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Chapter 18Lecture Outline

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A 50-million-year-old fossil of a unicorn fish – just one of the many different kinds of organisms that have

existed during the history of life on Earth 2

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Chapter 18

The Origin and Historyof Life on Earth

Origin of Life on Earth

The Fossil Record

History of Life on Earth

Chapter Outline:

13.8 bya – Universe began with the Big Bang

4.6 bya – Our solar system began

The Earth is 4.55 billion years old

4 bya – Earth had cooled enough for outer layers to solidify and oceans to form

between 4 and 3.5 bya – Life emerged

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Origin of Life on Earth

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84 μm

(b) Modern cyanobacteria(a) Fossil prokaryote

112 μm

a: © Stanley M. Awramik/Biological Photo Service; b: © Michael Abbey/Visuals Unlimited

Origin of life – four overlapping stages

Nucleotides and amino acids produced prior to the existence of cells

Nucleotides and amino acids became polymerized to form DNA, RNA and proteins

Polymers became enclosed in membranes

Polymers enclosed in membranes acquired cellular properties

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1.

2.

3.

4.

Stage 1: Origin of organic molecules

Conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules

Prebiotic or abiotic synthesis Little free oxygen gas so not oxidized Formed prebiotic soup (primordial soup)

Several hypotheses on where and how organic molecules originated

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Reducing atmosphere hypothesis Based on geological data available at the time

Assumed an atmosphere rich in water vapor, H2, CH4, NH3 (and little O2)

Stanley Miller used a chamber apparatus to simulate this atmosphere and bolts of lightning

Formed precursor molecules – amino acids, sugars and nitrogenous bases

First attempt to apply scientific experiments to understand origin of life

Since 1950s, ideas about the early atmosphere of Earth have changed

Still, similar results 8

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H2O

Electrical discharge Electrodes

To vacuum

Boiling water Trap

Sample containingorganic moleculessuch as amino acids

Precipitatingdroplets

Condenser

Cold water

Gases

NH3

CH4

H2

Extraterrestrial hypothesis Meteorites brought organic carbon to Earth

Including amino acids and nucleic acid bases

Opponents argue that most of this would be destroyed in the intense heat of collision

Deep-sea vent hypothesis Biologically important molecules may have been

formed in the temperature gradient between extremely hot vent water and cold ocean water

Supported by experiments and ancient fossils Complex biological communities found here that

derive energy from chemicals in the vent (not the sun)10

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Cold H2O

H2O temperaturesuitable for organicchemistry

HotH2O

Hot H2S gas

Vent

Ocean floor

Crack inEarth’s crust

(a) Deep-sea vent hypothesis

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BIOLOGY PRINCIPLE

Biology is an experimental science

By conducting experiments, Miller and Urey were able to demonstrate the feasibility of the synthesis of organic molecules prior to

the emergence of living cells.

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Stage 2: Organic polymers

Prebiotic synthesis of polymers was thought to be impossible in aqueous solutions Due to hydrolysis competing with polymerization

Experiments have shown formation of nucleic acid polymers and polypeptides on the negative silicate surface of clay

However, in 2004 Leman, Orgel and Ghadiri showed that polymers CAN also form in aqueous solutions Showed formation of peptides under mild conditions in water So polymer synthesis could have occurred in the prebiotic soup

Stage 3: Formation of boundaries

Protobiont An aggregate of prebiotically produced molecules

and macromolecules Has a boundary, such as a lipid bilayer, that allows it

to maintain an internal chemical environment distinct from that of its surroundings

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Four characteristics of a protobiont:

1. Boundary separated external environment from internal contents

2. Polymers inside the protobiont contained information

3. Polymers inside the protobiont had enzymatic function

4. Protobionts capable of self-replication

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Living cells may have evolved from

Coacervates Droplets that form spontaneously from the

association of charged polymers Enzymes trapped inside can perform primitive

metabolic functions

Liposomes Vesicles surrounded by a lipid layer Clay can catalyze formation of liposomes that

grow and divide Can enclose RNA

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Stage 4: RNA world

Majority of scientists favor RNA as the first macromolecule of protobionts

Three key RNA functions:Ability to store information

Capacity for self-replication

Enzymatic function (ribozymes)

DNA and proteins cannot do all 3 functions

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1.

2.

3.

Chemical selection

A chemical within a mixture has special properties that cause it to increase in number compared to other chemicals in the mixture

Hypothetical scenario with two steps:1. One of the RNA molecules mutates and has

enzymatic ability to attach nucleotides together Advantage of faster replication

2. Second mutation produces enzymatic ability to synthesize nucleotides No reliance on prebiotic synthesis

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Advantages of DNA/RNA/protein world

Information storage DNA relieves RNA of informational role and allows

RNA to do other functions DNA is less likely to suffer mutations

Metabolism and other cellular functions Proteins have greater catalytic potential and efficiency Proteins can perform other tasks – cytoskeleton,

transport, etc.

