principles we discuss apply not onlyto the evolution of the human body, but are equally applicable to the evolutionof any species.

170 CHAPTER 5. LIFE

Comparative Anatomy of Modern Humans and other Pri-mates

Biologists assign the diversity of life to taxonomic groups such as kingdoms,phyla, classes, families, orders, genera, and species. Organisms are assigned tothese groups based on their anatomical, developmental, and genetic characteris-tics. These traits interestingly also provide strong indications of their respectiveevolutionary histories, and relationships between them and other groups of liv-ing things. In this part of today’s reading we will review observations obtainedmainly through the comparative anatomy of modern humans and other living,non-human primates.

Anatomical traits shared by all living primates

All members of the mammalian order Primates share a body plan that isadapted to an arboreal (tree-dwelling) lifestyle.

Primates:A taxonomic group of mam-mals which includes humans,apes, and monkeys. This conclusion is supported by the observation that all primates, with the

exception of humans, live in tropical or subtropical forests, and most speciesspend a significant amount of their time in trees. As you review these charac-teristics, ask yourself whether humans have these traits or not.

1. Large brain-to-body ratio, especially the neocortex:

• The neocortex (the outer layer of the brain) functions in sensory per-ception, spatial orientation, hand-eye coordination, conscious thought,and, in humans, language.

2. Unique hands and feet:

• Hands and feet with five digits (fingers and toes).

• Tactile pads on tips of digits fleshy tips of fingers and toes provideexcellent grip.

• Flat keratin fingernails instead of claws provide the ability to gripsmall branches where claws just get in the way.

• Opposable thumb - useful for picking up and manipulating smallobjects (precision grip).

• Long inward closing fingers: together with the opposable thumb pro-vide a powerful grasping hand/foot.

3. Perception dominated by touch and vision: Primates have a reduced senseof smell and hearing compared to most other mammals.

• Binocular vision - two forward-facing eyes on the front of the skullhave overlapping fields of view that provide depth perception andthree-dimensional vision.

• Color vision - trichromatic cones (able to differentiate between reds,blues, and greens) or dichromatic cones (able to differentiate bluesand greens) in the retina of the eye allows for color vision. Primatesare the only placental mammals with color vision.

4. Collar bone functions as part of the pectoral girdle (shoulder):

• The arms of primates can reach behind and over the head.

5. Reduced litter size:

• Most primates have just one offspring at a time. This allows arborealforms increased mobility when moving through trees with a clingingyoung, and allows more individual attention to be given to eachindividual offspring.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 171

Evolutionary theory states that the reason modern humans share so manycharacteristics with other primates is that human and non-human primatesshare a common ancestor. Likewise, differences between modern humans andother primates indicate where our shared evolutionary history diverged fromthose of other primates.

Anatomical traits that make modern humans different than otherliving primates

The following list describes some of the anatomical differences between modernhumans (Homo sapiens) and other living primates. Most of these characteristicsare adaptations for bipedalism (walking on two legs), complex and abstractthought, and language:

1. Skull and jaw anatomy:

• Location of the foramen magnum - the opening through which thespinal cord enters the skull:This opening is located at the base ofthe skull in humans, allowing their eyes to look forward while theindividual is standing upright. It is located at the back of the skullin non-human primates that are quadrupeds allowing them to easilylook forward when they are down on all four legs.

• Brow ridge: Modern humans lack a prominent brow ridge, whileother primates have a prominent brow ridge.

• Forehead shape: Modern humans have a tall/high forehead. Otherprimates have a sloped or flat forehead.

• Nose shape: Modern humans have a prominent nose that projectsfrom the plane of the face and features downward pointing nostrils.Monkeys, apes, and chimps have a broad, flat nose that does notproject significantly from the plane of the face, and features forwardopening nostrils .

• Jaw shape: Modern humans have a thinner jaw that does not projectbeyond the plane of the face. Non-human primates have a broaderjaw that extends beyond the plane of the face.

• Dentition: Modern humans have smaller teeth, and the canines arethe same size as other teeth. Other primates have larger teeth, withespecially prominent canines.

• Brain size: Modern humans have a larger brain than any living pri-mate, with an adult brain size of about 1350 cubic centimeters (cc).Only Neanderthals had a larger brain size, at about 1450 cc. Alarge brain is believed to be a precondition for language and abstractthought. Other primates have significantly smaller brains than mod-ern humans, e.g., the brain of a chimp is about 400 cc and the brainof a gorilla is about 500 cc. To help you understand how big thesebrain sizes are, the volume of a baseball is 405 cc.

2. Pelvis and femur anatomy (see Figure 5.30):

• Pelvis shape: Modern humans have a tall, bowl-shaped pelvis whichsupports the weight of the organs of the abdominal cavity, and allowsa large surface for the attachment of a large gluteus maximus muscleneeded for effective walking. Other primates have a tall, flat pelvisthat is horizontal to the ground when they are on all fours, theseanimals use strong abdominal muscles to support the organs of theabdominal cavity, and the lack of a bowl-shaped pelvis allows onlya small gluteus maximus muscle to be attached there.

172 CHAPTER 5. LIFE

• Location of the ball-and-socket joint of the femur on the pelvis: Inmodern humans this joint is located midway back along the pelvis,directly under the center of gravity of the body when standing up-right. In other primates this joint is located at the back of the pelviswhen the animal is on all fours, and is behind of the center of gravitywhen the animal is upright.

• Angle of the femur relative to the midline of the body: In modernhumans the femur angles inward from the ball and socket joint to-ward the midline of the body, thus placing the feet just on eitherside of the midline, directly under the center of gravity, and allowinga smooth walking gait In other primates the femur extends nearlystraight down from the pelvis. The feet are consequently fartherremoved from the center of gravity when they are upright, so theywaddle when they walk on two legs

Figure 5.30: Knee and pelvis joints in primate species. The three specimensshown include a modern chimpanzee (left), Australopithecus afarensis (center),and a modern human (right). (Image is in the public domain.)

3. Knee structure:

• In modern humans the patella (knee cap) is able to lock the kneewhen the leg is fully extended. The knee of other primates does notlock and the leg remains slightly bent, even when fully extended.

4. Foot shape (see Figure 5.31):

• The foot of modern humans has a forward pointing big toe, an el-evated arch, and is not a prehensile structure. The foot of otherprimates has a big toe that points to the side and functions as anopposing digit, like a thumb, and is flat, lacking an arch all together.

5. Arm and leg length ratio:

• Modern humans have longer legs than arms. Apes and monkeys havearms that are as long or longer than their legs.

By comparing similarities and the differences among any group of organisms,we can place them into taxonomic groups (i.e., scientifically named groups) andestimate how closely related they are to each other. One common way to do

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 173

Figure 5.31: Ape feet compared to human feet. The blue line represents theareas where the weight is supported during walking motion. The quadrupedsape support their weight entirely on one side of the foot. Bipeds transfer theweight across the ball of the foot and to the big toe. (Image is in the publicdomain.)

this is by developing a diagram called a phylogenetic tree or cladogram.This approach to determining and indicating evolutionary relatedness betweengroups will be addressed in more detail in the next section of the text. HoweverFig. 5.32 shows a phylogenetic tree of the living mammals, with an emphasison the primates.

Phylogenetic Tree:A diagram representing evo-lutionary linkages betweenspecies.Remember that, as scientists, we make testable, verifiable predictions, and

phylogenetic trees represent predictions of relatedness between groups of or-ganisms. This means that the idea that hominids and other primates share acommon ancestry is a testable, potentially falsifiable hypothesis. The methodsand data used to test this or any other scientific hypothesis must be rigorousand be held to the highest scientific standard. If any one of the predictionsrelated to the hypothesis fails, that result will lead to a re-evaluation of thehypothesis. If, however, after rigorous tests are performed and the predictionsare confirmed time and time again we gain stronger confidence in the predic-tions stated in the hypothesis until all researchers in the field universally acceptit. Some scientific explanations are so well established that no new evidenceis likely to alter them. It is at this point the explanation becomes a scientifictheory, a firmly established scientific fact.

Many predictions, tests, and even challenges have been made over the past150 years with regard to the theory of relatedness between modern humans andother primates. This association was first implied by the principles outlined inDarwin’s book, “On the Origin of Species”. Some of the observations used toshow relatedness between humans and other primates are included in the listsof anatomical similarities and differences shown above. We will now consideranatomical data from the fossil record that can be used to test the hypothesisthat the modern human body is the result of evolutionary change.

The Fossil RecordIntermediate form:A species that is intermedi-ate in form and time betweentwo other species.Our first prediction, based on the hypothesis that modern humans and apes

share a common ancestor, is that there should be intermediate forms betweenmodern humans and their ancestral species if the human body is indeed theresult of evolutionary change. Intermediate forms should demonstrate ways in

174 CHAPTER 5. LIFE

Figure 5.32: A phylogenetic tree or cladogram including the primates and rep-resentatives of other orders of mammals. The more closely animals are relatedto each other, the more closely linked they are on the tree. This means thatour most distant living mammal relative is the group containing the duck-billed platypus, and our closest relations are chimpanzees and bonobos. (Imagecourtesy of Fred Hsu, used under the terms of the GNU Free DocumentationLicense)

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 175

which ancient fossil species could have given rise to modern species. In orderfor an organism to be considered an intermediate form it must meet at leasttwo criteria. First, the organism must be intermediate in its anatomy in thatit must have some traits found in earlier species, and it must also display atleast some anatomical structures that make it different that its ancestral species.Second, the intermediate form must be intermediate in time. Scientists test thisprediction by using the relative and absolute dating methods that we discussedin sections 4.1 and 4.2 of this text. Fossils of the intermediate forms mustbe dated to a period that occurs after the appearance of an ancestral species,but also earlier than the younger species for which it is an intermediate form.Additionally, if fossils are spread over a geographic area, fossils of intermediateforms should generally be found in intermediate geographical locations that liebetween the ranges of the older and younger forms. In order to assess the valueof a fossil as an intermediate form between modern humans and their commonancestor with apes, scientists will also consider the basic anatomical differencesbetween apes and modern hominids that were discussed earlier in this reading.

At this point we will examine a number of fossil species that have beendiscovered that show evidence of being intermediate forms between modernhumans and a non-hominid common ancestor that humans share with otherprimates. As we begin this part of this case study be careful not to fall into themisconceived notion that modern humans evolved from apes. Modern humansand apes are both living species, and the actual prediction is that modern apesand modern humans both evolved from a common ancestor.

Before we examine evidence of intermediate forms between modern humansand our last shared common ancestor with chimpanzees you need to be awarethat science has not yet discovered a fossil that has been widely recognized asthe link between the ancestral line that gave rise to modern humans and the linethat gave rise to chimpanzees. Using a technique called a molecular clock (thatwill not be described here), however, scientists concluded that the hominid lineand the chimpanzee line probably diverged from each other between about 8and 6 million years ago.

Before we begin our review of selected intermediate forms, take a look at thethree phylogenetic trees in Figure 5.33. Each tree presents a slightly differentinterpretation of how intermediate hominid forms may have been related toeach other. While there are slight differences between the trees, take note thatall experts in the field affirm the conclusion that the bodies of modern humansare the result of evolutionary change, and that there are certain aspects of everytree that are consistent within all three trees.

Introduction to selected intermediate forms Ancestral traits:traits that are shared with anancestral species.This section of the reading presents information about selected intermediate fos-

sil forms between hominids and their last common ancestor with chimpanzees.We will start with the forms that appeared longest ago in the fossil record andmove forward until we reach modern humans. As you review the characteristicsassociated with each intermediate form you will see a table of anatomical traitsthat shows characteristics that are considered to be ancestral, or similar totraits the hominid and chimp common ancestor most likely had, on the left sideof the table. Characteristics considered to be derived, or more modern, areon the right side of each table. Derived traits that are new for a particularintermediate form are highlighted in red font.

Derived traits:traits that a descendantspecies has, but that its an-cestral species did not have.Before we start our review of intermediate forms, take a moment to review

the list of characteristics that a common ancestor of both hominids and chimpswould most likely have had (see Table 5.5). All of these characteristics areconsidered to be ancestral, so there are no derived traits in this table.

176 CHAPTER 5. LIFE

Figure 5.33: These phylogenetic trees show different interpretations of rela-tionships between intermediate forms of hominids. Though there are slightdifferences between the trees, there is no disagreement among the three leadingspecialists who produced these trees about whether there are intermediate formsor that modern humans are the result of evolutionary change. You should alsonote that the question marks in the trees show areas where the specialists stillhave questions about details of the tree. The uppermost tree is based on thework of Donald Johanson of the University of Arizona, and a co-discoverer of thefossil “Lucy”. The middle tree is based on the work of Ian Tattersall, a curatorat the American Museum of Natural History. And the bottommost tree is basedon the work of Bernard Wood of George Washington University. (Images cour-tesy of http://www.becominghuman.org/node/interactive-documentary whichprovides this information as part of an interactive download.)

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 177

Ancestral Traits• Basic body plan is quadrupedal• Arms as long or longer than the legs• Organs of the abdominal cavity supported by strong abdominal muscles• Skull and jaw◦ Foramen magnum at back of skull◦ Pronounced brow ridge◦ Forehead flat or sloped◦ Brain probably less than 350 cc in volume◦ Flat face, with forward opening nostrils◦ Wide (tall) lower jaw that projects beyond the plane of the face◦ Large teeth, especially canines• Pelvis, leg, and foot◦ Pelvis tall, flat, horizontal to the ground when animal is on all fours◦ Small gluteus maximus muscle attachment surfaces on the flat pelvis◦ Femur ball and socket joint at the back of the pelvis◦ Femur extends straight down from pelvis◦ Leg cannot lock at the knee◦ Foot flat, with big toe to the side of the foot• Lower spine shape straight, no significant curvature

Table 5.5: Summary of anatomical traits that a common ancestor of hominidsand chimps are theorized to have had.

Sahelanthropus tchadensis

We start with the oldest known possible hominid form, Sahelanthropus tchaden-sis (see Figure 5.34). This species lived 6.9 6.2 Ma (million years ago). Enoughof the skull has been recovered to show its general form, and the strong likeli-hood that it was bipedal. Too little of the rest of its skeleton has been found,however, to make definite statements about other anatomical traits it had.Some paleontologists have suggested that this species could be a common an-cestor between hominids and chimps, but others contend that this species mayhave gone extinct before the hominid and chimp lines diverged, others statethat the chimp-hominid split occurred before this species appeared in the fossilrecord. Until more complete fossil remains are located, however, the sugges-tion that this is the common ancestor of humans and chimps remains firmlyentrenched in the realm of hypothesis, not theory.

Figure 5.34: Fossil skull of Sahelanthropus tchadensis. (Image courtesy of DidierDescouens, Creative Commons Attribution - Share Alike 3.0 Unported license.)

