The Tree of Life

Introduction to Biological Diversity

Tools for Studying Historyof Life: Phylogenies and the Fossil Record

The evolutionary history of a group of organisms is called a phylogeny

A phylogenetic tree shows ancestor-descendant relationships among evolutionary groups (usually species or populations).

Fossils are physical evidence left by organisms from the past. The fossil record includes all fossils that have been found and recorded.

The Parts of a Phylogenetic Tree

A Field Guide to Reading Phylogenetic Trees

Populations are represented by branches Nodes show where ancestral groups split into descendant groups.

a polytomy is a node where more than two descendant groups branch off.

Adjacent branches are sister taxa a taxon is any named group of organisms

Tips are branch endpoints represent living groups or a group’s end in extinction.

Field Guide Continued Rooted

The most ancient node of the tree is shown at the bottom Location of this node is determined using an out groupa taxonomic group that diverged before the rest of the taxa being studied.

An ancestor and all its descendants form a monophyletic group

a clade or lineage

Phylogeneticic Tree Illustrating Some of the Great Apes

Alternative Phylogenetic Treesrepresenting the same evolutionary relationships

Ways to Estimate Phylogenies

Morphological and genetic characteristics are used to estimate phylogenetic relationships among species.

Two approaches Phenetic approach Cladistic approach

The Phenetic Approach

Computes a statistic that summarizes the overall similarity among populations based on the data. A computer program then compares the similarities among populations and builds a tree

clusters the most similar populations places more divergent populations on more distant branches.

The Cladistic Approach

Focus on synapomorphiesthe shared derived characters of the species under study

A computer program is used to identify which traits are unique to each monophyletic group

many such traits measuredthen place the groups on a tree in the correct relationship to one another

Difficulties with the Cladistic Approach

Cases of convergent evolution. Biologists then use parsimony to try to identify the phylogenetic tree that minimizes the overall number of convergent evolution events.

Principle of logic stating that the most likely explanation or pattern is the one that implies the least amount of change or the least complexity. Assumes that convergent evolution should be much rarer than similarity due to shared descent.

Whale Evolution: A Case History

Traditionally, phylogenetic trees based on morphological data place whales outside of the artiodactyls

mammals such as cows, deer, and hippos have hooves an even number of toes unusual pulley-shaped ankle bone (astralagus)

Whale Evolution

DNA sequence datasuggest a close relationship between whales and hippos

A phylogenetic tree showing closely related whales and hippos is less parsimonious than the tree based on morphological data because it requires the evolution and then loss of the astralagus in whales.

Recent Data Recent data on short interspersed nuclear elements (SINEs) show that whales and hippos share several that are absent in other artiodactyl groups.

These SINEs are shared derived traits (synapomorphies) and support the hypothesis that whales and hippos are indeed closely related.

SINE Genes

Environment Changes Life/Life Changes Environment

Geological events that alter environments Change the course of biological evolution

Conversely, life changes the planet that it inhabits

EARLY EARTH

Earth formed about 4.5 billion years ago Along with the rest of the solar system

Earth’s early atmosphere Contained water vapor and many

chemicals released by volcanic eruptions

A. Oparin and J. Haldane In the 1920s, independently postulated that

conditions on the early Earth favored the synthesis of organic compounds from inorganic ones. The environment in the early atmosphere

would have promoted the joining of simple molecules to form more complex ones.

The energy required to make organic molecules could be provided by lightning and UV radiation in the primitive atmosphere.

The lack of an ozone layer in the early atmosphere would have allowed this radiation to reach the Earth.

They reasoned that this cannot happen today because high levels of oxygen in the atmosphere attack chemical bonds.

Stanley Miller and Harold Urey, 1953

tested the Oparin-Haldane hypothesis creating, in the laboratory, the conditions that had

been postulated for early Earth

They discharged sparks in an “atmosphere” of gases and water vapor. H2O, H2, CH4, and NH3 a more strongly reducing environment than is

currently believed to have existed on early Earth

Produced a variety of amino acids and other organic molecules

Miller and Urey experiment

CH4

NH3

H2

Water vapor Electrode

Condenser

Cold Water

Cooled water containing organic molecules

Sample for chemical analysis

H2O

Deep-Sea Vents

Instead of forming in the atmosphere The first organic compounds on Earth may

have been synthesized near submerged volcanoes and deep-sea vents

Extraterrestrial Sources of Organic Compounds

Some of the organic compounds from which the first life on Earth arose May have come from space

