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2007-2008

Life’s Origin & Early Evolution

(Ch. 20)

“…sparked by just the right combination of physical events & chemical processes…”

Bacteria Archae- bacteria

Animalia Fungi Protista Plantae

4500

4000

3500

3000

2500

2000

500

1500

0

1000

Formation of earth

Molten-hot surface of earth becomes cooler

Oldest definite fossils of prokaryotes

Appearance of oxygen in atmosphere

Oldest definite fossils of eukaryotes

First multicellular organisms

Appearance of animals and land plants

Colonization of land by animals Paleozoic

Mesozoic

Cenozoic

Millio

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AR

CH

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PR

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AM

BR

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The evolutionary tree of life can be documented with evidence. The Origin of Life on Earth is another story…

What is Life?

• First we have to define LIFE… – organized as cells

– respond to stimuli

– regulate internal processes • homeostasis

– use energy to grow • metabolism

– develop • change & mature within lifetime

– reproduce • heredity

– DNA / RNA

• adaptation & evolution

Life comes from Non-Life

• Where is the line between living and non-living?

• “A vehicle (organism) built by design information (DNA) for the purpose of replicating that information”

• Good– but too inclusive??? (computer viruses, etc.)

• Terrestrial life is cellular (sorry viruses).

No worries— Alive or Not,

I exist!!!

The Origin of Life is a Hypothesis

• Special Creation – Life was created by a supernatural or

divine force

– not testable

• Extraterrestrial Origin – The original source of organic (carbon)

materials was comets & meteorites striking early Earth

– testable

• Spontaneous Abiotic Origin – Life evolved spontaneously from

inorganic molecules

– testable

The Big Bang Model

• The universe began in an instant, with a big bang about 13 to 15 billion years ago

• The universe is still expanding

• Earth formed from dust and debris orbiting the sun, about 4.6 billion years ago

Conditions on Early Earth

• Came from gas released by volcanoes

• Reducing atmosphere – water vapor (H2O), CO2, N2, NOx, H2, NH3, CH4,

H2S

– lots of available H & its electron

– no free oxygen

• Energy source – lightning, UV radiation,

volcanic

low O2 =

organic molecules

do not break down

as quickly

What’s missing from that

atmosphere?

Early Earth

5 Steps to go from Non-Life to Life

1. Formation (or presence) of biological molecules

2. Isolation of biological molecules from surroundings (aka “cells”)

3. Development of metabolism

4. Development of information molecules

5. Reproduction

Origin of Organic Molecules

• Abiotic synthesis – 1920

Oparin & Haldane propose reducing atmosphere hypothesis

– 1953 Miller & Urey test hypothesis • formed organic

compounds – amino acids

– adenine

Stanley Miller

• Produced – amino acids

– Hydrocarbons

– nitrogen bases

– other organics

University of Chicago

It’s ALIVE!

Possible Origins of Complex Organic Compounds

• Clay templates on tidal flats; iron-sulfide rocks at hydrothermal vents on the deep ocean floor

Origin of Cells (Protobionts)

• Bubbles separate inside from outside metabolism & reproduction

Bubbles… Tiny bubbles…

Origin of Genetics

• An RNA-based system of inheritance may have preceded DNA-based systems

• RNA world – A time when RNA both stored genetic information

and functioned like an enzyme in synthesis

• Ribozymes – Synthetic, self-replicating RNAs

A ribozyme capable of replicating RNA

Ribozyme

(RNA molecule)

Template

Nucleotides

Complementary RNA copy

3

5 5

Key Events in Origin of Life

• Key events in evolutionary history of life on Earth – life originated 3.5–

4.0 bya

– “Heterotroph Hypothesis”: cells eating other cells for ~700 million years.

Prokaryotes

• Prokaryotes dominated life on Earth from 3.5–2.0 bya

3.5 billion year old

fossil of bacteria

chains of one-celled cyanobacteria

modern bacteria

Stromatolites

Fossilized mats of prokaryotes resemble modern microbial colonies (cyanobacteria)

Lynn Margulis

Oxygen Atmosphere

• Oxygen begins to accumulate 2.7 bya – reducing oxidizing atmosphere

• evidence in banded iron in rocks = rusting

• makes aerobic respiration possible

– photosynthetic bacteria (blue-green algae)

The Rise of Eukaryotes

• Oldest eukaryotic fossils: 2.1 billion years

Origins of Nucleus, ER, and Golgi Body

• The nucleus and ER may have arisen through modification of infoldings of plasma membrane

Evolution of Mitochondria and Chloroplasts

• Mitochondria and chloroplasts descended from bacteria that were prey or parasites of early eukaryotic cells

• Endosymbiosis – The symbiont lives out its life inside a host

– Eventually, host and symbiont become incapable of living independently

internal membrane

system aerobic bacterium mitochondrion

Endosymbiosis

Evidence of Endosymbiosis

• Evidence – structural

• mitochondria & chloroplasts resemble bacterial structure

– genetic • mitochondria & chloroplasts

have their own circular DNA, like bacteria

– functional • mitochondria & chloroplasts

move freely within the cell • mitochondria & chloroplasts

reproduce independently from the cell

Theory of Endosymbiosis

Lynn Margulis

Cambrian Explosion

• Diversification of Animals – within 10–20 million years most of the major phyla of animals

appear in fossil record

543 mya

Ca

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Nu

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f fam

ilies ( )

Number of

taxonomic

families Extinction rate

Cretaceous

mass extinction

Permian mass

extinction

Millions of years ago E

xtin

ctio

n r

ate

( )

Paleozoic Mesozoic

0

20

60

40

80

100 600 500 400 300 200 100 0

2,500

1,500

1,000

500

0

2,000

Ceno-

zoic

Diversity of life & periods of mass extinction

Cambrian

explosion

Cretaceous Extinction

• The Chicxulub impact crater in the Caribbean Sea near the Yucatan Peninsula of Mexico indicates an asteroid or comet struck the earth and changed conditions 65 million years ago

Early Mammal Evolution

• 125 mya mammals began to radiate out & fill niches

3 Domain system • Domains = “Super” Kingdoms

– Bacteria – Archaea

• extremophiles = live in extreme environments – methanogens – halogens – thermophiles

– Eukarya • eukaryotes

–protists –fungi –plants –animals

Kingdom

Protist

Kingdom

Fungi

Kingdom

Plant

Kingdom

Animal

Kingdom

Archaebacteria

Kingdom

Bacteria