178 CHAPTER 5. LIFE

Sahelanthropustchadensis

Ancestral

traitsD

erivedtraits

•B

ipedalismstrongly

suggested•

Arm

sas

longor

longerthan

thelegs

(?)•

Organs

ofthe

abdominal

cavitysupported

bystrong

abdominal

muscles

(?)•

Skulland

jaw•

Skulland

jaw◦

Foramen

magnum

atbase

ofskull

◦P

ronouncedbrow

ridge◦

Foreheadflat

orsloped

◦B

rainvolum

e320-380

cc◦

Flat

face,w

ithforw

ardopening

nostrils◦

Wide

(tall)low

erjaw

thatprojects

beyondthe

planeof

theface

◦Sm

allcanine

teeth•

Pelvis,

leg,and

foot◦

Pelvis

tall,flat,

horizontalto

theground

when

animal

ison

allfours

(?)◦

Small

gluteusm

aximus

muscle

attachment

surfaceson

theflat

pelvis(?)

◦Fem

urball

andsocket

jointat

theback

ofthe

pelvis(?)

◦Fem

urextends

straightdow

nfrom

pelvis(?)

◦L

egcannot

lockat

theknee

(?)◦

Footflat,

with

bigtoe

tothe

sideof

thefoot

(?)•

Low

erspine

shapestraight,

nosignificant

curvature(?)

•B

odym

ass(?)

•B

odyheight

(?)

Table

5.6:A

natomical

traitsof

S.tchadensis.

The

questionm

arksindicate

traitsfor

which

thereis

nofossil

evidence.T

raitsindicated

inred

representobserved,

derived(i.e.

more

modern)

traitsthat

firstappeared

inS.

tchadensis.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 179

Ardipithecus ramidus

The next intermediate form we will review is Ardipithecus ramidus (nicknamed“Ardi”; see image, below). This species lived about 4.4 million years ago,well after the divergence of hominid and chimp lines. An amazingly completeskeleton of this species was discovered in 1994, and after several years of dedi-cated analysis of that fossil the entire October 9th, 2009, issue of the prestigiousjournal Science (http://www.sciencemag.org/ardipithecus/) was devoted to theformal announcement of this discovery and eleven reports on its anatomy andpossible position within the ancestry of hominids. Note: If you are interested,you may peruse those articles by clicking on the link provided above. The arti-cles describe the anatomy of the skull, limbs, pelvis, legs, and other anatomicalstructures that are useful in showing the relationship between “Ardi” and otherspecies.

Though there are still intense discussions regarding where “Ardi” fits in thehominid line, most paleontologists firmly include this species in the hominidline. The skeletal reconstruction in the figure at the right shows an extremelyinteresting combination of anatomical traits. This combination of ancestral andderived traits is shown in Table 5.7.

Figure 5.35: Left: A digital reconstruction of the fossil skull of Ardipithecusramidus. (Image courtesy T. Michael Keesey, Creative Commons Attribution2.0 general license) Right: An artist’s concept of Ardipithecus ramidus. (Imagecourtesy Jason Sunnar)

180 CHAPTER 5. LIFE

Ardipithecus

ramidus

Ancestral

traitsD

erivedtraits

•B

ipedalismstrongly

suggested•

Arm

sas

longor

longerthan

thelegs

•O

rgansof

theabdom

inalcavity

supportedby

pelvis•

Skulland

jaw•

Skulland

jaw◦

Foramen

magnum

atbase

ofskull

◦P

ronouncedbrow

ridge◦

Foreheadflat

orsloped

◦B

rainvolum

e300-350

cc◦

Flat

face,w

ithforw

ardopening

nostrils◦

Wide

jawthat

projectsbeyond

theplane

ofthe

face◦

Small

canineteeth

•P

elvis,leg,

andfoot

•P

elvis,leg,

andfoot

◦Short,

bowl-shaped

pelvisthat

cansupport

abdominal

organs◦

Small

gluteusm

aximus

muscle

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urextends

straightdow

nfrom

pelvis◦

Knee

possiblyable

tolock

◦Foot

flat,w

ithbig

toeto

theside

ofthe

foot•

Low

erspinal

column

somew

hatcurved

•B

odym

ass∼

110lbs.

•B

odyheight

∼3’11”

Table

5.7:A

natomical

traitsof

A.

ramidus.

Traits

indicatedin

redrepresent

observed,derived

(i.e.m

orem

odern)traits

thatfirst

appearedin

thisspecies.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 181

Australopithecus afarensis

The next intermediate form we will examine is Australopithecus afarensis (seeimage below). If you take another look at Figure 5.33 of the family trees ofhumans you will see that most scientist place A. afarensis at the base of thetree that gave rise to modern humans. A. afarensis lived between 3.8 and 2.9Ma. This was an extremely successful species, and it is well represented in thefossil record with specimens collected from over 300 individuals. In the 1970sresearchers found one unusually complete skeleton of an adult female. The fossilfind included a complete lower jaw, parts of the skull, most of the pelvis, onefemur, and many other skeletal structures that provided information about hersize, shape, and anatomy. Interestingly, the paleontologists who discovered herfelt that she deserved a name, so they named her “Lucy”, after the popularBeatles song “Lucy in the Sky with Diamonds”.

“Lucy” has an interesting set of characteristics that make this species animportant intermediate form. These traits are listed in Table 5.8.

Figure 5.36: Fossil remains and artist’s concepts of Australopithecus afarensis.The first image is of a cast found at the Museum National d’Historie Naturellein Paris, France. The second image is an assembled cast of the same specimenfound at the Senckenberg Museum in Frankfort, Germany. The third image isfrom a display at the Cosmocaixa Museum in Barcelona, Spain. It is worthnoting that additional details on the shape and size of the skull can be ac-quired from other A. afarensis specimens. (Images licensed under the GNUFree Documentation License.)

182 CHAPTER 5. LIFE

Australopithecus

afarensisA

ncestraltraits

Derived

traits•

Bipedalism

clearlydem

onstrated•

Arm

sas

longor

longerthan

thelegs

•O

rgansof

theabdom

inalcavity

supportedby

pelvis•

Skulland

jaw•

Skulland

jaw◦

Foramen

magnum

atbase

ofskull

◦P

ronouncedbrow

ridge◦

Foreheadflat

orsloped

◦B

rainvolum

e400

cc◦

Flat

face,w

ithforw

ardopening

nostrils◦

Wide

jawthat

projectsbeyond

theplane

ofthe

face◦

Small

canineteeth

•P

elvis,leg,

andfoot

•P

elvis,leg,

andfoot

◦Short,

bowl-shaped

pelvisthat

cansupport

abdominal

organs◦

Small

gluteusm

aximus

muscle

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urangles

toward

them

idlineof

thebody

◦K

neepossibly

ableto

lock◦

Foothas

anarch

andforw

ardpointed

bigtoe

•L

ower

spinalcolum

nsom

ewhat

curved•

Body

mass∼

65-90lbs.

•B

odyheight

∼3.5-5’

Table

5.8:A

natomical

traitsof

A.

afarensis.T

raitsindicated

inred

representobserved,

derived(i.e.

more

modern)

traitsthat

firstappeared

inthis

species.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 183

Homo habilis

The next intermediate form is Homo habilis, nicknamed “Handy Man” (Figure5.37). This species lived 2.4-1.4 Ma, and is placed on the direct line that gaverise to modern humans. It is nicknamed “Handy Man” because the earliestknown stone tools are found in the same rock strata and location as fossils ofH. habilis. You should note the general shape of the skull. It is larger and morehuman-like than any preceding species, such as that of A. afarensis (Figure5.38).

Figure 5.37: Skull and artist’s reconstruction of Homo habilis. (Left image cour-tesy of “Lillyundfreya”, Creative Commons Attribution Share-Alike license)

Figure 5.38: Reconstructed skull of A. afarensis, for comparison to that of H.habilis above. (Image is in the public domain.)

184 CHAPTER 5. LIFE

Hom

ohabilis

Ancestral

traitsD

erivedtraits

•B

ipedalismclearly

demonstrated

•L

egsas

longor

longerthan

arms

•O

rgansof

theabdom

inalcavity

supportedby

pelvis•

Skulland

jaw•

Skulland

jaw◦

Foramen

magnum

atbase

ofskull

◦P

ronouncedbrow

ridge◦

Foreheadless

slopedthan

previousspecies

◦B

rainvolum

e510-600

cc◦

Flat

face,w

ithforw

ardopening

nostrils◦

Wide

jawthat

projectsbeyond

theplane

ofthe

face◦

Small

teeth,including

canines•

Pelvis,

leg,and

foot•

Pelvis,

leg,and

foot◦

Short,bow

l-shapedpelvis

thatcan

supportabdom

inalorgans

◦Sm

allgluteus

maxim

usm

uscle(?)

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urangles

toward

them

idlineof

thebody

◦K

neeable

tolock

◦Foot

hasan

archand

forward

pointedbig

toe•

Low

erspinal

column

somew

hatcurved

•B

odym

ass∼

70lbs.

•B

odyheight

∼3.5-4.5’

•T

oolm

aking

Table

5.9:A

natomical

traitsof

H.

habilis.T

raitsindicated

inred

representobserved,

derived(i.e.

more

modern)

traitsthat

firstappeared

inthis

species.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 185

Homo ergaster

The next species is Homo ergaster. This species lived 1.9-1.4 Ma, and is nearlyuniversally accepted as being the species that is a direct line ancestor of youngerHomo species (see the three phylogenetic trees in Figure 5.33). It also gave riseto Homo erectus, a species that was a side-branch off of the main hominid treethat lived from about 1.4 Ma until perhaps as recently as 70,000 years ago, andwas the first hominid species to move out of Africa into Europe and Asia.

H. ergaster is known for having a thinner skull and higher forehead than itspredecessors.

Figure 5.39: Reconstructed skull of Homo ergaster. The key features to noteare the higher forehead compared to those of previous species. (GNU FreeDocumentation License.)

Figure 5.40: Reconstruction of Homo ergaster by Nikolas Zalotockyj. (Imageis in the public domain.)

186 CHAPTER 5. LIFE

Hom

oergaster

Ancestral

traitsD

erivedtraits

•B

ipedalismclearly

indicated•

Legs

aslong

orlonger

thanarm

s•

Organs

ofthe

abdominal

cavitysupported

bypelvis

•Skull

andjaw

•Skull

andjaw

◦Foram

enm

agnumat

baseof

skull◦

Pronounced

browridge

◦Forehead

lesssloped

thanprevious

species◦

Brain

volume

800-900cc

◦D

ownw

ardpointing

nostrils◦

Narrow

erjaw

◦Sm

allteeth,

includingcanines

•P

elvis,leg,

andfoot

◦Short,

bowl-shaped

pelvisthat

cansupport

abdominal

organs◦

Large

gluteusm

aximus

muscle

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urangles

toward

them

idlineof

thebody

◦K

neeable

tolock

◦Foot

hasan

archand

forward

pointedbig

toe•

Low

erspinal

column

somew

hatcurved

•B

odym

assto

150lbs.

•B

odyheight

∼5’10”

•T

oolm

aking

Table

5.10:A

natomical

traitsof

H.

ergaster.T

raitsindicated

inred

representobserved,

derived(i.e.

more

modern)

traitsthat

firstappeared

inthis

species.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 187

Homo heidelbergensis

Homo heidelbergensis is the next intermediate form we will consider. Thisspecies lived between about 1.3 Ma and 200,000 years ago, and is the commonancestor of modern humans and of Homo neanderthalensis. Scientists concludedthat modern humans and Neanderthals are related to each other only by thiscommon ancestor. H. heidelbergensis is the first hominid known to routinelyuse fire, build shelters, and use spears to hunt large animals. Fossils of thisspecies have been found in Africa, Europe, and parts of Asia.

Figure 5.41: Artist’s rendition of Homo heidelbergensis (left, image courtesy ofJose Luis Martinez Alvarez, Creative Commons Attribution - Share Alike 2.0Generic license), and a model of a H. heidelbergensis lower jaw at the Instituteof Earth Sciences, University of Heidelberg (right, image courtesy of “Gerbil”,Creative Commons Attribution-Share Alike 3.0 Unported license).

188 CHAPTER 5. LIFE

Hom

oheidelbergensis

Ancestral

traitsD

erivedtraits

•B

ipedalismclearly

indicated•

Legs

aslong

orlonger

thanarm

s•

Organs

ofthe

abdominal

cavitysupported

bypelvis

•Skull

andjaw

•Skull

andjaw

◦Foram

enm

agnumat

baseof

skull◦

Pronounced

browridge

◦Forehead

lesssloped

thanprevious

species◦

Brain

volume

1,100-1,400cc

◦D

ownw

ardpointing

nostrils◦

Narrow

erjaw

◦Sm

allteeth,

includingcanines

•P

elvis,leg,

andfoot

◦B

owl-shaped

pelvisthat

cansupport

abdominal

organs◦

Large

gluteusm

aximus

muscle

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urangles

toward

them

idlineof

thebody

◦K

neeable

tolock

◦Foot

hasan

archand

forward

pointedbig

toe•

Low

erspinal

column

somew

hatcurved

•B

odym

ass∼

110-150lbs.

•B

odyheight

∼5-6’

•T

oolm

aking◦

Fire

◦B

uildingstructures

◦F

irstspears,

usedto

huntlarge

animals

•L

ossof

bodyhair

(determined

bystudies

onthe

evolutionof

pubiclice

andbody

lice)

Table

5.11:A

natomical

traitsof

H.

heidelbergensis.T

raitsindicated

inred

representobserved,

derived(i.e.

more

modern)

traitsthat

firstappeared

inthis

species.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 189

Homo neanderthalensis

The next species we will consider, Homo neanderthalensis (Neanderthal man),is not an intermediate form, but is a species that existed at same time as modernhumans. H. neanderthalensis lived between 200,000 and 28,000 years ago andis related to humans by their shared common ancestor, H. heidelbergensis. Ne-anderthals had mostly modern looking facial features, but had a heavier, moremuscular body than Homo sapiens. Paleontologists agree that Neanderthalswere not the ancestors of H. sapiens, but were a side-branch group that evolvedand migrated out of Africa before modern humans did.

Anatomical traits of H. neanderthalensis are listed in Table 5.12.The leading explanation about why H. neanderthalensis went extinct in-

cludes environmental and physiological explanations. The physiological partof the explanation states that because of the heavier, more muscular bodies ofNeanderthals, they would have had to expend about 30% more energy to getfrom one place to another than the more slightly built H. sapiens. Also, whilea big brain probably seems like a good adaptation, the brain consumes about20% of all the energy used by the body. As you may have noted in Table 5.12Neanderthals have big brains - up to 1450 cc in size. So, when you combinethe extra energy required to move a heavier body and run a big brain, Nean-derthals needed hundreds of calories more each day than modern human bodieswould have needed. If something happened that affected Neanderthals’ foodsupply, Neanderthals would suffer the effects of a food shortage very quickly.This is probably what happened. Paleontologists and archaeologists discoveredthat Neanderthals ate large mammals almost exclusively. Other evidence showsthat there was a global cooling event that occurred starting about 70,000 yearsago, and that those changes probably caused a shift in the ranges and abun-dances of their prey species, and may have even included indirect competitionfor the same food with other predators. As a result food became scarce, andNeanderthals most likely starved and froze to death because of their inability orunwillingness to shift to eat other kinds of food. The last known Neanderthalsite was in southern Spain, at the Straits of Gibraltar, about 28,000 years ago.

