ch.27.presentation

The Origin and Diversity of Life

Chapter 27

Deep Time• The Earth formed as a hot mass of molten rock

about 4.6 billion years ago (BYA)

• The first 700 million years of Earth’s history is called the Hadean eon– No rocks remain from Hadean Earth (melted)– Hadean Earth was pummeled by asteroids, which

could potentially vaporize entire oceans– Early atmosphere had high CO2 levels

– As CO2 levels dropped, temperatures shifted from 2000°C to -200°C; ocean froze

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Periods

Eons

Eras

Cen

ozoi

cM

esoz

oic

Phan

eroz

oic

Pale

ozoi

c

Prot

eroz

oic

Late

Mid

dle

Early

Late

Mid

dle

Early

Arc

hean

Had

ean

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Present

50 MYA

100 MYA

150 MYA

200 MYA

250 MYA

300 MYA

350 MYA

400 MYA

450 MYA

500 MYA

1000 MYA

1500 MYA

2000 MYA

2500 MYA

3000 MYA

3500 MYA

4000 MYA

4500 MYA

Appearance of humans

First primate

Bird radiationMammal radiation

Pollinating insectsDiversification of flowering plants

First flowering plants, birds, marsupial mammals

First dinosaurs

First gymnosperms

First reptiles

First amphibians

Bony fish, tetrapods, seed plants, and insects appear

Early vascular plants diversify

Invertebrates dominateFirst land plantsCambrian explosion; increase in diversity

Appearance of animals and plants

First multicellular organisms

Oldest definite fossils of eukaryotes

Appearance of oxygen in atmosphere

Cyanobacteria

Oldest fossils of prokaryotes

Molten-hot surface of Earth becomes somewhat coolerOldest rocks

Formation of Earth

North and SouthAmerica joinedby land bridge.Uplift of theSierra Nevada.Worldwide glaciation.

Gondwana beginsto break apart;interior less arid.

Pangea intact.Interior of Pangeaarid. Climatevery warm.

Supercontinent ofLaurentia to thenorth andGondwana to thesouth. Climate mild.

Supercontinent of Gondwana forms. Oceans cover much of North America. Climate not well known.

Most of Earth is covered in ocean and ice.

0°

GondwanaGondwana

PangeaPangeaPangea

LaurentiaLaurentia

RodiniaRodinia

0°

0°

0°

0°

30°

0°

30°

South PoleSouth Pole●






GondwanaGondwana

GondwanaGondwana

• Continents moved over geological time– Earth’s crust formed rigid slabs of rock called

plates• Under continents and oceans

• Two supercontinents formed– Rodinia (all continents) – Gondwana (all current Southern Hemisphere

continents)– Pangea (formed from Gondwana)

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• Evidence of life first appears in the Archean eon fossils– prokaryotes

• Proterozoic eon followed– oxygen appeared in atmosphere– eukaryotes and multicellular organisms appeared– Rodinia broke up

Hadean+Archean+Proterozoic = “PreCambrian Time”

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The past can be reconstructedfrom the fossil record

• Fossils are the preserved remains of once-living organisms

• Rock fossils are created when three events occur– Organism buried in sediment– Calcium in bone or other hard tissue

mineralizes– Surrounding sediment hardens to form rock

• Process of fossilization is rare event7

Determining the age of fossils• Fossils are the preserved remains of once-living organisms• Rock fossils are created when three events occur

– Organism buried in sediment– Calcium in bone or other hard tissue mineralizes– Surrounding sediment hardens to form rock

• Process of fossilization is rare event

• Relative age: age of fossils is estimated by position of the fossil in the sediment

• Absolute age: age of fossils is estimated by rates of radioactive decay

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Absolute Age• Isotopes transform at precisely known rates

into nonradioactive forms• The rate of decay is known as an isotope’s

half-life– Amount of time needed for one-half of the

original amount to be transformed• Types of Isotopes used in aging fossils

– Potassium isotopes: 1.25 billion year half-life– Carbon isotopes: 5700 billion year half-life

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10

0 1 3 4 50

.25

.50

.75

1

Amount of daughter isotope

radioactive decay

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41

81

161

Time in half-lives

Prop

ortio

n of

par

ent

isot

ope

rem

aini

ng

Amount ofparent isotope

parentisotope

2

daughterisotope


Organic molecules may have originated on early Earth

• Few geochemists agree on exact composition of early atmosphere– Popular view of early atmosphere

• Carbon dioxide (CO2)

• Nitrogen gas (N2)

• Water vapor (H2O)

• Hydrogen gas (H2)• Other sulfur, nitrogen, and carbon compounds

– Atmosphere lacked oxygen gas (O2)11

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The Miller–Urey Experiment

Water vapor

Samples testedfor analysis

Electrodesdischargesparks(lightningsimulation)