Carbon compounds Have been found in some of the

meteorites that have landed on Earth

Protobionts Laboratory experiments demonstrate

that protobionts aggregates of abiotically produced

molecules surrounded by a membrane or membrane-like structure

Could have formed spontaneously from abiotically produced organic compounds

Example: small membrane-bounded droplets called liposomes Can form when lipids or other organic

molecules are added to water

Liposomes20 m

(a)Simple reproduction. This lipo-some is “giving birth” to smallerliposomes (LM).

(b) Simple metabolism. If enzymes—in this case, phosphorylase and amylase—are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products.

Glucose-phosphate

Glucose-phosphate

Phosphorylase

Starch

Amylase

Maltose

Maltose

Phosphate

The “RNA World” and the Dawn of Natural Selection

The first genetic material Was probably RNA, not DNA RNA

molecules called ribozymes have been found to catalyze many different reactions, including

Self-splicing Making complementary copies of short

stretches of their own sequence or other short pieces of RNA

Ribozyme(RNA molecule)

Template

Nucleotides

Complementary RNA copy

3

5 5

How life actually began is speculative

There are clues in the molecules and anatomical developments of each species. These clues together with the fossil record have produced several theories about how life evolved on Earth.

Earth formed about 4.5 billion years ago,

The oldest fossils embedded in rocks from western Australia about 3.5 billion years old. resemble bacteria, so scientists think that life originated

much earlier. may have been as early as 3.9 billion years ago

when Earth began to cool to a temperature at which liquid water could exist.

Using the Fossil Record

The fossil record is the only source of direct evidence about what prehistoric organisms looked like, where they lived, and when they existed.

Careful study of fossils opens a window into the lives of organisms that existed long ago and provides information about the evolution of life over billions of years.

Most Fossils Form When an Organism is Buried in Sediment Before Decomposition Begins

Four Types of Fossils

Index fossils

Are similar fossils found in the same strata in different locations

Allow strata at one location to be correlated with strata at another location

Absolute Ages of Fossils

Can be determined by radiometric dating

Limitations of the Fossil Record

There are several features and limitations of the fossil record that must be recognized

habitat biastaxonomic biastemporal biasand abundance bias

Limitations are Recognized

Paleontologists recognize that they are limited to asking questions about tiny and scattered segments on the tree of life Yet analyzing fossils is the only way scientists have of examining the physical appearance of extinct forms and inferring how they lived.

The Geologic Record

By studying rocks and fossils at many different sites Geologists have established a geologic

record of Earth’s history Three Eonss the Archaean the Proterozoic the Phanerozoic

Life’s Timeline Major events in the history of life are marked on the timeline which has been broken into four segments

the Precambrianthe Paleozoicthe Mesozoicthe Cenozoic).

Precambrian

Almost all life was unicellular. Little or no oxygen in atomosphere.

Paleozoic

Paleozoic era

Mesozoic

Ended with extinction of the dinosaurs

Cenozoic

The Cambrian Explosion

Animals first originated around 565 million years ago

Animals diversified into almost all the major groups extant today

Known as the Cambrian explosion.

Cambrian Fossils

•Three major fossil beds record this explosion of animal life

The Doushantuo Microfossils

Researchers identified Sponges Cyanobacteria Multicellular algae

Samples dated 570–580 Ma

Also Animal Embryos

In early stages of development Samples contained

one-celled, two-celled, four-celled, and eight-celled fossilsindividuals containing larger cell numbers whose overall size was the same exactly the pattern that occurs during cleavage in today’s animals.

The Ediacaran Faunas

found in these Australian deposits Sponges Jellyfish Comb jellies

Traces of other animals dated 544–565 Ma

Indicate that shallow-water marine habitats contained a diversity of animal species

•Virtually every major animal group is represented in the Burgess Shale fossils

Compelling Picture of Life in the Oceans 525–515 Ma

Few, if any, species in the Ediacaran faunas are also found in the Burgess Shale–type assemblages 20–40 million years later

New species of sponges, jellyfish, and comb jellies are abundant Entirely new groups as well

arthropods mollusks.