Figure 5.42: An artist’s recreation of Homo sapiens-neanderthal. (Image is inthe public domain.)

190 CHAPTER 5. LIFE

Hom

oneanderthalensis

Ancestral

traitsD

erivedtraits

•B

ipedalismclearly

indicated•

Legs

aslong

orlonger

thanarm

s•

Organs

ofthe

abdominal

cavitysupported

bypelvis

•Skull

andjaw

•Skull

andjaw

◦Foram

enm

agnumat

baseof

skull◦

Pronounced

browridge

◦Forehead

lesssloped

thanprevious

species◦

Brain

volume

1,400-1,450cc

◦D

ownw

ardpointing

nostrils◦

Narrow

erjaw

◦Sm

allteeth,

includingcanines

•P

elvis,leg,

andfoot

◦B

owl-shaped

pelvisthat

cansupport

abdominal

organs◦

Large

gluteusm

aximus

muscle

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urangles

toward

them

idlineof

thebody

◦K

neeable

tolock

◦Foot

hasan

archand

forward

pointedbig

toe•

Low

erspinal

column

somew

hatcurved

•B

odym

ass∼

120-145lbs.

•B

odyheight

∼5’

-5’6”

•T

oolm

aking◦

Fire

◦B

uildingstructures

◦Spears,

usedto

huntlarge

animals

•L

ossof

bodyhair

•E

videnceof

symbolism

andabstract

thinking

Table

5.12:A

natomicaltraits

ofH

.neanderthalensis.

Traits

indicatedin

redrepresent

observed,derived(i.e.

more

modern)

traitsthat

firstappeared

inthis

species.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 191

Figure 5.43: Comparison of Homo sapiens and Homo sapiens-neanderthalskulls. (Image courtesy of “hairymuseummatt”, used under the terms of theCreative Commons Attribution Share-Alike license)

Homo sapiens

Homo sapiens, “modern humans”, are the last group we will examine. Thisspecies evolved in Eastern Africa about 200,000 years ago during a time ofmajor environmental upheavals in that part of the world. Sometime after itevolved H. sapiens also migrated out of Africa, and eventually became thesole surviving species of hominids on the planet. H. sapiens demonstrate aresilient ability to adapt to different conditions, food sources, etc. When onetype of food is scarce, they readily switch to other prey, and have a relativelyindiscriminate diet. The lighter body build and slightly smaller brain gives anenergetic advantage when compared to Neanderthals.

Characteristics of modern humans are listed in Table 5.13.

Summary

The observations included in this reading show that the modern human bodyhas many anatomical similarities and many differences when compared to otherprimates, as well as to other hominids. The abundance of intermediate forms,each displaying changes in anatomical structures becoming less like the ancestralform and more like the derived (modern) form provides strong evidence thatthe human body is the result of evolutionary change.

So far all of the empirical anatomical evidence discovered to date supportsthe hypothesis that the physical human body is the result of a process of evo-lutionary change. In the next reading we will investigate additional sourcesof evidence, namely genetic evidence, to continue to test the hypothesis thatpatterns and processes of evolution produced the bodies of modern humans.

192 CHAPTER 5. LIFE

Hom

osapiens

Ancestral

traitsD

erivedtraits

•B

ipedalismclearly

indicated•

Legs

aslong

orlonger

thanarm

s•

Organs

ofthe

abdominal

cavitysupported

bypelvis

•Skull

andjaw

•Skull

andjaw

◦Foram

enm

agnumat

baseof

skull◦

Greatly

reducedbrow

ridge◦

Tall

forehead◦

Brain

volume

1,300cc

◦D

ownw

ardpointing

nostrils◦

Narrow

erjaw

◦Sm

allteeth,

includingcanines

•P

elvis,leg,

andfoot

◦B

owl-shaped

pelvisthat

cansupport

abdominal

organs◦

Large

gluteusm

aximus

muscle

◦P

elvis-femur

jointin

linew

ithspinal

column

when

standingupright

◦Fem

urangles

toward

them

idlineof

thebody

◦K

neeable

tolock

◦Foot

hasan

archand

forward

pointedbig

toe•

Low

erspinal

column

somew

hatcurved

•B

odym

ass∼

100-200lbs.

•B

odyheight

∼5’

-6’

•T

oolm

aking◦

Fire,

buildingstructures

◦D

istancehunting

weapons

(arrows,

throwing

spears),farm

ing,language,

modern

technologies•

Loss

ofbody

hair

Table

5.13:A

natomical

traitsof

H.

sapiens.T

raitsindicated

inred

representobserved,

derived(i.e.

more

modern)

traitsthat

firstappeared

inthis

species.

5.5. HUMAN EVOLUTION I: ANATOMICAL EVIDENCE 193

REVIEW QUESTIONS

1. Why are humans considered primates?

2. Name some of the similarities and differences in the anatomy of mod-ern humans and apes. Discuss the advantages and disadvantages ofthese anatomical features.

3. What does a phylogenetic tree tell you?

4. What is an intermediate form?

5. Is there some other way to explain the similarities and differencesin human and ape anatomies? Is there some other way to explainwhat we see in the fossil record? Do these explanations qualify asscientific hypotheses or theories?

194 CHAPTER 5. LIFE

5.6 Human Evolution II: Anatomy and Genet-ics

OVERVIEW

Summary: A continuation of the discussion on human evolution, focus-ing on vestigial structures, homologous structures, and genetic evidence.

Learning Objectives:

• List vestigial structures that exist in the human body, and explainwhy the existence of these structures represents evidence of the evo-lutionary history of humans.

• List examples of homologous anatomical structures, and explain howthese structures help scientists to identify relatedness between groupsof species.

• Explain what a pseudogene is, and carry out a phylogenetic analysisof DNA, explaining how the results of that analysis show evidenceof the degree of relatedness between species.

• List a few examples of how small differences in the timing and du-ration of developmental regulatory gene activity can produce signif-icant anatomical differences between closely related species.

• Recognize that the conclusion that the human physical body is theresult of evolutionary change does not contradict or challenge anyauthoritative statements on the origin of man made by First Presi-dencies of the Church.

Vocabulary:

• Vestigial structures

• Pseudogenes

• Homologous structures

This reading is a continuation of the previous section: a case study usingthe physical human body as an example of evolutionary change. The first partof this section addresses additional material related to anatomical evidence thatthe human body has changed over time: vestigial structures and homologousstructures. The latter part of this reading focuses on evidence of genetic change.

Vestigial Structures

Scientists observe that the anatomy of any living species retains some hints ofits ancestry. Therefore, if the human body has undergone evolutionary change,some clues of that change should still exist in our modern bodies. Some ofthis evidence may be puzzling at first, since these structures sometimes haveno known function in the body, yet the same structure is known to play a vitalrole in the bodies of other species. This type of non-functional organ is referredto as a vestigial structure.

Vestigial structure:An anatomical structurethat played a vital role inthe body of an ancestralspecies, but no longer playsa vital role or may be com-pletely non-functional inthe body of the descendantspecies.

Before we address the question of whether there are vestigial structures inthe human body, let’s consider an example of a vestigial structure in a non-human species. Did you know that some species of whales have hips? Thatis, these whales have a pelvis, as well as femur bones. The forelimbs of whalesevolved into flippers that play an important role in maintaining stability asthey swim, but they have no hind limbs, just the tail that developed into thepowerful flukes that they use for generating thrust as they swim. As you readin section 5.1 of this text, whales are air-breathing mammals that evolved from

5.6. HUMAN EVOLUTION II: ANATOMY AND GENETICS 195

a land-dwelling ancestor. Since this is the case, you may predict that modernwhales have at least some vestigial structures associated with their terrestrialancestry. When early comparative anatomists first examined skeletons of largewhales they were surprised to discover that they had a vestigial pelvis and femurbones. These bones are covered completely by tissues of the body so there isno external evidence of their existence, and they have no known function (seeFigure 5.44).

Figure 5.44: The upper drawing shows the skeleton of a bowhead whale, andthe letter “C” marks the location of a vestigial pelvis and femur. The lowerdrawing is of the skeleton of a sperm whale, the largest species of toothed whale,and the letter “p” indicates a vestigial pelvis in that species. (Images are inthe public domain.)

Let’s now turn our attention to the question about vestigial structures inhumans. Several such structures have been identified, and some of these aredescribed in Table 5.14. The characteristics listed in this table clearly showthat we share characteristics in common with other organisms, and that someof these traits have become non-functional in our bodies. This evidence ofchange supports the prediction that the human body has undergone evolution-ary change. This is the case, because vestigial structures provide evidence of aconnection to other species via a common ancestor, as well as of evolutionarychange in the species of interest; in this case, humans.

Homologous Traits

Anatomical features other than vestigial structures also provide insights intoevolutionary changes that a species has experienced. These include homolo-gous traits, which are features that come from the same developmental origin,but produce structures of different sizes, shapes, and functions in different re-lated species.

Homologous traits:traits that are similar be-cause they are inherited froma common ancestor.

When we look at the skeletal anatomy of humans and other mammals wesee many structures that are homologous traits. The first example we willexamine is the structure of the mammalian/reptilian forelimb (Figure 5.45).The homology of bones of the forelimb is obvious in that each forelimb has thesame bones in the same general orientation to each other, even thought thelarge-scale structures produced carry out significantly different functions.

Because all mammals and reptiles came from the same ancestral species,it is not surprising that the basic skeletal anatomy of mammalian forelimbs isthe same. That is, they all have a humerus bone in the upper arm, radiusand ulna forearm bones, wrist bones, and bones of the digits (fingers). At thesame time, it is perfectly logical that the sizes and lengths of these bones will

196 CHAPTER 5. LIFE

Trait

Fu

nction

inn

on-h

um

ans

Fu

nction

inhu

man

sA

ppendix(a

closed-endedpouch

Used

byherbivores

todigest

Mostly

non-functional,but

some

evidencesuggests

attachedto

theintestine)

cellulosefrom

plantm

aterialthat

itm

ayhelp

retainbacteria

Coccyx

(thetail

bone)L

ongtail

usedfor

balanceN

on-functionalas

atail,

butis

anattachm

entpoint

forsom

em

uscles.W

isdomteeth

Grinding

offibrous,

toughU

suallynon-functional

foodM

usclesof

theexternal

earO

rientsthe

eartow

ardsounds

Non-functional

Plica

semilum

inaris(a

small

Nictiating

mem

brane(extra

Non-functional

foldin

thetissue

inthe

eyelid),in

some

fishesand

cornerof

theeye

reptilesH

air/furInsulation

andspecies

Mostly

non-functional,but

hairon

thehead

recognitionprovides

insulation,and

hairunder

thearm

sand

inthe

groinalong

with

oilysw

eatreduce

chaffing

Arrector

pili(sm

allm

usclesR

aiseshair

totrap

airN

on-functional,but

causesgoose

bumps

attachedto

thebase

ofhair

between

theskin

andfur

tofollicles

increaseinsulation,

orto

raisethe

furto

appearlarger

asa

bluffing

displayP

almer

graspreflex

(infantsA

llows

infantsto

hangonto

aN

on-funtional,but

researchshow

sthat

37%of

graspthings

thattouch

theirparent’s

furinfants

testedcan

supporttheir

own

weight

inpalm

thism

anner

Table

5.14:A

listof

afew

vestigialstructures

foundin

them

odernhum

anbody.

5.6. HUMAN EVOLUTION II: ANATOMY AND GENETICS 197

differ between species to reflect the evolutionary changes different species wentthrough to produce the different sizes, shapes, and functions of forelimbs wesee today. The human arm is used for reaching and grasping, the mole forelimbis used for digging and clawing, the whale forelimb is used for swimming, andthe bat forelimb is used for flight. Groups of homologous traits become obviousonce you start to compare traits among any related group of organisms. And,since humans share a huge number of homologous traits with other organisms,these traits provide evidence of shared evolutionary histories. These traits alsoshow the degree to which each species that shares the same ancestry has evolvedfrom the other species that shares that same history.

Further study of mammalian skeletons reveals equally impressive similaritiesbetween the pelvis and legs of humans and pelvis and hind limbs of othermammals, bones of the skull, spinal column, and so forth. Let’s consider asomewhat extreme comparison. What trait strikes you as being one of the moststriking anatomical differences between giraffes and humans? If you ask me,it’s the length of our necks. Are the necks of humans and giraffes homologous?What do you predict? To help you get started thinking about this, a humanneck is about three to four inches long. On the other hand, the neck of anadult giraffe may be six to eight feet long. How many cervical vertebrae wouldyou predict are in the neck of a giraffe? If you look at Figure 5.46 you cancount the number of cervical vertebrae in humans and giraffes. What doesthis observation tell you about human and giraffe necks? Are they homologousstructures?

About now you may be asking yourself where these kinds of homologoustraits come from. The blueprint for each organism’s anatomy is found in itsgenetic make-up, and this genetic make-up is inherited from an organism’sancestors. It therefore follows that if more than one species shares a commonancestor, that they are likely to have inherited similar traits.

The genetic code carried in each species’ DNA determines the kind of speciesit is, and their development from fertilized egg to mature adult is regulated bygenetic factors. It is this process of development that produces the physicalsimilarities and differences we see between species. We will therefore spend therest of this reading examining genetic evidence of evolutionary change.

Genetic Evidence

Do you think that it would make sense that species that are closely relatedto each other would be more similar genetically than more distantly relatedspecies? That’s a completely logical and correct assumption. The data inTable 5.15 show the degree of genetic similarity between modern humans andsome other species. You will note that the degree of genetic similarity indicateshow closely species are related.

Differences in genetic similarities between taxa (i.e., groups of organisms)reflect the amount of time that passed since the taxa shared a common ancestor.It is therefore no surprise that humans have a smaller genetic similarity toroundworms than they do to any of their mammalian relatives: the last commonancestor of humans and roundworms lived much, much longer ago than the lastcommon ancestor of humans and Neanderthals. At the same time, you maybe surprised to see that we share as much as a 50% genetic similarity withroundworms. Actually, this isn’t all that surprising, since all animals use thesame basic set of genes to carry our cellular activities, make and use the samekinds of molecules, etc. Once the amount of genetic differences between taxaare known, scientists can use those data to generate figures called phylogenetictrees, also known as cladograms, to show the degree of relatedness between taxa(see Figure 5.47).

198 CHAPTER 5. LIFE

Percentage ofSpecies genetic similarity

to humansRoundworm 50%Fruit fly 55%Chicken 60%Horse 71%Cow 80%Mouse 85%Cat 90%Monkey 93%Ape 97%Chimpanzee 98.8%Homo neanderthalensis 99.7%

Table 5.15: Percentages of similarity between human DNA and DNA of selectedspecies. Note: These data are difficult to standardize because of the differentmethods that are used to test for genetic similarity and because not all of everyspecies’ genomes are known. These percentages may represent comparisons be-tween specific segments of DNA or comparisons of entire genomes. The numberspresented in the table represent the highest percentages of similarity reported.