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Preserved remains of past life on Earth

Paleontologists study fossils

Many rocks with fossils are sedimentary Sediments pile up and become rock Organisms buried quickly, hard parts replaced by

minerals

Older rock is deeper and older organisms are deeper in the rock bed

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The Fossil Record

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Radioisotope dating

Fossils can be dated using elemental isotopes in accompanying rock

Half-life – length of time required for exactly one-half of original isotope to decay

Measure amount of a given isotope as well as the amount of the decay product

Usually igneous rock is dated

Expect fossil record to underestimate actual date species came into existence

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00

Iso

top

e (%

)

1 2 3 4

100

75

50

25

Buildup of decay product

Decay of radioisotope

Time (half-lives)

(a) Decay of a radioisotope

RadioisotopeDecayproduct

Half-life(years)

Useful datingrange (years)

100–50,000Carbon-14

Potassium-40

Rubidium-87

Uranium-235

Uranium-238

100,000–4.5 billion

10million–4.5 billion

10million–4.5 billion

10million–4.5 billion

Nitrogen-14

Argon-40

Strontium-87

Lead-207

Lead-206

5,730

1.3 billion

47 billion

4.5 billion

710 million

(b) Radioisotopes that are useful for geological dating

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Geological time scale Origin 4.55 bya to present

Four eons Hadean Archaean Proterozoic Phanerozoic

Each eon further divided into eras28

History of Life on Earth

Precambrian

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Changes in living organisms are the result of Genetic changes Environmental changes

Can allow for new types of organisms Responsible for many extinctions

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Major environmental changes

Temperature Atmospheric composition (amount of O2) Land masses shifting Flood Glaciation Volcanic eruptions Meteorite impacts

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Era

s

GondwanaE

on

s

0

PA

LE

OZ

OIC

PR

OT

ER

OZ

OIC

PH

AN

ER

OZ

OIC

CE

NO

ZO

ICM

ES

OZ

OIC

PacificOcean

Australia

Asia

IndianOcean

Europe

Africa

NorthAmerica

AtlanticOcean

SouthAmerica

Antarctica

Cenozoic period (modern Earth)

Laurasia

Millionsof yearsago (mya)

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248

PacificOcean

Mesozoic period

Tethys OceanPangaeaPanthalassicOcean

Paleozoic period (Pangaea)

PanthalassicOceanRodinia

PanafricanOcean

543

750

Pre-Paleozoic period

Tethys Ocean

Mass extinctions

5 large mass extinctions

Near end of Ordovician, Devonian, Permian, Triassic, and Cretaceous periods

Boundaries between geologic time periods are often based on these events

Rapid extinction of many modern species due to human activities is sometimes referred to as the sixth mass extinction

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Prokaryotic cells arose during Archaeon Eon

Archaeon Eon – when diverse microbial life flourished in primordial oceans

First known fossils 3.5 bya

First cells prokaryotic

All life forms prokaryotic during Archaeon Eon

Hardly any free oxygen so organisms were anaerobic

Biologists are undecided about whether heterotrophs or autotrophs came first

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Stromatolites

Autotrophic cyanobacteria

Form stromatolites – layered structure of calcium carbonate

Cyanobacteria produce oxygen as a waste product of photosynthesis

Spelled doom for many prokaryotic groups that were anaerobic

Allowed the evolution of aerobic species

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(a) Fossil stromatolite (b) Modern stromatolitesa: © Dirk Wiersma/SPL/Photo Researchers, Inc.; b: © Roger Garwood & Trish Ainslie/Corbis

The origin of eukaryotic cells is hypothesized to involve a union between bacterial

and archaeal cells

Origin of first eukaryotic cell matter of debate

In modern eukaryotes, DNA found in nucleus, mitochondria and chloroplasts

Examine properties of this DNA and modern prokaryotes

Nuclear genome – both bacteria and archaea contributed substantially Symbiotic relationship – two species live in direct contact Endosymbiotic – one organism lives inside another

Data support this origin

EVOLUTIONARY CONNECTIONS

EVOLUTIONARY CONNECTIONS

EVOLUTIONARY CONNECTIONS

Proterozoic Eon

Multicellular eukaryotes arise 1.5 bya

Two possible origins Individuals form a colony Single cell divides and stays stuck together

Volvocine green algae display variations in the degree of multicellularity

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Flagella

(a) Chlamydomonas reinhardtii,a unicellular alga

3 μm 10 μm 30 μm 100 μm

(d) Volvox aureus, composedof about 1,000 to 2,000 cells,has 2 cell types, somatic andreproductive

(c) Pleodorina californica,composed of 64 to 128 cells,has 2 cell types, somatic andreproductive

(b) Gonium pectorale, composedof 16 identical cells

a: Courtesy of Dr. Barbara Surek, Culture Collection of Algae at the University of Cologne (CCAC); b: © Bill Bourland/micro*scope; c-d: © Dr. Cristian A. Solari, Department of Ecology and Evolutionary Biology, University of Arizona

BIOLOGY PRINCIPLE

New properties emerge fromcomplex interactions

The formation of different cell types is an emergent property of multicellularity.