Reducing atmosphere mixture (H2O, N2, NH3,

CO2, CO, CH4, H2)

Condenser

Cool water

Condensed liquid withcomplex molecules

Many cyclesduring one

week

Heated water(“ocean”)

Heat sourceSmall organic moleculesincluding amino acids

Boiler

In 1953, Miller and Urey did an experiment that reproduced early atmosphere• Atmosphere placed over liquid water• Temperature below 100ºC• Simulate lightning with sparks

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The First Cells• Microfossils are fossilized forms of

microscopic life– Oldest are 3.5 billion years old (Archean eon)– Seem to resemble present-day prokaryotes

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100 μm

Picture courtesy of E. Javaux

• Living things are selective in the carbon isotopes used– Living things incorporate carbon-12– Higher level of carbon-12 than nonliving

things• Isotopic analysis of carbon-12 in fossils

suggests that carbon fixation (ie photosynthesis?) was active as much as 3.8 BYA

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Earth’s Changing System • Climate (temperature and water

availability) and atmosphere are among the many factors that affect survival

• Dramatic shifts in all these factors led to mass extinctions influencing the course of evolution– Earth has been cooling since its formation– Extreme drops in temperature resulted in glacial ice

covering Earth

• Continental motion affected evolution17

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Three global glaciation events occurred during the Proterozoic


Phanerozoic

Solar System Origin

Regional glaciationSnowball Earth

0

500

1000

1500

2000

2500

3000

3500

4000

4500

ProterozoicTi

me

(MYA

)

Archaean

Hadean

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Atmospheric O2 levels over time


01.0Time (BYA)

2.03.0

40

30

20

10

Atm

osph

eric

oxy

gen

leve

l (%

)

Cambrian explosion

Ever-Changing Life on Earth • Life evolved into three domains

– Eubacteria, Archaea, and Eukaryotes

• Prokaryotic fossils appear 3.5 bya (Archean eon)

• Eukaryotes appear 1.5 bya (Proterozoic eon)

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Six supergroups have been identified within the Eukaryote domain, one of three domains of life on Earth

Arc

haea

Chr

omal

veol

ates

Rhi

zaria

Arc

haep

last

ida

Exca

vata

Am

oebo

zoa

Opi

stho

kont

a

Euba

cter

ia


Includes algae and Kingdom Plantae

Includes Kingdoms Fungi and Animalia

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• Eukaryotes have compartmentalization

– an endomembrane system (from infolded plasma membranes)

– mitochondria and chloroplasts are derived from engulfed purple-sulfer bacteria and cyanobacteria (“endosymbiosis”)

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Multicellularity leads to cell specialization

• A unicellular body plan is tremendously successful– Unicellular prokaryotes and eukaryotes

constitute about half of the biomass on Earth– Single cell has limits with cell specialization

• Multicellularity allowed organisms to deal with environment in novel ways through differentiation/specialization

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Eukaryotes form colonial aggregates of many cells. Aggregation of Dictyostelium discoideum forms a colonial organism


1.5 mm© Rupert Mutzel

Sexual reproduction increases genetic diversity

• First eukaryotes were probably haploid

• Diploids seem to have arisen on several separate occasions, via fusion of haploid cells, followed by mitotic divisions.

• Sexual reproduction allows greater genetic diversity (meiosis and fusion of gametes)

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Rapid diversification occurredduring the Cambrian period

• Cambrian period marks the beginning of the Phanerozoic eon

• The “Cambrian explosion” (or “Cambrian radiation”) was confined to ocean– First multicellular animals appeared 50 million

years following Cambrian radiation

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Fossil from the Cambrian explosion


3 mmWith permission of the Royal Ontario Museum and Parks Canada © ROM. Photo Credit: J.B. Caron

Major innovations allowedfor the move onto land

• Plants and then animals colonized terrestrial environments after Cambrian radiation (land plants, then arthropods, then tetrapods)

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Key Eukaryotic Characteristics

• Compartmentalization– Allows for increased subcellular specialization– Nuclear membrane allows for additional levels

of control of transcription and translation• Multicellularity

– Allows for differentiation of cells into tissues• Sexual reproduction

– Allows for greater genetic diversity

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Naming diverse organismsis essential in biology

• Emphasis is on constructing evolutionary hypotheses to explain the relatedness of species

• Organisms are given genus and species names (binomial system)

• Related organisms are grouped into clusters– Family, order, class, phylum, kingdom

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Hierarchical system used in classifying the eastern gray squirrel

DomainEukarya

KingdomAnimalia

PhylumChordata

SubphylumVertebrata


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ClassMammalia

OrderRodentia

FamilySciuridae

GenusSciurus

SpeciesSciurus

carolinensis

Sciuruscarolinensis


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