5.6. HUMAN EVOLUTION II: ANATOMY AND GENETICS 199

Figure 5.45: This figure shows the skeletal anatomy of forelimbs of (startingon the top row) salamanders, turtles, crocodiles, birds, bats, whales, moles,and humans. The humerus (upper arm bone) in each forelimb is indicatedby the letter “o”, the radius and ulna (bones of the forearm) by “a” and “s”,respectively, the bones of the wrist by “h”, and bones of the digits by the letters“m” and “f”. (This diagram originally appeared in a text by Wilhelm Leche.It is now in the public domain.)

200 CHAPTER 5. LIFE

Figure 5.46: The image on the left shows the cervical vertebrae and skull of agiraffe, and the image on the right is an x-ray of the human neck region showingthe cervical vertebrae. Humans and giraffes both have seven cervical vertebrae.(Image sources: giraffe after Richard Owen, 1866, in the public domain; humanx-ray courtesy “Hellerhoff”, Creative Commons Attribution - Share Alike 3.0Unported License.)

5.6. HUMAN EVOLUTION II: ANATOMY AND GENETICS 201

Figure 5.47: This phylogenetic tree shows the degree of relatedness between themajor forms of life on earth. The lengths of line segments between branchesindicate how closely or distantly related taxa are. The names in green areBacteria that survive under typical conditions on earth. The names in turquoiseare the Archaea, groups of bacteria that require unique, hostile environments,such as acidic conditions, high temperatures, or extremely salty conditions. Thenames in blue are the Eucaryota, groups of organisms that have cells with anucleus, and include the protists, fungi, plants, and animals. (Image is in thepublic domain.)

Historically these kinds of trees were based solely on anatomical and de-velopmental traits. More recently, however, genetic traits including DNA se-quences (the specific order of As, Ts, Cs, and Gs) and amino acid sequences ofhomologous proteins have been used to test the accuracy of older phylogenetictrees. By combining anatomical, developmental, and genetic traits we now pro-duce trees that give us the best insights available about relatedness betweengroups of organisms. Sometimes trees need to undergo significant revision. Inother cases, such as the traditional phylogenetic tree of primates, only minoradjustments were needed.

Further research into the genetics of organisms uncovered an extremely sur-prising result, the existence of vestigial genes, also known as pseudogenes.

Pseudogenes:Vestigial genes, which wereonce functional, but havebeen disabled by randommutations.

Pseudogenes were once functional genes, but they became disabled by randommutations. Once a gene becomes a pseudogene it remains in the DNA of thespecies, it is not expressed, and is therefore no longer subject to selection. Thatmeans that a pseudogene will be retained for a long, long time. It also meansthat once a gene becomes a pseudogene the only genetic changes it will expe-rience are the result of random mutations. Some pseudogenes are homologousgenetic traits, since a particular pseudogene will be found only in organismsthat are direct ancestors of the organism in which the original pseudogene wasproduced. Many pseudogenes are known to exist in primates, and some of themare found only in primates (Table 5.16).

The last gene listed in Table 5.16, the GULO pseudogene, makes it impos-sible for primates to make their own Vitamin C. The production of Vitamin Cis a multi-step process requiring multiple enzymes (a kind of protein), but inprimates the GULO pseudogene does not produce a necessary enzyme neededto complete the synthesis of Vitamin C. This is why the diets of primates mustinclude foods that have vitamin C in them. If they don’t have access to Vita-min C they will eventually weaken and die as a result of a malnutrition diseasecalled scurvy. The primate GULO pseudogene has the same disabling pointdeletion mutation in exactly the same place in the genetic code of all primate

202 CHAPTER 5. LIFE

Pseudogene Lost functionEnolase pseudogene Lost ability to synthesize a specific kind of

organic compound called an enol, we have otherfunctional enolase genes

Hemoglobin pseudogene Lost ability to produce a specific kind ofhemoglobin protein, we have other functionalhemoglobin genes

Sulfatase pseudogene Lost ability to form bonds needed to makesteroids, proteins and other molecules, wehave other functional sulfatases

Steroid-21 pseudogene Loss of ability to produce compounds likealdosterone (manages ion balance in thekidney) and cortisol (an adrenal glandsecretion used to deal with stress), we haveother functional steroid-21 genes

GULO pseudogene Lost ability to synthesize the last step ofthe production of Vitamin C, there is nofunctional GULO gene in primates

Table 5.16: A few pseudogenes found only in primates.

species (see Figure 5.48).

Figure 5.48: A segment of the GULO gene, which produces an enzyme thatcreates vitamin C in most mammals. In primates, the gene has been disabledby a mutation and is a pseudogene. (Image is in the public domain.)

This observation provides additional genetic evidence that all primates arerelated. That is, all primates share a common ancestor that experienced amutation that disabled the GULO gene. The existence of pseudogenes like theGULO gene allow us to state with a high degree of confidence that the mutationproducing the GULO pseudogene occurred at some time after the divergenceof the evolutionary line that gave rise to rats and the line that gave rise toprimates, but before the ancestral primate diverged into different species. Thisobservation supports the conclusion that all primates have a common ancestor(see Figure 5.49).

One other extremely interesting observation on the evolutionary history ofhumans relative to other primates has to do with the number of chromosomes wehave. Humans have 23 pairs of chromosomes. Interestingly, all other primateshave 24 pairs of chromosomes. Where did this difference come from? Didhumans simply lose one pair of chromosomes? That seems unlikely, since losingand entire pair of chromosomes would also mean that we had lost about 1/24thof our entire genetic code. Since our genetic code is made up of about threebillion base pairs of nucleotides, if we lost 1/24th of that, we would have lostabout 125 million base pairs, and who knows how many genes! Some researcherspredicted that we didn’t lose a chromosome, but that two of our chromosomes

5.6. HUMAN EVOLUTION II: ANATOMY AND GENETICS 203

Figure 5.49: A cladogram indicating the timing of the GULO mutation. Allspecies possessing the same mutation evolved from a common ancestor. Themutation happened somewhere around the time indicated by the red star. (Im-age is in the public domain.)

must have fused together to form a combined single chromosome. In 2005researchers discovered that human chromosome #2 is a fused chromosome, andsolved the mystery of the missing chromosome. You will have a chance to learnmore about this discovery during class discussion. Suffice it to say that ourfused chromosome is a derived trait that makes humans different than otherprimates, yet provides further evidence of our common ancestry.

The last topic we will consider in this reading is how even closely relatedspecies can produce significantly different looking bodies.

Developmental regulatory genes

Sometimes people wonder how closely-related species, like humans and chim-panzees, can be so similar genetically, yet produce bodies that are so different.For example, how can science explain why the arms of chimps are propor-tionally longer than those of humans, why chimps have flatter noses, slopedforeheads, large canines, etc., when humans and chimps have close to a 99%similarity in their genetic make-up? At first glance this appears to be a per-plexing question. Recent advances in the field of developmental-evolutionarybiology, a union between the fields of evolutionary biology and developmentalbiology, have, however, provided some answers.

All mammals share the same basic body plan, with essentially the samestructures in the same places relative to each other. With that thought inmind, consider the question of how chimps and humans can have different legto arm lengths, and different skull shapes. Do these differences have to beexplained by genetic mutations? No! Scientists have learned that specializedcells that appear during development produce signal molecules called regulatorygene products that have specific effects on neighboring masses of cells. Theseregulatory gene products cause these neighboring tissues to develop into some-times extremely complex structures, like an arm, or a leg, or even an eye. Theonly thing that has to happen for one species to produce longer legs than armsis to have these regulatory genes active longer at the end of the developing armsthan at the end of developing legs.

Just in case you are curious, here is a specific example of how limbs areproduced in vertebrates (animals with a backbone). During embryonic devel-opment a small ridge of cells called a limb bud forms where a limb will beproduced. The cells at the tip of the limb bud are called the Apical EctodermalRidge (AER). These cells produce a chemical that causes neighboring cells todivide rapidly, causing the limb bud to elongate. As long as the cells of the AERactively produce their signal molecules, the limb will continue to elongate. Inthe meantime, other regulatory genes drive the formation and organization of

204 CHAPTER 5. LIFE

bone, muscle, and other tissues of the limb. In chimpanzees the AER of thearm limb buds are active longer than the AER of the leg limb buds, so by thetime a chimp is fully developed it has longer arms than legs. The reverse is truein humans. There is no known mutational change involved in producing thesestructures; only the duration of regulatory gene activity differs between thesespecies. In this way, the timing and duration of activity by regulatory genescan produce markedly different structures including differences in limb length,skull size and shape, pelvic size and shape, etc.

These differences in timing during development are collectively referred toas heterochrony (i.e, different timing). Through heterochrony even species thatare genetically similar can produce bodies that have markedly different propor-tions without having to rely on genetic mutations. Another example of hete-rochrony during development produces the differently shaped skulls of humansand chimpanzees (Figure 5.50).

Figure 5.50: The top row of figures shows the shape of a chimpanzee skull atthe fetal stage of development (left), at birth (middle), and then at adulthood(right). The lower row shows the shape of the fetal human skull before birth(left) and at adulthood (right). The shapes of the cells in the figures of theadult skulls indicate the degree of shape change that has occurred relative tothe fetal skull.

It is obvious the timing and duration of regulatory genes are different in hu-mans and chimpanzees. In humans these signals produce a taller forehead andlarger brain cavity than in chimpanzees. At the same time, in chimpanzees, reg-ulatory genes produce a more elongated skull with a protruding jaw and smallerbrain cavity. These kinds of differences may be the result of only very smalldifferences in genetic information, or perhaps only species-specific differencesin the timing and length of expression of identical regulatory genes. Scientistshope that further work in this field will yield additional insights into how bodiesof different species are produced, as well as additional information about theevolutionary history of life on earth.

Let’s look at one last example, the development of the homologous structuresof the hand of a human and the flipper of a whale. Can scientists explainhow whales produce a broad, tissue covered structure, while humans producea hand with separate fingers and a thumb? Scientists discovered that duringdevelopment of the mammalian limb, the digits at the end of the limb alwaysform in paddle-like structures. Bones and muscles develop within the paddle,so all of the parts and pieces of digits are there, but they are not separate fromeach other during early stages of limb development. Then, in humans, after the

5.6. HUMAN EVOLUTION II: ANATOMY AND GENETICS 205

internal structures of the hand are fully formed a regulatory gene produces asubstance sends a signal to cells in the tissue between fingers to die. In this wayyou could say that the fingers of your hand were sculpted out of a paddle-likestructure, or, in other words, the reason that your fingers are independent ofeach other is that the tissue between them died. Whales, however, don’t havefingers. So in order to produce its pectoral fins the regulatory gene that signalscell death between fingers is simply not activated, and the solid fin structure,with bones and muscles homologous to our hand, is produced.

It is exciting to see how scientific research is starting to reveal the interplaybetween genetics, development, and evolutionary change. Further studies on thedevelopment of living things should provide additional insights into homologousstructures, their developmental origins, and their evolutionary significance.

Summary

Scientific discoveries have provided an incredible number of observations show-ing that the physical human body is the result of evolutionary change:

• Humans have the same molecular and genetic structure as other organisms(universal genetic code).

• Humans use the same cell signaling pathways as other animals (proteinsynthesis, genetic overlap, and regulatory developmental genes).

• Humans have the same anatomical features as other animals (comparativeanatomy of living and fossil forms, homologous traits).

• There is variation in the human population (a prerequisite for evolutionto occur)

• Humans go through similar patterns of development as other species (de-velopmental biology)

• Humans have vestigial organs (comparative anatomy of living forms)

• Humans have pseudogenes that carry genetic information of ancestralcharacteristics found in other organisms (genetics)

• There are abundant intermediate species that exist in the fossil record.Intermediate in time, anatomical structure, and even in culture (compar-ative anatomy of intermediate forms).

• Fossil evidence shows that the human body plan did not appear suddenly,but was produced via evolutionary change the same as all of the otherforms of life on earth (comparative anatomy of intermediate forms).

REVIEW QUESTIONS

1. What is a vestigial structure? Name a few vestigial structures foundin humans and explain how they are used by other animals.

2. What is a homologous structure? Name a few that were not men-tioned in this reading.

3. How similar is human DNA to that of other primates?

4. What is a pseudogene?

5. How do pseudogenes help confirm genetic and evolutionary related-ness between two groups?

6. How can differing times and durations of regulatory genes producesignificantly different body forms?

206 CHAPTER 5. LIFE

Appendix A

Views on Science andReligion

A.1 Reconciling Scientific and Religious Viewsof Nature

Dr. Dan Moore, BYU-Idaho, 2009

“I have been announced as a student of science. But I also like tothink of myself as one who loves the Gospel of Jesus Christ. Forme there has been no serious difficulty in reconciling the princi-ples of true science with the principles of true religion, for both areconcerned with the eternal verities of the Universe. And yet thereare many people, and particularly among our youth, who regardthe field of science and the field of religion as two wholly differentspheres, the one entirely separated from and unrelated to the other.In fact, there are those in both fields who have done themselves andthe causes to which they give their interests a distinct disservice inteaching that the two are opposed and that they cannot be harmo-nized one with the other.” Dr. Henry D. Eyring“We have a dilemma, however, because God has left messages allover in the physical world that scientists have learned to read. Thesemessages are quite clear, well-understood, and accepted in science.That is, the theories that the earth is about four-and-one-half billionyears old and that life evolved over the last billion years or so areas well established scientifically as many theories ever are. So, ifthe word of God found in the scriptures and the word of God foundin the rocks are contradictory, must we choose between them, or isthere some way they can be reconciled?” Dr. Henry D. Eyring

Many are confused about the origin and history of the natural world. In-dividuals hear scientific and religious assertions about the Earth and Universeand are unsure how to reconcile them. Most commonly, they attempt to solvethe apparent discrepancy by rejecting either religious or scientific truths aboutnature or by compartmentalizing them, i.e., not allowing them to be compared.Both of these solutions are flawed. Since all truth is compatible with all othertruth and God is the author of all truthscientific and religious, the goal shouldbe to bring together the truth from science and the truth from religion intoone harmonious whole. There are three aspects of nature where the confusionabout the truth is the greatest: the origin of the universe, the age of the Earth,and the origin of life. These aspects of nature provide us with an excellentopportunity to better understand two important aspects of the relationship be-

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208 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

tween science and religion. The first relates to the complementary nature ofscientific and religious truth. The second has to do with why scientific study ofthe physical world identifies governing laws not the hand of the Creator.

Science and Religion: Hand in Glove

Clearly, scientific truth and religious truth are distinct. These two paths totruth are commonly perceived to be in opposition to each other. However,when seen from the correct perspective their relationship is complementary,not antagonistic. Let’s take a look at that perspective. Both religion andscience contain truth revealed from God to man or discovered by man withGod’s help. Even so, the nature of truth in religion is distinct from the natureof truth in science.