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Multicellular animals emerge toward the end of the eon

First animals were invertebrates Bilateral symmetry facilitates locomotion

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Left

Right

Anterior

Posterior

Mouth

50 µm

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© We thank Prof. Jun-yuan Chen for permission to use this image

Phanerozoic Eon

Proliferation of multicellular eukaryotic life extensive (543 mya to today)

Paleozoic Era

Mesozoic Era

Cenozoic Era

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Phanerozoic Eon – Paleozoic Era

543-248 mya

Cambrian period

Ordovician period

Silurian period

Devonian period

Carboniferous period

Permian period

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Phanerozoic Eon – Paleozoic Era – Cambrian Period

543-490 mya

Warm and wet with no ice at poles

Cambrian explosion – abrupt increase in diversity of animal species Cause unknown – shell evolution, atmospheric

oxygen, “arms race”?

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Phanerozoic Eon – Paleozoic Era – Cambrian Period

All existing major types of marine invertebrates plus many others that no longer exist

Although new species have arisen since, no major reorganizations of body plans

First vertebrates 520 mya

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Phanerozoic Eon – Paleozoic Era – Ordovician Period

490-443 mya

Warm temperatures and atmosphere very moist

Diverse group of marine invertebrates including trilobites and brachiopods

Primitive land plants and arthropods first invade land

Toward end, abrupt climate change (large glaciers) resulting in mass extinction

Over 60% of marine invertebrates became extinct

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2 cm

3 cm

(b) Brachiopod

(a) Trilobite

a: © Francois Gohier/Photo Researchers, Inc.; b: © DK Limited/Corbis

Phanerozoic Eon – Paleozoic Era – Silurian Period

443-417 mya

Relatively stable climate

Glaciers largely melted

No new major invertebrates

Significant new vertebrates and plants

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Phanerozoic Eon – Paleozoic Era – Silurian Period

Many new fish

Coral reefs appeared

Large colonization by terrestrial plants and animals Evolved adaptations to prevent drying out

Spiders and centipedes

Earliest vascular plants54

Phanerozoic Eon – Paleozoic Era – Devonian Period

417-354 mya

Generally dry across north but southern hemisphere mostly covered by cool, temperate oceans

Major increase in number of terrestrial species

Ferns, horsetails and seed plants (gymnosperms) emerge

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Phanerozoic Eon – Paleozoic Era – Devonian Period

Insects emerge

Tetrapods – amphibians emerge

Invertebrates flourish in the oceans

The Age of Fishes

Near end, prolonged series of extinctions eliminate many marine species

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Phanerozoic Eon – Paleozoic Era – Carboniferous Period

354-290 mya

Rich coal deposits formed

Cooler, land covered by forested swamps

Plants and animals further diversified Very large plants and trees prevalent First flying insects Amphibians prevalent Amniotic egg emerges - reptiles

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Psaronius

© Ken Lucas/Visuals Unlimited

Phanerozoic Eon – Paleozoic Era – Permian Period

290-248 mya

Continental drift formed supercontinent Pangaea

Interior regions dry with seasonal fluctuations

Forest shift to gymnosperms

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Phanerozoic Eon – Paleozoic Era – Permian Period

Amphibians prevalent but reptiles became dominant

First mammal-like reptiles appeared

At the end, largest known mass extinction event

90-95% of all marine species and large proportion of terrestrial species eliminated

Glaciations or volcanic eruptions blamed

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Phanerozoic Eon – Mesozoic Era

Permian extinction marks boundary between Paleozoic and Mesozoic eras

The Age of Dinosaurs

Consistently hot climate, dry terrestrial environments, little if any ice at poles

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Phanerozoic Eon – Mesozoic Era – Triassic Period

248-206 mya

Reptiles plentiful

First dinosaurs

First true mammals

Gymnosperms dominant land plant

Volcanic eruptions led to global warming and mass extinctions near the end

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Phanerozoic Eon – Mesozoic Era – Jurassic Period

206-144 mya

Gymnosperms continued to be dominant

Dinosaurs dominant land animal Some attained enormous size

First known bird

Mammals present but not prevalent

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Phanerozoic Eon – Mesozoic Era – Cretaceous Period

144-65 mya

Dinosaurs still dominant on land

Earliest flowering plants, angiosperms

Another mass extinction at the end of the period

Dinosaurs and many other species died out

Large meteorite/asteroid or volcanic eruptions blamed

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Phanerozoic Eon – Cenozoic Era

Spans most recent 65 million years

Tropical conditions replaced by a colder, drier climate

Sometimes called The Age of Mammals

Amazing diversification of birds, fish, insects, and flowering plants

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Phanerozoic Eon – Cenozoic Era – Tertiary Period

65-1.8 mya

Mammals that survived expanded rapidly

Birds and terrestrial insects diversified

Angiosperms become the dominant land plant

Fish diversified, sharks become abundant

Whales appeared

Hominids appeared about 7 mya

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Phanerozoic Eon – Cenozoic Era – Quaternary Period

1.8 mya to present

Periodic ice ages cover much of Europe and North America

Widespread extinction of many species

Certain hominids become more human-like

Homo sapiens appear 130,000 years ago

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