Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 26 The Tree of Life

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Chapter 26

• The Tree of Life


1. How Life Began

• Geological events that alter environments

– Change the course of biological evolution

• Conversely, life changes the planet that it inhabits

Figure 26.1


• Conditions on early Earth made the origin of life possible

• Hypothesis:

– Chemical and physical processes on early Earth produced very simple cells through a sequence of stages


1. Atmosphere

• Earth formed ~ 4.6 billion years ago

• Earth’s early atmosphere

– Contained water vapor + many chemicals released by volcanic eruptions

– Little or no O2 reducing atmosphere

– But it did have: CO, CO2, H2, N2, H2O, S, and HCl,


2. SEAS

• First organic compounds may have formed near submerged volcanoes and deep-sea vents

Figure 26.3


As material circulated through the apparatus, Miller and Urey periodically collected samples for analysis. They

identified a variety of organic molecules, including amino acids such as alanine and glutamic acid that are common in the proteins of organisms. They also found many other amino acids and complex,oily hydrocarbons.

RESULTS

Figure 26.2

Miller and Urey set up a closed system in their laboratory to simulate conditions thought to have existed on early Earth. A warmed flask of water simulated the primeval sea. The

strongly reducing “atmosphere” in the system consisted of H2,

methane CH4), ammonia (NH3), and water vapor. Sparks were

discharged in the synthetic atmosphere to mimic lightning. A condenser cooled the atmosphere, raining water and any dissolved compounds into the miniature sea.

EXPERIMENT

Electrode

Condenser

Cooled watercontaining

organic molecules

H2O

Sample forchemical analysis

Coldwater

Water vaporCH4

H 2NH

3

CONCLUSIONOrganic molecules, a first step in the origin of

life, can form in a strongly reducing atmosphere.

3. Complex Molecules

• Lab simulations of early Earth atmosphere:


• Some organic cmpds. may have come from space

• Carbon cmpds. have been found in some of the meteorites


4. Abiotic synthesis of polymers

• Small organic molecules

– Polymerize when they are concentrated on hot sand, clay, or rock


5. Protobionts

– Aggregates of abiotically produced molecules surrounded by membrane


• Protobionts could have formed spontaneously from abiotically produced organic cmpds

• e.g., small membrane-bounded droplets called liposomes form when lipids are added to water


20 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

Figure 26.4a, b


• RNA molecules (ribozymes) catalyze many different reactions, including

– Self-splicing

– Making copies of short stretches of their own sequence

Figure 26.5

Ribozyme(RNA molecule)

Template

Nucleotides

Complementary RNA copy

3

5 5


• Early protobionts with self-replicating, catalytic RNA

– used resources and increased in number through natural selection


6. Heterotrophs

• As prokaryotes evolved, they exploited and changed young Earth

• Oldest known fossils are stromatolites

– Rocklike structures w/ many layers of bacteria and sediment

– 3.5 billion years old


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


• Prokaryotes were Earth’s sole inhabitants

– From 3.5 to about 2 billion years ago


• Electron transport systems of a variety of types

– Essential to early life

– Some aspects that possibly precede life itself


7. Photosynthesis

• Earliest types of photosynthesis

– Did not produce O2

– Cyanobacteria are autotrophic prokaryotes that obtain their energy and manufacture organic compounds by photosynthesis


8. Oxygen Revolution

• Oxygenic photosynthesis

– Evolved ~ 3.5 bya in cyanobacteria

Figure 26.12


• O2 began to accumulate in the atmosphere ~ 2.7 bya

– Challenge for life

– Opportunity to gain energy from light

– Allowed organisms to exploit new ecosystems


• As the ozone formed the UV light was absorbed by the atmosphere disrupting the energy available for abiotic synthesis of organic compounds.


9. Eukaryotes

• Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes


• Oldest fossils of eukaryotic cells

– 2.1 billion years


• Endosymbiosis

– Mitochondria and plastids were formerly small prokaryotes living within larger host cells


• Probably undigested prey or internal parasites

Figure 26.13

CytoplasmDNA

Plasmamembrane

Ancestralprokaryote

Infolding of

plasma membrane

Endoplasmicreticulum

Nuclear envelope

Nucleus

Engulfingof aerobic

heterotrophicprokaryote

Cell with nucleusand endomembranesystem

Mitochondrion

Ancestralheterotrophiceukaryote Plastid

Mitochondrion

Engulfing ofphotosyntheticprokaryote insome cells

Ancestral Photosyntheticeukaryote


• Eventually host and endosymbionts would have become a single organism


• Supporting evidence:

– Similar inner membrane structures and functions

– Both have their own circular DNA (genes)

– Own ribosomes


• Chinese paleontologists recently described 570-million-year-old fossils

– probably animal embryos

Figure 26.15a, b

150 m 200 m(a) Two-cell stage (b) Later stage


• The first multicellular organisms were colonies

– Collections of autonomously replicating cells

Figure 26.1610 m


• Some cells in the colonies

– Became specialized for different functions

• The first cellular specializations

– Had already appeared in the prokaryotic world

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 26 The Tree of Life