In religion we can know things absolutely, without understanding how theywork. Take your testimony of the atonement, for example: You know absolutelythat the atonement is real and true, but you don’t understand the details ofhow it works, only how you can gain access to its power. By contrast, inscience we can understand how something works without knowing that it is theultimate truth. Take the theory of plate tectonics, for example: It allows us tounderstand how the Earth works, and yet we cannot prove that it is the ultimatetruth that it will never be improved. In fact, we expect it to be modified asour knowledge grows. Because of their unique characteristics, religious andscientific truths complement each other. In short, religious truth allows youto know absolutely without understanding the mechanism(s) by which thattruth operates, and scientific truth allows you to understand the operation ofmechanisms without the ability to know absolutely.

God has revealed truth about nature through scientists and through prophets.Understanding the complementary nature of these two sources of truth facil-itates appreciating and making use of both. Religious truths about natureinclude revelation about who the Creator is and why He created what He did.Scientific truths about nature are descriptions of how and when things werecreated and how they work. Many people see conflict and disagreement whenthey compare scientific and religious truths about nature. I do not. Instead, Isee compatible truth. To me, their similarities testify that they came from thesame source, God. Apparent discrepancies typically arise when people try touse the truth from one of these sources inappropriately like turning to religioustruth to understand how or when the Earth was made, or turning to science tounderstand the purpose of the Earth or who made the Earth. We must choosethe path to truth science or religion that is appropriate for the question weare asking! Of course, the source of all truth is the same: God. Moreover,we must recognize that each source of truth places constraints on how truthsfrom the other source should be interpreted; that is, a correct understanding ofscientific truths must be compatible with religious truths and vice-versa. If thetruths from each source are used appropriately, they fit together like a handin a glove.

Evidence of the Creator’s Hand

“Behold, all these are kingdoms, and any man who hath seen any orthe least of these hath seen God moving in his majesty and power.I say unto you, he hath seen him; nevertheless, he who came untohis own was not comprehended. The light shineth in darkness, andthe darkness comprehendeth it not; nevertheless, the day shall comewhen you shall comprehend even God, being quickened in him andby him. Then shall ye know that ye have seen me, that I am, andthat I am the true light that is in you, and that you are in me;otherwise ye could not abound.” Jehovah (D&C 88: 47-50)

A.1. RECONCILING SCIENTIFIC AND RELIGIOUS VIEWS OF NATURE209

You and I know that the Gods created the Universe and everything within it,including the Earth (Abraham Ch 4-5). Alma teaches that all things, includingthe earth, “denote there is a God” (Alma 30:44). The Creator himself Jehovahhas declared that observing his creations is equivalent to seeing His glory (D&C88:47-50; see above). When scientists study the physical world, we see physicalsystems and physical causes. Why don’t scientists see the Creator’s hand? Howdo we reconcile the above declarations with the observations of scientists? Theanswer to this apparent conundrum is found in the declarations of the Creatorhimself:

“...by the word of my power, have I created them, which is mineOnly Begotten Son...” Jehovah (Moses 1:32)

“...my word, which is my law...” Jehovah (D&C 132:12)

“And the light which shineth, which giveth you light, ... The lightwhich is in all things, which giveth life to all things, which is thelaw by which all things are governed, even the power of God whositteth upon his throne, ...” Jehovah (D&C 88:11-13)

“...he hath given a law unto all things...and their courses are fixed”Jehovah (D&C 88:42-43)

“All kingdoms have a law given; and there are many kingdoms; forthere is no space in the which there is no kingdom; and there isno kingdom in which there is no space, either a greater or a lesserkingdom. And unto every kingdom is given a law; and unto everylaw there are certain bounds also and conditions.” Jehovah (D&C88:36-39)

“All truth is independent in that sphere in which God has placedit, to act for itself, as all intelligence also; otherwise there is noexistence.” Jehovah (D&C 93:30)

These and other verses suggest that Gods do not create like humans do.Gods create by establishing law (with bounds and conditions) in a system(kingdom). Examples of physical systems include the atom, the cell, an or-ganism, a solar system, and a galaxy. The law established by Gods in systemsis independent, it acts for itself. Is it any wonder, then, that scientists find law,not the Creator’s hand, when they study physical systems? Discovering theselaws the processes and mechanisms by which physical systems are governedis the central aim of science. Still, how do we reconcile scientific observationswith scriptural declarations that indicate that God can be seen’ as we observenature? As with all things spiritual, believing is seeing. Finding God in natureis something that is felt more than seen. The spiritual world, including knowl-edge of the existence of God, lies outside the realm of science. Still, for thosewho believe that He created the physical world, observing His creations as ona starry night causes us to be in awe of His glory, the power and understandingof truth that allowed Him to create the physical world. To summarize, when westudy the physical world, we observe physical systems being governed by thelaws that God placed to govern those systems (kingdoms). To see the Lawgiverrequires the heart. We come to know the Creator through His Spirit, as weapproach him with a sincere heart and real intent, exercising faith in Christ(Moro 10:3-5).

210 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

End Note

It is important for you and I not to succumb to the common, easy, and falsenotion that science and religion are opposed, that we must reject one to acceptthe other. God is in both. Even so, there is a clear distinction between the rel-ative importance of scientific and religious truth: both are eternally important,though in different ways religious truth is essential to our salvation; scientifictruth is not. If ever you find yourself convinced that religious and scientifictruth cannot be reconciled, that you must reject one to accept the other, I hopeyou will remember that all truth is compatible with all other truth. Find afaithful scientist or a faithful member of the church that is scientifically literateand ask them for guidance in working through your questions. If, after that,you still don’t feel that you can reconcile things, set the scientific explanationaside for a day when you can see things more clearly and cling to the centraltruths of the gospel.

Readings

1. Reconciling Science and Religion: Eyring, H.D., 1967, The Faith of aScientist, Bookcraft, 53 p.; Eyring, H.D., 1983, Reflections of a Scientist,Shadow Mountain, 111 p. Both of these books are classics. Dr. Eyringstrikes a wonderful balance as he reconciles faith and reason.

2. Eyring, H.J., 2008 Mormon Scientist: The Life and Faith of Henry Eyring,Deseret Book, 320 p. This is a great biography, written by Dr. Eyring’sgrandson (who, at the time of this writing, was serving in the adminis-tration at BYU-Idaho).

A.2 Making Sense of Scientific and ReligiousAssertions

Dr. Dan Moore, BYU-Idaho, 2009

Have you ever heard anyone say something like this, “The Big Bang Theoryproves that life has no purpose,” or conversely, like this, “It is impossible tobelieve in the Bible and believe that the Earth is old”? Both of these statementsare typical of the kind of mud-slinging that (unfortunately) has been part ofthe science and religion debate for centuries. All this mud-slinging has lead tosome very muddy water’ that is difficult for most people to see through. Thepurpose of this brief essay is to help to more clearly and effectively analyze theassertions that you are presented with each day.

Science and religion debates typically pit one against the other in a way thatencourages you to choose one or the other path to truth, but not both. Clearly,this is false, for God is the source of all truthboth scientific and religious, and alltruth is compatible with all other truth. Both science and religion are avenuesto truth and speak with authority, but in different areas. The authority ofreligion is absolute in all areas where God has revealed truth and a true prophethas clearly taught how we are to understand that truth. Science is limited toareas of inquiry that are physically falsifiable. Science speaks with authority forexplanations that have withstood many independent tests. (Science does not,and cannot, prove things true; rather, it proves things false. Our confidencein a particular scientific explanation grows as the number of independent testsit has withstood grows.) Ideas that are not solidly grounded on the authorityof religion or science are opinions, and we are free to agree or disagree withopinions according to our pleasure though hopefully we have a good reason fordoing so. Authoritative religious and scientific declarations carry much weight,while opinions do not.

A.3. THE BYU EVOLUTION PACKET 211

Problem arise and the waters are muddied for unknowing or lazy hearerswhen scientists or religionists make proclamations outside their realm of au-thority; that is, when someone tries to state an opinion as if it is founded onthe authority of either science or religion. This would not be a problem ifthe hearers of such statements would recognize that the declaration was non-authoritative, an opinion. Sadly, most hearers do not. To illustrate, let me walkyou through several examples. Some scientists and philosophers have used BigBang theory or the theory of evolution to suggest that human lives have nopurpose or that there is no God. When most religionists hear such statements,they typically do not see that what they’ve heard is a misapplication of science;rather, they react something like this: if you’re going to tell me that the bigbang or evolution suggests that life has no meaning or that God doesn’t exist,then I must conclude that those theories are false, because I know absolutelythat God exists and that life has a purpose. Instead, the reaction should be,simply: I reject your opinion; it is a misapplication of science and is withoutauthority; I will evaluate those theories on their own merits, untainted by youropinion. Unfortunately, many religionists try to paint their opinions as au-thoritative declarations as well. They will say, for example, that the scripturesprove that the Earth is very young, or that it is not possible to have a testimonyof the atonement and believe in evolution. Unfortunately, many of those whohear these statements make the mistake of rejecting what is true (the scripturesand the atonement, in these cases) because of someone’s opinion, which mayor may not be true. My experience indicates that most of the problems in thescience and religion debates are like those described above: people talk at crosspurposes and then, as a reaction throw the baby out with the bathwater’. You,however, as a lover of truth, as a religionist with a big R’, and as a scientist witha small s’, will do better. You and I must critically evaluate each assertioncomefrom whatever source it doesto determine whether it rests on the authority ofreligion, rests on the authority of science, or whether it is merely opinion. If anopinion, we should analyze its merits and agree or disagree with it according toour pleasure, but we should not assign to it the clout of authoritative religiousor scientific declarations. When you and I share opinions, we must clearly in-dicate to those that are listening that we are not speaking with authority, andwe must not engage in silly, heated arguments about opinions. It saddens medeeply to think about the amount of wasted energy and brainpower and thenumber of people that have been led astray because of the empty, inane natureof the so-called science and religion debates. You and I must rise above theintellectual and spiritual poverty that typically characterize these debates! AsVoltaire once said, “opinion has caused more trouble on this little earth thanplagues or earthquakes.”

A.3 The BYU Evolution Packet

The following materials are taken from the BYU “evolution packet”. Theyhave been reformatted to conform to the style of this text, but are otherwiseunchanged.

Evolution Packet Defined

BYU Daily Universe Nov. 12, 1992 p.3

In the interest of clarifying the background and purpose of the library packeton evolution and the origin of man, which was announced in The Daily Uni-verse on Thursday, Oct. 29. I provide the following information about thedevelopment of this packet and the motivation for it.

As appropriate at any university, the subject of organic evolution and theorigin of man comes up in BYU courses in several departments. In these courses,

212 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

students naturally wish to know the official position of the LDS Church on thissubject. Some faculty members in the sciences and in Religious Educationhave gathered material on these topics to distribute to their students. Studentsmight receive one set of statements by Church leaders from one professor anda different set from another professor.

Several faculty members and administrators felt the diversity of materialson these subjects, which were often selected to emphasize the views of the pro-fessor, tended to create confusion in the minds of the students and accentuatethe potential for controversy about the Church’s position. In 1991, in responseto questions from students about the Church position on evolution, PresidentRex E. Lee authorized that one of these packets be placed in the HBLL ReserveLibrary as a source for information about the Church’s position on evolutionand the origin of man.

Purpose of packet

Because of my experience in preparing the evolution article for the “Ency-clopedia of Mormonism,” I was asked by Provost Bruce Hafen to consider apacket that could be made available to students as the official and fundamentalChurch position on this subject. It was immediately clear that the selection ofmaterial for such a packet could not depend on the content of the statements.The goal is not to achieve some kind of “balance” among the views that havebeen expressed, but to give students the full range of official views so thatthey can judge the different positions they encounter. The full range of officialviews should provide the basis for the evaluation of other views that have beenexpressed but that do not have the status of official Church positions.

In line with this philosophical stance, I prepared an initial draft of thepacket, which contained the First Presidency statements and all published state-ments made by presidents of the Church during the time they held that office.It also included the speech given in 1931 by Elder James E. Talmage of theQuorum of the Twelve, which was reviewed and approved by the First Presi-dency and officially published by the Church. Finally, this draft packet includedthe “Encyclopedia of Mormonism” article because of the excerpt from the FirstPresidency Minutes in 1931 about the Church’s stance toward scientific studiesof evolution and the origin of man. This packet was made entirely of materialswith official status and included all of the statements published by or with theauthorization of the First Presidency.

The draft packet’s contents were discussed amicably with Dean Robert Mil-let of Religious, Education and Provost Hafen. After considerable discussion,we agreed that the official university packet should contain only those itemsthat represent the official position of the Church, i.e. statements from the FirstPresidency. The encyclopedia article was kept because of the First PresidencyMinutes item included in it, which is not otherwise available to the public. Thefinal packet was then reviewed by BYU’s Board of Trustees-consisting of TheFirst Presidency, many members of the Quorum of the Twelve and other gen-eral authorities and officers. They approved the packet.

Balance not the issue

Again, I emphasize that balance was not the issue. The issue was provid-ing only those materials that could clearly be said to be the official, declaredposition of the Church.

None of us involved in preparing this packet for Board review anticipate thatprofessors will be limited from distributing other materials to their students. Itis only requested that BYU faculty members refer students to the materials inthis specific packet along with the other items they may choose to distribute.When other items are distributed, they should be clearly separated and given

A.3. THE BYU EVOLUTION PACKET 213

as a supplement to this material and include a fair sampling of the diverseviewpoints among LDS leaders. For example, if one included statements byLDS apostles in a handout on evolution, the range of views would include somestatements against evolution, some sympathetic to evolution and several shadesof opinion in between. We want to avoid the implication that a greater senseof unanimity or resolution of this topic exists than is actually the case, and weare eager to avoid contention. The university has also suggested that facultymembers limit supplemental LDS material on the subject of evolution and theorigin of man to published documents, avoiding private letters or other privatematerial.

The process was one of constructive and harmonious effort to provide mate-rials from which students could see clearly the foundation of LDS doctrine onthis subject and distinguish it from the wide variety of opinions encountered inLDS literature.

by William E. Evenson

Dean, College of Physical and Mathematical Sciencesprofessor of Physics

Evolution and the Origin of Man

October, 1992

This packet contains, as far as could be found, all statements issued bythe First Presidency of the Church of Jesus Christ of Latter-day Saints on thesubject of evolution and the origin of man, and a statement on the Church’sattitude toward science. The earliest First Presidency statement, “The Originof Man,” was issued during the administration of President Joseph F. Smith in1909. This was followed by a First Presidency Message in 1910 that includedbrief comments related to the study of these topics. The second statement,“Mormon View of Evolution,” was issued during the administration of PresidentHeber J. Grant in 1925. Although there has never been a formal declarationfrom the First Presidency addressing the general matter of organic evolutionas a process for development of biological species, these documents make clearthe official position of the Church regarding the origin of man.

This packet also contains the article on evolution from the Encyclopediaof Mormonism, published in 1992. The current First Presidency authorizedinclusion of the excerpt from the First Presidency minutes of 1931 in the 1992Encyclopedia article.

Various views have been expressed by other Church leaders on this subjectover many decades; however, formal statements by the First Presidency are thedefinitive source of official Church positions. It is hoped that these materialswill provide a firm foundation for individual study in a context of faith in therestored gospel.

Approved by the BYU Board of TrusteesJune, 1992

The Origin of Man

BY THE FIRST PRESIDENCY OF THE CHURCH.IMPROVEMENT ERA.