Oldest Fossils

As prokaryotes evolved, they exploited and changed young Earth

The oldest known fossils are stromatolites Rocklike structures composed of many

layers of bacteria and sediment Which date back 3.5 billion years ago

Stromatolites

Lynn Margulis (top right), of the University of Massachussetts, and Kenneth Nealson, of the University of Southern California, are shown collecting bacterial mats in a Baja California lagoon. Themats are produced by colonies of bacteria that live in environments inhospitable to most other life. A section through a mat (inset) shows layers of sediment that adhere to the sticky bacteria asthe bacteria migrate upward.

Some bacterial mats form rocklike structures called stromatolites,such as these in Shark Bay, Western Australia. The Shark Baystromatolites began forming about 3,000 years ago. The insetshows a section through a fossilized stromatolite that is about3.5 billion years old.

(a)

(b)

Figure 26.11a, b

Oxygenic photosynthesis

Probably evolved about 3.5 billion years ago in cyanobacteria

Figure 26.12

The First Eukaryotes

The oldest fossils of eukaryotic cells Date back 2.1 billion years

Endosymbiotic Origin of Mitochondria and Plastids

The theory of endosymbiosis Proposes that mitochondria and plastids

were formerly small prokaryotes living within larger host cells

Probably gained entry to the host cell as undigested prey or internal parasites

The host and endosymbionts would have become a single organism

Eukaryotic Cells as Genetic Chimeras

Additional endosymbiotic events and horizontal gene transfers May have contributed to the large

genomes and complex cellular structures of eukaryotic cells

The Earliest Multicellular Eukaryotes

Molecular clocks Date the common ancestor of

multicellular eukaryotes to 1.5 billion years

The oldest known fossils of eukaryotes Are of relatively small algae that lived

about 1.2 billion years ago

The Colonial Connection The first multicellular organisms were

colonies Collections of autonomously replicating cells

Figure 26.1610 m

Specialized Cells

Some cells in the colonies Became specialized for different

functions The first cellular specializations

Had already appeared in the prokaryotic world

Mass Extinctions

The rapid extinction of many groups

loss of at least 60% of all species within 1 million yearscaused by catastrophic episodes. traditionally recognize five mass extinctions

Background and Mass Extinctions

Background extinctions typically occur when normal environmental change or competition reduces a population to the point where it dies out. Mass extinctions occur when unusual large-scale environmental change causes the extinction of many normally well-adapted species. Natural selection causes most background extinctions, whereas random chance plays a large

role in mass extinctions.

Permian

The Permian extinction Claimed about 96% of marine animal

species and 8 out of 27 orders of insects Is thought to have been caused by

enormous volcanic eruptions

Cretaceous The Cretaceous extinction (K-T)

Doomed many marine and terrestrial organisms, most notably the dinosaurs

Is thought to have been caused by the impact of a large meteor

Figure 26.9

NORTHAMERICA

ChicxulubcraterYucatán

Peninsula

Evidence Conclusive evidence—including iridium, shocked quartz, and microtektites found in rock layers dated to 65 million years ago, as well as a huge crater off the Yucatán Peninsula—has led researchers to accept the impact hypothesis. The large-scale environmental change triggered by the asteroid impact caused the extinction of 60% to 80% of all species.

Selectivity Some evolutionary lineages were better able than others to withstand the environmental change brought on by the asteroid impact. Why certain groups survived while others perished is still a mystery.

Recovery Ferns appear to have replaced diverse woody and flowering plants in many habitats following the K-T extinction. Mammals diversified to fill the niches left empty following the dinosaur extinctions.

Reconstructing the Tree of Life: A Work in Progress

A three domain system Has replaced the five kingdom system Includes the domains Archaea, Bacteria,

and Eukarya Each domain

Has been split by taxonomists into many kingdoms

Adaptive Radiations One broad pattern that can be observed in the tree of life

Dense groups of branches scattered throughout the tree Star phylogenies

represent major diversification over a relatively short period of time

a process known as adaptive radiation.

Colonization Events as a Trigger

Adaptive radiations occurred following the colonization of unoccupied island habitats Example: Anolis lizards of the Caribbean islands

The Role ofMorphological Innovation

Morphological innovation Opportunities for adaptive radiation

Examples: Important new traits such as limbs, wings, flowers, and jaws Allowed descendants to live in new areas, move in new ways, and exploit new sources of food