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The Origin and Diversity of Life
Chapter 27
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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●
South PoleSouth Pole●
South PoleSouth Pole●
South PoleSouth Pole●
South PoleSouth Pole●
South PoleSouth Pole●
GondwanaGondwana
GondwanaGondwana
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• 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
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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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0 1 3 4 50
.25
.50
.75
1
Amount of daughter isotope
radioactive decay
21
41
81
161
Time in half-lives
Prop
ortio
n of
par
ent
isot
ope
rem
aini
ng
Amount ofparent isotope
parentisotope
2
daughterisotope
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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
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• 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
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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
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01.0Time (BYA)
2.03.0
40
30
20
10
Atm
osph
eric
oxy
gen
leve
l (%
)
Cambrian explosion
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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
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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
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1.5 mm© Rupert Mutzel
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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
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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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