Vol. XIII. NOVEMBER, 1909. No. 1.Editor’s Table.

“God created man in his own image.”

214 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

Inquiries arise from time to time respecting the attitude of the Church ofJesus Christ of Latter-day Saints upon questions which, though not vital froma doctrinal standpoint, are closely connected with the fundamental principlesof salvation. The latest inquiry of this kind that has reached us is in relationto the origin of man. It is believed that a statement of the position held by theChurch upon this important subject will be timely and productive of good.

In presenting the statement that follows we are not conscious of putting forthanything essentially new; neither is it our desire so to do. Truth is what wewish to present, and truth - eternal truth - is fundamentally old. A restatementof the original attitude of the Church relative to this matter is all that will beattempted here. To tell the truth as God has revealed it, and commend it tothe acceptance of those who need to conform their opinions thereto, is the solepurpose of this presentation.

“God created man in his own image, in the image of God created he him;male and female created he them.” In these plain and pointed words the inspiredauthor of the book of Genesis made known to the world the truth concerningthe origin of the human family. Moses, the prophet historian, “learned,” aswe are told, “in all the wisdom of the Egyptians,” when making this impor-tant announcement, was not voicing a mere opinion, a theory derived from hisesearches into the occult lore of that ancient people. He was speaking as themouthpiece of God, and his solemn declaration was for all time and for allpeople. No subsequent revelator of the truth has contradicted the great leaderand law-giver of Israel. All who have since spoken by divine authority uponthis theme have confirmed his simple and sublime proclamation. Nor could itbe otherwise. Truth has but one source, and all revelations from heaven areharmonious with each other. The omnipotent Creator, the maker of heaven andearth - had shown unto Moses everything pertaining to this planet, includingthe facts relating to man’s origin, and the authoritative pronouncement of thatmighty prophet and seer to the house of Israel, and through Israel to the wholeworld, is couched in the simple, clause: “God created man in his own image”(Genesis 1:27; Pearl of Great Price - Book of Moses, 1:27-41.)

The creation was two-fold - firstly spiritual, secondly temporal. This truth,also, Moses plainly taught - much more plainly than it has come down to us inthe imperfect translations of the Bible that are now in use. Therein the factof a spiritual creation, antedating the temporal creation, is strongly implied,but the proof of it is not so clear and conclusive as in other records held bythe Latter-day Saints to be of equal authority with the Jewish scriptures. Thepartial obscurity of the latter upon the point in question is owing, no doubt, tothe loss of those “plain and precious” parts of sacred writ, which, as the Bookof Mormon informs us, have been taken away from the Bible during its passagedown the centuries (I Nephi 13: 24-29). Some of these missing parts the ProphetJoseph Smith undertook to restore when he revised those scriptures by the spiritof revelation, the result being that more complete account of the creation whichis found in the book of Moses, previously cited. Note the following passages:

And now, behold I say unto you, that these are the generations of theheaven and the earth, when they were created in the day that I, theLord God, made the heaven and the earth,

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And every plant of the field before it was in the earth, and every herbof the field before it

For I, the Lord God, created all things of which I have spoken,spiritually, before they were naturally upon the face of the earth. ForI, the Lord God, had not caused it to rain upon the face of the earth.

And I, the Lord God, had created all the children of men, and notyet a man to till the ground; for in heaven created I them, and therewas not yet flesh upon the earth, neither in the water, neither in the air.

But I, the Lord God, spake, and there went up a mist from the earth,and watered the whole face of the ground.

And I, the Lord God, formed man from the dust of the ground, andbreathed into his nostrils the breath of life; and man became a livingsoul, the first flesh upon the earth, the first man also.

Nevertheless, all things were before created, but spiritually were theycreated and made, according to my word. (Pearl of Great Price - Bookof Moses, 3:4-7. See also chapters 1 and 2, and compare with Genesis1 and 2).

These two points being established, namely, the creation of man in the imageof God and the two-fold character of the creation, let us now inquire: Whatwas the form of man, in the spirit and in the body, as originally created? In ageneral way the answer is given in the words chosen as the text of this treatise.“God created man in his own image.” It is more explicitly rendered in the Bookof Mormon thus: “All men were created in the beginning after mine own image”(Ether 3:15). It is the Father who is speaking. If, therefore, we can ascertainthe form of the “Father of spirits,” “The God of the spirits of all flesh,” we shallbe able to discover the form of the original man.

Jesus Christ, the Son of God, is “the express image” of His Father’s person(Hebrews 1:3). He walked the earth as a human being, as a perfect man, andsaid, in answer to a question put to Him: “He that hath seen me hath seen theFather” (John 14: 9). This alone ought to solve the problem to the satisfactionof every thoughtful, reverent mind. The conclusion is irresistible, that if the Sonof God be the express image (that is, likeness) of His Father’s person, then HisFather is in the form of man; for that was the form of the Son of God, not onlyduring His mortal life, but before His mortal birth, and after His resurrection.It was in this form that the Father and the Son, as two personages, appearedto Joseph Smith, when, as a boy of fourteen years, he received his first vision.Then if God made man - the first man - in His own image and likeness, he musthave made him like unto Christ, and consequently like unto men of Christ’stime and of the present day. That man was made in the image of Christ, ispositively stated in the Book of Moses: “And I, God, said unto mine OnlyBegotten, which was with me from the beginning, Let us make man in ourimage, after our likeness; and it was so....And I, God, created man in mine ownimage, in the image of mine Only Begotten created I him, male and femalecreated I them” (Moses 2:26, 27).

The Father of Jesus is our Father also. Jesus Himself taught this truth, whenHe instructed His disciples how to pray: “Our Father which art in heaven,” etc.Jesus, however, is the first born among all the sons of God - the first begottenin the spirit, and the only begotten in the flesh. He is our elder brother, andwe, like Him, are in the image of God. All men and women are in the similitudeof the universal Father and Mother, and are literally the sons and daughters ofDeity.

216 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

“God created man in His own image.” This is just as true of the spirit as itis of the body, which is only the clothing of the spirit, its complement; the twotogether constituting the soul. The spirit of man is in the form of man, andthe spirits of all creatures are in the likeness of their bodies. This was plainlytaught by the Prophet Joseph Smith (Doctrine and Covenants, 77: 2).

Here is further evidence of the fact. More than seven hundred years beforeMoses was shown the things pertaining to this earth, another great prophet,known to us as the brother of Jared, was similarly favored by the Lord. Hewas even permitted to behold the spirit-body of the foreordained Savior, priorto His incarnation; and so like the body of a man was His spirit in form andappearance, that the prophet thought he was gazing upon a being of flesh andblood. He first saw the finger and then the entire body of the Lord - all in thespirit. The Book of Mormon says of this wonderful manifestation:

And it came to pass that when the brother of Jared had said thesewords,behold, the Lord stretched forth His hand and touched thestones one by one with His finger; and the veil was taken from off theeyes of the brother of Jared, and he saw the finger of the Lord; and itwas as the finger of a man, like unto flesh and blood; and the brotherof Jared fell down before the Lord, for he was struck with fear.

And the Lord saw that the brother of Jared had fallen to the earth;and the Lord said unto him, Arise, why hast thou fallen? And he saithunto the Lord, I saw the finger of the Lord, and feared lest he shouldsmite me for I knew not that the Lord had flesh and blood.

And the Lord said unto him, Because of thy faith thou hast seen thatI shall take upon me flesh and blood; and never has man come beforeme with such exceeding faith as thou hast; for were it not so, ye couldnot have seen my finger. Sawest thou more than this?

And he answered, Nay, Lord, show thyself unto me.

And the Lord said unto him, Believest thou the works which I Shallspeak?

And he answered, Yea, Lord, I know that thou speakest the truth, forthou art a God of truth and canst not lie.

A.3. THE BYU EVOLUTION PACKET 217

And when he had said these words, behold, the Lord showed himselfunto him, and said, Because thou knowest these things ye are redeemedfrom the fall; therefore ye are brought back into my presence; thereforeI show myself unto you.

Behold, I am He who was prepared from the foundation of the worldto redeem my people. Behold, I am Jesus Christ, I am the Father andthe Son. In me shall all mankind have light, and that eternally, eventhey who shall believe on my name; and they shall become my sonsand my daughters.

And never have I showed myself unto man whom I have created, fornever hath man believed in me as thou hast. Seest thou that ye arecreated after mine own image? Yea, even all men were created in thebeginning after mine own image.

Behold, this body, which ye now behold, is the body of my spirit, andman have I created after the body of my spirit; and even as I appearunto thee to be in the spirit, will I appear unto my people in the flesh”(Ether, 3:6-16).

What more is needed to convince us that man, both in spirit and in body,is the image and likeness of God, and that God Himself is in the form of man?

When the divine Being whose spirit-body the brother of Jared beheld, tookupon Him flesh and blood, He appeared as a man, having “body, parts andpassions,” like other men, though vastly superior to all others, because He wasGod, even the Son of God, the Word made flesh: in Him “dwelt the fulness ofthe Godhead bodily.” And why should He not appear as a man? That was theform of His spirit, and it must needs have an appropriate covering, a suitabletabernacle. He came into the world as He had promised to come (III Nephi,1:13), taking an infant tabernacle, and developing it gradually to the fulness ofHis spirit stature. He came as man had been coming for ages, and as man hascontinued to come ever since. Jesus, however, as shown, was the only begottenof God in the flesh.

Adam our great progenitor, “the first man,” was, like Christ, a pre-existentspirit, and like Christ he took upon him an appropriate body, the body ofa man, and so became a “living soul.” The doctrine of the pre-existence, -revealed so plainly, particularly in latter days, pours a wonderful flood of lightupon the otherwise mysterious problem of man’s origin. It shows that man, as aspirit, was begotten and born of heavenly parents, and reared to maturity in theeternal mansions of the Father, prior to coming upon the earth in a temporalbody to undergo an experience in mortality. It teaches that all men existed inthe spirit before any man existed in the flesh, and that all who have inhabitedthe earth since Adam have taken bodies and become souls in like manner.

It is held by some that Adam was not the first man upon this earth, and thatthe original human being was a development from lower orders of the animalcreation. These, however, are the theories of men. The word of the Lorddeclares that Adam was “the first man of all men” (Moses 1:34), and we aretherefore in duty bound to regard him as the primal parent of our race. It wasshown to the brother of Jared that all men were created in the beginning afterthe image of God; and whether we take this to mean the spirit or the body, orboth, it commits us to the same conclusion: Man began life as a human being,in the likeness of our heavenly Father.

True it is that the body of man enters upon its career as a tiny germ orembryo, which becomes an infant, quickened at a certain stage by the spiritwhose tabernacle it is, and the child, after being born, develops into a man.There is nothing in this, however, to indicate that the original man, the first ofour race, began life as anything less than a man, or less than the human germ

218 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

or embryo that becomes a man.Man, by searching, cannot find out God. Never, unaided, will he discover

the truth about the beginning of human life. The Lord must reveal Himself,or remain unrevealed; and the same is true of the facts relating to the originof Adam’s race - God alone can reveal them. Some of these facts, however, arealready known, and what has been made known it is our duty to receive andretain.

The Church of Jesus Christ of Latter-day Saints, basing its belief on di-vine revelation, ancient and modem, proclaims man to be the direct and linealoffspring of Deity. God Himself is an exalted man, perfected, enthroned, andsupreme. By His almighty power He organized the earth, and all that it con-tains, from spirit and element, which exist co-eternally with Himself. He formedevery plant that grows, and every animal that breathes, each after its own kind,spiritually and temporally - “that which is spiritual being in the likeness of thatwhich is temporal, and that which is temporal in the likeness of that which isspiritual.” He made the tadpole and the ape, the lion and the elephant; butHe did not make them in His own image, nor endow them with Godlike reasonand intelligence. Nevertheless, the whole animal creation will be perfected andperpetuated in the Hereafter, each class in its “distinct order or sphere,” andwill enjoy “eternal felicity.” That fact has been made plain in this dispensation(Doctrine and Covenants 77:3).

Man is the child of God, formed in the divine image and endowed withdivine attributes, and even as the infant son of an earthly father and mother iscapable in due time of becoming a man, so the undeveloped offspring of celestialparentage is capable, by experience through ages and aeons, of evolving into aGod.

JOSEPH F.SMITH,JOHN R.WINDER,ANTHON H.LUND,

First Presidency of the Church of Jesus Christ of Latter-day Saints.

Words in Season from the First Presidency

Deseret Evening News December 17, 1910, part 1, p. 3

In this Christmas message, the First Presidency devoted several sentencesto the Church’s position with regard to questions-raised by science:

Diversity of opinion does not necessitate intolerance of spirit, nor should itembitter or set rational beings against each other. The Christ taught kindness,patience, and charity.

Our religion is not hostile to real science. That which is demonstrated, weaccept with joy; but vain philosophy, human theory and mere speculations ofmen, we do not accept nor do we adopt anything contrary to divine revelationor to good common sense. But everything that tends to right conduct, thatharmonizes with sound morality and increases faith in Deity, finds favor withus no matter where it may be found.

Mormon View of Evolution

IMPROVEMENT ERA.Vol. XXVIII. SEPTEMBER, 1925. No. 11.

Editor’s Table.

A Statement by the First Presidency of the Church of Jesus Christ ofLatter-day Saints

A.3. THE BYU EVOLUTION PACKET 219

“God created man in his own image, in the image of God created he him;male and female created he them.”

In these plain and pointed words the inspired author of the book of Genesismade known to the world the truth concerning the origin of the human family.Moses, the prophet-historian, who was “learned” we are told, “in all the wisdomof the Egyptians,” when making this important announcement, was not voicinga mere opinion. He was speaking as the mouthpiece of God, and his solemndeclaration was for all time and for all people. No subsequent revelator of thetruth has contradicted the great leader and law-giver of Israel. All who havesince spoken by divine authority upon this theme have confirmed his simple andsublime proclamation. Nor could it be otherwise. Truth has but one source,and all revelations from heaven are harmonious one with the other.

Jesus Christ, the Son of God, is “the express image” of his Father’s person(Hebrews 1:3). He walked the earth as a human being, as a perfect man, andsaid, in answer to a question put to him: “He that hath seen me hath seen theFather” (John 14:9). This alone ought to solve the problem to the satisfactionof every thoughtful, reverent mind. It was in this form that the Father and theSon, as two distinct personages, appeared to Joseph Smith, when, as a boy offourteen years, he received his first vision.

The Father of Jesus Christ is our Father also. Jesus himself taught thistruth, when he instructed his disciples how to pray: “Our Father which art inheaven,” etc. Jesus, however, is the first born among all the sons of God - thefirst begotten in the spirit, and the only begotten in the flesh. He is our elderbrother, and we, like him, are in the image of God. All men and women arein the similitude of the universal Father and Mother, and are literally sons anddaughters of Deity.

Adam, our great progenitor, “the first man,” was, like Christ, a pre-existentspirit, and, like Christ, he took upon him an appropriate body, the body ofa man, and so became a “living soul.” The doctrine of pre-existence pourswonderful flood of light upon the otherwise mysterious problem of man’s origin.It shows that man, as a spirit, was begotten and born of heavenly parents, andreared to maturity in the eternal mansions of the Father, prior to coming uponthe earth in a temporal body to undergo an experience in mortality.

The Church of Jesus Christ of Latter-day Saints, basing its belief on divinerevelation, ancient and modem, proclaims man to be the direct and lineal off-spring of Deity. By his Almighty power God organized the earth, and all thatit contains, from spirit and element, which exist co-eternally with himself.

Man is the child of God, formed in the divine image and endowed withdivine attributes, and even as the infant son of an earthly father and mother iscapable in due time of becoming a man, so the undeveloped offspring of celestialparentage is capable, by experience through ages and aeons, of evolving into aGod.

HEBER J. GRANT,ANTHONY W. WINS,CHARLES W. NIB-LEY,

First Presidency.

Encyclopedia of Mormonism: Evolution

The position of the Church on the origin of man was published by the FirstPresidency in 1909 and stated again by a different First Presidency in 1925:

The Church of Jesus Christ of Latter-day Saints, basing its belief ondivine revelation, ancient and modem, declares man to be the direct andlineal offspring of Deity.... Man is the child of God, formed in the divineimage and endowed with divine attributes (see Appendix, “DoctrinalExpositions of the First Presidency”).

220 APPENDIX A. VIEWS ON SCIENCE AND RELIGION

The scriptures tell why man was created, but they do not tell how, thoughthe Lord has promised that he will tell that when he comes again (D&C 101:32-33). In 1931, when there was intense discussion on the issue of organic evolution,the First Presidency of the Church, then consisting of Presidents Heber J.Grant, Anthony W. Ivins, and Charles W. Nibley, addressed all of the GeneralAuthorities of the Church on the matter, and concluded,

Upon the fundamental doctrines of the Church we are all agreed. Ourmission is to bear the message of the restored gospel to the world. Leavegeology, biology, archaeology, and anthropology, no one of which has todo with the salvation of the souls of mankind, to scientific research,while we magnify our calling in the realm of the Church.... Upon onething we should all be able to agree, namely, that Presidents Joseph F.Smith, John R. Winder, and Anthon H. Lund were right when they said:“Adam is the primal parent of our race” [First Presidency Minutes, Apr.7, 1931].

WILLIAM E. EVENSON(Encyclopedia of Mormonism, Vol. 2)

Appendix B

Study Helps

B.1 How to Take Notes Effectively

B.2 Good Environments

B.3 Concept Mapping

221

222 APPENDIX B. STUDY HELPS

Appendix C

Image Licensing

C.1 GNU Free Documentation License 1.3

Version 1.3, 3 November 2008

Copyright c© 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

<http://fsf.org/>

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

Preamble

The purpose of this License is to make a manual, textbook, or other functional and useful document “free” in thesense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, eithercommercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit fortheir work, while not being considered responsible for modifications made by others.

This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free inthe same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs freedocumentation: a free program should come with manuals providing the same freedoms that the software does. But thisLicense is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether itis published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that contains a notice placed by the copyrightholder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license,unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any suchmanual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy,modify or distribute the work in a way requiring permission under copyright law.

A “Modified Version” of the Document means any work containing the Document or a portion of it, either copiedverbatim, or with modifications and/or translated into another language.

A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively withthe relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters)and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook ofmathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historicalconnection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political positionregarding them.

The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of InvariantSections, in the notice that says that the Document is released under this License. If a section does not fit the abovedefinition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero InvariantSections. If the Document does not identify any Invariant Sections then there are none.

223

224 APPENDIX C. IMAGE LICENSING

The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, inthe notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, anda Back-Cover Text may be at most 25 words.

A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specificationis available to the general public, that is suitable for revising the document straightforwardly with generic text editorsor (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, andthat is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to textformatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arrangedto thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent ifused for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.

Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format,LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScriptor PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaqueformats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XMLfor which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript orPDF produced by some word processors for output purposes only.

The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold,legibly, the material this License requires to appear in the title page. For works in formats which do not have any title pageas such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning ofthe body of the text.

The “publisher” means any person or entity that distributes copies of the Document to the public.A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains

XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section namementioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preservethe Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” accordingto this definition.

The Document may include Warranty Disclaimers next to the notice which states that this License applies to theDocument. These Warranty Disclaimers are considered to be included by reference in this License, but only as regardsdisclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on themeaning of this License.

2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either commercially or noncommercially, provided thatthis License, the copyright notices, and the license notice saying this License applies to the Document are reproducedin all copies, and that you add no other conditions whatsoever to those of this License. You may not use technicalmeasures to obstruct or control the reading or further copying of the copies you make or distribute. However, you mayaccept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow theconditions in section 3.

You may also lend copies, under the same conditions stated above, and you may publicly display copies.

3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numberingmore than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry,clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover.Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present thefull title with all words of the title equally prominent and visible. You may add other material on the covers in addition.Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions,can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many asfit reasonably) on the actual cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include amachine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard networkprotocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you musttake reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparentcopy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaquecopy (directly or through your agents or retailers) of that edition to the public.

C.1. GNU FREE DOCUMENTATION LICENSE 1.3 225

It is requested, but not required, that you contact the authors of the Document well before redistributing any largenumber of copies, to give them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above,provided that you release the Modified Version under precisely this License, with the Modified Version filling the role ofthe Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. Inaddition, you must do these things in the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those ofprevious versions (which should, if there were any, be listed in the History section of the Document). You may usethe same title as a previous version if the original publisher of that version gives permission.

B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modificationsin the Modified Version, together with at least five of the principal authors of the Document (all of its principalauthors, if it has fewer than five), unless they release you from this requirement.

C. State on the Title page the name of the publisher of the Modified Version, as the publisher.

D. Preserve all the copyright notices of the Document.

E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.

F. Include, immediately after the copyright notices, a license notice giving the public permission to use the ModifiedVersion under the terms of this License, in the form shown in the Addendum below.

G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’slicense notice.

H. Include an unaltered copy of this License.

I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, newauthors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History”in the Document, create one stating the title, year, authors, and publisher of the Document as given on its TitlePage, then add an item describing the Modified Version as stated in the previous sentence.

J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of theDocument, and likewise the network locations given in the Document for previous versions it was based on. Thesemay be placed in the “History” section. You may omit a network location for a work that was published at leastfour years before the Document itself, or if the original publisher of the version it refers to gives permission.

K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preservein the section all the substance and tone of each of the contributor acknowledgements and/or dedications giventherein.

L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers orthe equivalent are not considered part of the section titles.

M. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.

N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.

O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and containno material copied from the Document, you may at your option designate some or all of these sections as invariant. To dothis, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinctfrom any other section titles.

You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your ModifiedVersion by various parties—for example, statements of peer review or that the text has been approved by an organizationas the authoritative definition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-CoverText, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one ofBack-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includesa cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting onbehalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisherthat added the old one.

The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicityfor or to assert or imply endorsement of any Modified Version.

226 APPENDIX C. IMAGE LICENSING

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this License, under the terms defined in section 4above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the originaldocuments, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that youpreserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and multiple identical Invariant Sections may bereplaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make thetitle of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisherof that section if known, or else a unique number. Make the same adjustment to the section titles in the list of InvariantSections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History” in the various original documents, formingone section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled“Dedications”. You must delete all sections Entitled “Endorsements”.

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents released under this License, and replacethe individual copies of this License in the various documents with a single copy that is included in the collection, providedthat you follow the rules of this License for verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it individually under this License, providedyou insert a copy of this License into the extracted document, and follow this License in all other respects regardingverbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independent documents or works, in or on avolume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is notused to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Documentis included in an aggregate, this License does not apply to the other works in the aggregate which are not themselvesderivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is lessthan one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Documentwithin the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they mustappear on printed covers that bracket the whole aggregate.

8. TRANSLATION

Translation is considered a kind of modification, so you may distribute translations of the Document under the termsof section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders,but you may include translations of some or all Invariant Sections in addition to the original versions of these InvariantSections. You may include a translation of this License, and all the license notices in the Document, and any WarrantyDisclaimers, provided that you also include the original English version of this License and the original versions of thosenotices and disclaimers. In case of a disagreement between the translation and the original version of this License or anotice or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4)to Preserve its Title (section 1) will typically require changing the actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License.Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rightsunder this License.

However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated(a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently,if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifiesyou of the violation by some reasonable means, this is the first time you have received notice of violation of this License(for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

Termination of your rights under this section does not terminate the licenses of parties who have received copies orrights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copyof some or all of the same material does not give you any rights to use it.

C.2. CREATIVE COMMONS ATTRIBUTION 2.0 LICENSE 227

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Glossary

r-process Nucleosynthesis involving the rapid capture ofmany neutrons., 68

s-process Nucleosynthesis due to the capture of neutrons(when there aren’t many neutrons around)., 67

Absolute dating The process of determining the actual age of aspecimen, as opposed to relative ages of severalspecimens., 103

Accuracy When a measurement yields a similar result ev-ery time it is taken., 12

Active planet A planet whose surface is subject to change dueto convective processes beneath the surface., 113

Allele One of two copies of a particular gene carriedby an organism., 155

Alpha decay Spontaneous decay by the emission of a 42He nu-

cleus, or alpha particle., 88Alternative Hypothesis a researcher’s best explanation for an observa-

tion., 10Amino acids Molecules composed of carbon, oxygen, hydro-

gen, and nitrogen which serve as the basic build-ing blocks for life., 84

Ancestral traits traits that are shared with an ancestral species.,175

Anecdotal Evidence Evidence consisting of inferences based on thechronological relationship of events or personalfirsthand or spiritual experiences which have notbeen or cannot be tested empirically., 13

Anthropogenic forcings Climate forcings that occur as a result of humanactivity., 22

Atomic number The number of protons in the nucleus of anatom., 60

Base In scientific notation, the number representinghow many times the base should be multipliedby itself., 30

235

236 Glossary

Base The integer that serves as the basis for a numbersystem. The numbers you are accustomed toseeing are written in a base 10 format, meaningthat the digits represent, ones, tens, hundreds,and so on. Another common base is base two,in which the digits represent ones, two, fours,eights, etc... Base two numbers are also calledbinary numbers., 30

Beta decay See Beta minus decay or beta plus decay., 89Beta minus decay Spontaneous decay where a neutron is trans-

formed into a proton and electron, and the elec-tron is subsequently ejected from the nucleus.,89

Beta plus decay Spontaneous decay where a proton is trans-formed into a neutron and positron, and thepositron is subsequently ejected from the nu-cleus., 89

Big bang theory A theory which says that the universe startedvery small and very dense, and has been ex-panding for the last several billion years., 54

Bohr model An atomic model where the electrons are onlyallowed to have discrete set amounts of angularmomentum or energy., 74

Cap and trade a system where total carbon dioxide emissionsare limited, and entities are allowed carbonemissions through their purchase of “carboncredits.”, 23

Carbohydrates Molecules composed of carbon, oxygen, andhydrogen which have a distinct rigid ring-likestructure., 84

Carbon emissions Carbon dioxide gas released into the atmo-sphere., 22

Catastrophism A hypothesis stating that Earth’s geological his-tory could be accounted for by a series of catas-trophic events., 100

Cenozoic era The age of recent life., 127Chemical evolution Transmutation on a large scale over a long pe-

riod of time., 63Chromosome A set of genes; a linear strand of DNA., 156Cladogram See phylogenetic tree, 172Climate The conditions found in a given region, consist-

ing of many factors such as atmospheric compo-sition, temperature, tectonic activity, solar in-tensity, etc...., 120

Climate change A change in the average temperature, humidity,weather, etc... of a region., 20

Codon A group of three nitrogenous bases., 163Comparative anatomy Similarities and differences in the physical struc-

tures of two species., 169Compound (chemical) A chemical combination of atoms of two or more

elements., 77

Glossary 237

Convection Heat transfer that occurs when hot materialsrise., 113

Convergent boundary A region where two tectonic plates collide., 116Cosmic microwave background A persistent signal of microwaves coming at us

from all directions in space., 54Critical Review The process whereby a researcher’s work is scru-

tinized by the scientific community., 13Cross-cutting relationships A principle of relative dating which states that a

rock unit that is broken formed before the breakin it did., 99

Cyanobacteria Photosynthesizing bacteria that first appearedbillions of years ago, and are thought to be theorigin of oxygen in our atmosphere, 131

Cyclicity The characteristic and frequency of repetitionin a process., 35

Daughter (radioactivity) The nucleus that remains after a spontaneousdecay process., 88

Decay series A sequence of radioactive decays that ensueswhen the daughter nucleus of a decay processis itself radioactive., 91

Deoxyribonucleic acid A molecule made of nucleotides which carriesthe genetic code., 84

Derived traits traits that a descendant species has, but that itsancestral species did not have., 175

Divergent boundary A region where two tectonic plates are separat-ing., 116

DNA See Deoxyribonucleic acid., 84Dominant allele An allele whose trait will be expressed regard-

less of which other allele for the same trait ispresent., 156

Doppler shift The shifting of frequencies of sound when thesource of the sound is moving relative to an ob-server., 52

Duration the length of an event or time interval betweenevents., 35

Electron A fundamental subatomic particle with negativecharge and very little mass., 60

Electron capture The spontaneous decay process wherein anatomic electron and proton combine to form aneutron., 89

Element A fundamental type of matter, or in other wordsa type of matter that is not a composite of othertypes of matter., 60

Emission spectrum The unique set of specific wavelengths emittedby a hot rarefied gas., 53

Empirical based on measurement, as opposed to personalrecollection, 10

Empirical Evidence evidence consisting of measurements and obser-vations using the senses or instruments that ex-tend the senses., 12

238 Glossary

Epicycle An additional orbital path of planets in geocen-tric models used to explain retrograde motion.,45

Evolution The change and development of species overtime., 134

Exponent See “base”., 30

False Negative See “Type II Error”., 11False Positive See “Type I Error”., 11Fossil succession A principle of relative dating which states the

unique set of fossils found in rock units can beused to correlate formation times of geologic fea-tures which are geographically separated., 100

Gamma decay The spontaneous emission of a gamma ray byan excited nucleus., 90

Gene A discrete unit containing heritable traits; a seg-ment of DNA found on a chromosome., 155

General relativity An extension of special relativity that includesgravitational interactions., 52

Geocentric model A cosmological model where the Earth is at thecenter of the Universe., 45

Greenhouse effect When light energy is easily transmitted into anobject, but heat energy is retained., 20

Half life The amount of time it takes for half of a sampleof radioactive material to decay., 93

Hard sphere model An early atomic model that suggested all matterwas made of indivisible pieces called “atomos”.,71

Heliocentric model A cosmological model where the Sun is at thecenter of the Universe., 48

Homologous traits traits that are similar because they are inheritedfrom a common ancestor., 195

Hypothesis a reasoned possible explanation for an observa-tion or set of observations., 10

Inclusions A principle of relative dating which states thatmaterial wholly included inside a rock existedbefore the rock that surrounds it did., 99

Inheritance The mechanism whereby traits are passed fromparent to offspring., 155

Inheritance by acquired traits An idea which suggests animal populationsevolve when traits acquired during an individuallifetime are passed on to its offspring., 151

Initial daughter problem The quandary in radiometric dating associatedwith not always knowing whether and how muchof a daughter isotope was in existence when amaterial formed., 107

Intermediate form A species that is intermediate in form and timebetween two other species., 173

Ionizing radiation Radiation which carries sufficient energy toknock electrons from atoms., 87

Glossary 239

Isotopes Atoms with the same number of protons butdifferent numbers of neutrons., 61

Lateral continuity A principle of relative dating which states thatsedimentary rock layers that have been sepa-rated by erosion were once continuous., 99

Law A generalized description of observations; a gen-eral rule that explains what will happen in agiven set of circumstances., 11

Law of definite proportions When elements react to form compounds, theyreact in defined whole number ratios., 77

Law of independent assortment A scientific law which states that alleles for agiven trait are passed to offspring independentlyof alleles for other traits., 156

Law of segregation A scientific law which states that the allelespassed onto offspring are selected randomly.,156

Light year The distance that light will travel throughempty space over the course of one year., 33

Lipids Molecules composed of carbon, oxygen, and hy-drogen with a characteristic rod-like structure.,84

Macroevolution Large scale evolutionary change representingthe accumulation of many small scale changes.,167

Macroscopic dating Determining the age of things based on visiblecharacteristics or formations., 104

Mantissa The portion of a number expressed in scientificnotation that tells you information beyond theappropriate power of ten., 30

Mass number The total number of nucleons in the nucleus ofan atom. Equal to the sum of the atomic num-ber and neutron number., 61

Mesozoic Era The age of middle life., 126Microevolution Changes in allele frequencies that occur within a

population from generation to generation., 167Model A visualization or analogue that helps a scientist

understand a particular system., 12mRNA messenger RNA; a single stranded molecule

made of nucleotides that carries the genetic mes-sage out of the cell nucleus and into the body ofthe cell., 163

Mutation A change in the order of nitrogenous bases in agene., 164

Natural forcings Climate forcings (factors that influence climate)that occur independent of human activity., 22

Natural selection A theory which states that organisms evolveas ecologically favored genetic variations arepassed on to offspring., 152

240 Glossary

Neo-Platonism A philosophy that suggests all things in natureare based on perfect ideas, and thus living or-ganisms and fossils are just nature’s way of ex-pressing perfection., 148

Neutron An electrically neutral subatomic particle foundin the nucleus of atoms., 60

Noble gasses A group of elemental gasses that are chemicallyinert., 80

Nuclear reaction Any type of interaction involving the nucleus ofan atom., 63

Nucleosynthesis The synthesis of atoms (i.e. nuclei) through nu-clear reactions., 63

Nucleotides Molecules consisting of a carbohydrate (sugar),a group of phosphates, and a nitrogenous basewhich serve as the building blocks for DNA., 84

Nucleus (atoms) The small region at the center of an atom whichcontains most of the atom’s mass., 60

Null Hypothesis A hypothesis that states that the explanationfor the observation is something other than theresearch hypothesis., 10

Objective based on a measurable property and not a ques-tion of personal opinions or feelings., 9

Original horizontality A principle of relative dating which states thatthe sediments that make sedimentary rocks areoriginally deposited in horizontal layers., 99

Paleozoic Era The age of ancient life., 125Parent (radioactivity) The original radioactive nucleus in a sponta-

neous decay process., 88Phases (astronomy) Refers to the amount of a celestial body’s sur-

face illuminated by the Sun and visible to anEarth-based observer., 46

Phyletic gradualism Speciation that occurs gradually over largespans of time., 167

Phylogenetic Tree A diagram representing evolutionary linkagesbetween species., 172

Planetary model An atomic model where the electrons orbitaround a positively charged nucleus, much likeplanets orbiting a star., 73

Plum pudding model An atomic model wherein the electrons are em-bedded in a soft positively charged mass, muchlike plums or raisins in a pudding., 72

Precambrian era The age of early Earth., 124Precision When repeated measurements fall within a nar-

row range of values., 12Predictive Suggesting that there are additional phenomena

which could be observed., 16Primates A taxonomic group of mammals which includes

humans, apes, and monkeys., 170Processes that result in rapid macroevolution. , 167Proteins Long, folded chains of amino acids., 84

Glossary 241

Proton A positively charged subatomic particle foundin the nucleus of an atom., 60

Pseudogenes Vestigial genes, which were once functional, buthave been disabled by random mutations., 201

Quantum mechanics A theory which describes subatomic particles aslocalized waves., 51

Radiometric dating Using radioactive isotopes to determine the ageof a sample., 106

Rate How quickly a process proceeds., 35Rate constant process A process that proceeds always at the same rate

and with regular cyclicity., 36Recessive allele An allele whose trait is expressed only when a

dominant allele is not present., 156Relative dating Determining the sequence of events in Earth’s

history without knowing actual ages., 99Replication The process by which exact copies of DNA are

made., 159Reproducibility An experiment is reproducible when another re-

searcher can perform the same experiment andget the same result., 13

Research Hypothesis See “Alternative Hypothesis”., 10Retrograde motion The apparent periodic backward motion of the

planets relative to the stars., 45Ribosome The structure within a cell where the genetic

code is read and proteins are assembled., 163

Scaling Representing a system at a size other than itsactual size., 31

Scientific notation a compact way of writing numbers that are noteasy to express in standard format., 30

Sequence The ordering of events in time., 35Spacetime The four dimensional fabric of our universe in

which time and space are intrinsically linked to-gether., 39

Special creation A theologically based idea which states God cre-ated the various species the way that they aretoday., 147

Special relativity A theory based on two premises: that the laws ofphysics are the same for all observers, and thatlight traveling through vacuum has the samemeasured speed for all observers - regardless oftheir motion relative to the source. Within thecontext of this theory, space and time changeas object move at speeds close to the speed oflight., 52

Student honor following the path of discipleship and learningto be more like Christ - learning to think, tofeel, and to act more as He does., xxviii

Subjective subject to an individuals personal opinions, feel-ings, or tastes; the opposite of objective., 9

242 Glossary

Superposition A principle of relative dating which states thatolder sedimentary rock units in an undisturbedsequence are found below younger units., 99

System The portion of the universe of interest in a sci-entific investigation., 15

Tectonics The movement of Earth’s crust on the surface,driven by mantle convection., 113

Theory An attempt to explain, at a more fundamen-tal level, how or why a particular phenomenonhappens., 11

Transcription The process wherein the nitrogenous bases thatcode for a particular gene is copied into a strandof mRNA., 163

Transform boundary A region where two tectonic plates slide pastone another., 116

Translation The process in which proteins are formed basedon the genetic code carried by mRNA., 163

Transmutation Changing one type of element into anotherthrough nuclear reactions., 63

tRNA Transfer RNA; a single stranded molecule com-posed of nucleotides that carries amino acidsinto the ribosome., 164

Type I Error When the null hypothesis is rejected, eventhough it is actually correct., 11

Type II Error When the null hypothesis is accepted, eventhough it is actually incorrect., 11

Uniformitarianism A hypothesis stating that Earth’s geological his-tory can be explained by the same processes oc-curring today., 100

Vestigial structure An anatomical structure that played a vital rolein the body of an ancestral species, but nolonger plays a vital role or may be completelynon-functional in the body of the descendantspecies., 194

Index

s-process, 67Ardipithecus ramidus, 179Australopithecus afarensis, 181Homo ergaster, 185Homo habilis, 183Homo heidelbergensis, 187Homo neanderthalensis, 189Homo sapiens, 191Sahelanthropus tchadensis, 177

Absolute dating, 103Accuracy, 12Active planets, 113Alleles, 155Alpha decay, 88, 93Alpha particles, 73Amino acids, 84Anecdotal evidence, 13Anthropogenic forcings, 22Aristotle, 17, 98Atmospheric composition, 122Atomic Models, 70Atomic Nucleus, 73Atomic number, 60Atomic theory, 77Atoms, 40, 60, 70, 77

BacteriaCyanobacteria, 131

Base, 30Becquerel, Henri, 87Beta decay, 88Beta minus decay, 88, 93Beta plus decay, 89, 93Big bang theory, 51, 54, 63Bipedalism, 171Black hole, 68Bohr model, 74Bohr, Niels, 74Brigham Young University - Idaho

Honor code, xxviiMission statement, xxvii

Cap and trade, 23Carbohydrates, 84Carbon dating, 108

Carbon emissions, 22Catastrophism, 100Cathode rays, 71Cenozoic era, 127Chemical bonding, 82Chemical evolution, 63Chromosomes, 155, 156, 163, 202Cladogram, 173Climate, 20, 121Climate change, 20, 121Climate forcings, 22Clocks, 36, 103CNO cycle, 65Codon, 163Comparative anatomy, 170Conduction, 113Confidence level, 11Continental drift, 114Convection, 113, 116Convergent boundary, 116Copernicus, Nicolaus, 48Cosmic microwave background, 54Cosmological models, 43Coulomb force, 91Covalent bonding, 82Craters, 113Crick, Francis, 157Critical review, 13Cross-cutting relationships, 99Curie, Marie, 87Cuvier, Georges, 100, 149Cyclicity, 35

Dalton, John, 77Darwin, Charles, 151Daughter (radioactivity), 88Decay series, 91Deep time, 37, 151Democritus, 71Dendrochronology, 104, 108Discovery, 98Divergent boundary, 116DNA, 156, 157, 162, 197Dobereiner, Johann, 77Dominant allele, 156

243

244 INDEX

Doppler shift, 52Duration, 35

Earth, 113, 116Einstein, Albert, 18, 38Electron, 60Electron capture, 89, 93Electrons, 71Electrostatic force, 91Elements, 60Emission spectrum, 73Empedocles, 70Empirical evidence, 12Evolution, 123Evolution, criteria for, 166Experimentation, 12Exponent, 30Extinction, 149Eyring, Elder Henry B., xEyring, Henry B., 6

False Negative, 11False Positive, 11Fossil fuels, 22, 119Fossil succession, 100Foundations program, ixFour element model, 70Franklin, Benjamin, ix

Galactic evolution, 56Galileo Galilei, 17, 46Gamma decay, 90, 93General relativity, 39, 52Genes, 155, 156, 163Genetics, 155Geocentric model, 43, 45Geologic time scale, 101Geology, 98Glacial lake sediments, 104Glaciers, 106

seasonal layers in, 106Gold foil experiment, 73Gravity, 39Greenhouse effect, 20GULO gene, 201

Half life, 106Hard sphere model, 71Hardy, Godfrey, 166Hardy-Weinberg criteria, 166Heliocentric model, 48Hess, Harry, 115Holmes, Arthur, 111Honor code, xxviiHubble’s law, 53

Human development, 203Human evolution, 169Hutton, James, 100Hypothesis, 11

Alternative hypothesis, 10Null hypothesis, 10Research hypothesis, 10Two hypothesis method, 10

Inclusions, 99Inheritance, 155Inheritance of acquired traits, 148, 151Initial daughter problem, 107Intermediate form, 173Intuition, 37Ionic bonding, 82Ionizing radiation, 87Isotopes, 61

Kelvin, Lord William Thomson, 103

Lake sediments, 104Lamarck, Jean-Baptiste, 151Lateral continuity, 99Lavoisier, Antoine, 76Law, 11Law of independent assortment, 156Law of Segregation, 156Learning Model, xxvLee, Elder Rex E., xLeptons, 55Light

speed of, 32Light year, 33Lipids, 84Lorentz contraction, 39Lower case “s” scientists, ixLyell, Charles, 101

Macroevolution, 167Mantissa, 30Mass number, 61Mendel, Gregor, 155Mendeleyev, Dmitri, 79Merrill, Joseph F., 6Mesozoic era, 126Method of isochrons, 107Microevolution, 167Model, 12Molecular genetics, 165Molecules, 84Moon, phases of, 46mRNA, 163Mutation, 164, 165, 201

INDEX 245

Natural forcings, 22Natural selection, 152Nelson, Elder Russell M., xNeo-Platonism, 148Neutron, 60Neutron star, 68Neutrons, 55New synthesis, the, 166Newlands, John, 79Newton, Issac, 18Newtonian gravitation, 51Nuclear reactions, 63Nucleosynthesis, 55, 63

Big bang, 55Nucleosynthesis, big bang, 63Nucleotides, 84Nucleus, 60Nucleus (atomic), 73

Objective questions, 9Oceans, 122Original horizontality, 99Overview boxes, xv

Paleozoic era, 125Pangaea, 114Parent (radioactivity), 88Peer review, 13Periodic table, 77Phyletic gradualism, 167Phylogenetic tree, 173, 197Planetary model, 72Plate Tectonics, 114Plate tectonics, 122Playfair, John, 101Plum pudding model, 71Powers of ten, 29PP-I cycle, 64Precambrian era, 124Precision, 12Predictions, 16Priestley, Joseph, 76Primates, 170Proteins, 84, 163Proton, 60Protons, 55Proust, Joseph, 76Pseudogenes, 201Ptolemy, Claudius, 45Punctuated equilibrium, 167

Quantization, 74Quantum mechanics, 40, 51, 61, 75Quarks, 55

Roentgen, Wilhelm, 87Radioactivity, 87, 103, 106Radiocarbon dating, 108Radiometric dating, 106Rate, 35Rate constant process, 36Recessive allele, 156Redshift, 52Regulatory genes, 203Relative dating, 99Relativity, 38Replication, 159Reproducibility, 13Retrograde motion, 45Revelation, 2Review questions, xvRibosome, 163RNA, 163Rutherford, Ernest, 73, 87

Salinity of ocean, 104Scales of nature, 37Scaling, 31Science

Limitations of, 2Scientific Method, 2Scientific method, 16Scientific notation, 29Scientific questions, 9Scott, Elder Richard G., 2Sea level, 122Seafloor spreading, 115Sedimentation, 113Sequence, 35Shroud of Turin, 109Smith, William, 100Spacetime, 39Special creation, 147Special relativity, 39, 52Standard format, 30Stellar fuel cycle, 64Stellar nucleosynthesis, 64Stellarium, 43Steno, Nicholas, 99Strong nuclear force, 91Subjective questions, 9Supercontinents, 118Superposition, 99System, 15

Talmage, James E., 7Tectonics, 113Telescopes, 46Temperature

246 INDEX

of Earth, 103Theory, 11Thomson, J.J., 71Time, 35, 103Time dilation, 39Transcription, 163Transform boundary, 116Translation, 163Transmutation, 63Tree rings, 104Triads (chemistry), 77Triple alpha reaction, 65tRNA, 163Truth, 2, 98Type I Error, 11Type II Error, 11

Uncertainty principle, 41Uniformitarianism, 100Uranium-lead dating, 110Ussher, Archbishop James, 103

Venus, phases of, 46Vestigial structures, 194Volcanic activity, 122

Watson, James, 157Wave-particle duality, 41Wavefunctions, 75Wegener, Alfred, 114Weinberg, Wilhelm, 166White dwarf, 68Widtsoe, John A., 7