The History of Life on Earth
Chapter 25
Overview: Lost Worlds
• Past organisms were very different from those alive
• Fossil record shows macroevolutionary changes over large time scales – For example:
– Emergence of terrestrial vertebrates – Impact of mass extinctions– Origin of flight in birds
Where were these fossils found?
Cryolophosaurus skull
500 Mya the waters surroundingAntarctica were warm and fullof tropical invertebrates.
The continent was covered withForests.
These Cryolophosaurus, inhabitedThese forests.
Conditions on early Earth made the origin of life possible
• Chemical and physical processes on early Earth produced simple cells through these stages:
1. Abiotic synthesis of small organic molecules
2. Joining of small molecules into macromolecules
3. Packaging of molecules into protocells
4. Origin of self-replicating molecules
Synthesis of Organic Compounds
• Earth and solar system formed +/- 4.6 Bya
• Bombardment of Earth by rocks and ice, generating heat, vaporized water
• Earth’s early atmosphere contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)
Early Research• 1920s – Oparin and Haldane hypothesized that early atmosphere
was reducing (electron adding)
• 1953 – Miller & Urey showed that abiotic synthesis of organic molecules in reducing atmosphere is possible
• Evidence not yet convincing that early atmosphere was reducing
• 1st organic compounds may have been synthesized near volcanoes or deep-sea vents
• Amino acids have also been found in meteorites
Figure 25.2
Mas
s of
am
ino
acid
s (m
g)
Num
ber o
f am
ino
acid
s
20
10
01953 2008
200
100
01953 2008
Miller & Urey, 1953 and 2008
Amino acid synthesis in a simulated volcaniceruption
Abiotic Synthesis of Macromolecules
• RNA monomers have been produced spontaneously from simple molecules
• Small organic molecules polymerize when concentrated on hot sand, clay, or rock
Protocells
• Replication and metabolism are key properties of life and may have appeared together
• Protocells may have been fluid-filled vesicles with membrane-like structure
• In water, lipids and other organic molecules spontaneously form vesicles with lipid bilayer
• Adding clay can increase rate of vesicle formation• Vesicles exhibit simple reproduction & metabolism
and maintain internal chemical environment
(a) Self-assembly
Time (minutes)
Precursor molecules plusmontmorillonite clay
Precursor molecules onlyRe
lativ
e tu
rbid
ity,
an in
dex
of v
esic
le n
umbe
r
0
20 m
(b) Reproduction (c) Absorption of RNA
Vesicle boundary
1 m
0
0.2
0.4
4020 60
Presence of clayincreases rateof vesicleformation
Can divide ontheir own
This vesiclehas absorbed clay coated with RNA
Self-Replicating RNA & Dawn of Natural Selection
• First genetic material was probably RNA, not DNA• RNA molecules called ribozymes have been found to catalyze
many different reactions– Ribozymes can make complementary copies of short
stretches of RNA• Natural selection has produced self-replicating RNA• RNA molecules that were more stable or replicated more
quickly would have left most descendent RNA molecules• Early genetic material might have formed an “RNA world”
Protocells
• Vesicles with RNA capable of replication would have been protocells
• RNA could have provided template for DNA, a more stable genetic material
The fossil record documents the history of life
• Fossil record reveals changes in history of life on Earth
• Sedimentary rocks deposited into layers called strata; richest source of fossils
Dimetrodon
Stromatolites
Fossilizedstromatolite
Coccosteuscuspidatus
4.5 cm
0.5 m
2.5 cm
Present
Rhomaleosaurus victor
Tiktaalik
Hallucigenia
Dickinsonia costata
Tappania
1 cm
1 m
100 mya
175200
300
375400
500525
565600
1,500
3,500
270
• Few individuals fossilized; even fewer discovered
• Fossil record is biased in favor of species that– Existed for a long time– Were abundant and widespread– Had hard parts
• Fossil discoveries can be a matter of chance or prediction
– Paleontologists found Tiktaalik (early terrestrial vertebrate) by targeting sedimentary rock from specific time and environment
Animation: The Geologic Record Right-click slide / select “Play”
How Rocks and Fossils Are Dated • Sedimentary strata reveal relative ages of fossils• Absolute ages determined by radiometric dating• “Parent” isotope decays to “daughter” isotope at constant
rate
• Each isotope has a known half-life – time required for half of parent isotope to decay
• Radiocarbon dating of Carbon can be used to date fossils up to 75,000 years old
• For older fossils, some isotopes (ie. Uranium) can be used to date sedimentary rock layers above and below fossil
Accumulating “daughter”
isotope
Frac
tion
of p
aren
t is
otop
e re
mai
ning
Remaining “parent” isotope
Time (half-lives)1 2 3 4
1 2
1 41 8 1 16
Origin of New Groups of Organisms
• Mammals belong to group of animals called tetrapods
• Evolution of unique mammalian features can be traced through gradual changes over time
OTHERTETRA-PODS
Temporal fenestra
Hinge
†Dimetrodon
†Very late (non-mammalian) cynodonts
Mammals
Synapsids
Therapsids
Cynodonts
Reptiles (including dinosaurs and birds)
Key to skull bones
Articular
Quadrate Squamosal
Dentary
Temporal fenestra
HingeHinge
Hinge
Hinges
Temporal fenestra(partial view)
Early cynodont (260 mya)
Very late cynodont (195 mya)
Synapsid (300 mya)
Therapsid (280 mya)
Later cynodont (220 mya)
Figure 25.6
Key events in life’s history
• Geologic record divided into Archaean, Proterozoic, and Phanerozoic eons
• Phanerozoic encompasses multicellular eukaryotic life– Phanerozoic divided into three eras: Paleozoic, Mesozoic,
and Cenozoic– Major boundaries between geological divisions correspond
to extinction events in fossil record
Origin of solar system and Earth
Prokaryotes
Atmospheric oxygen
Archaean
4
3
Billions of years
ago
Origin of solar system and Earth
Prokaryotes
Atmospheric oxygen
Archaean
4
3
Proterozoic
2
Animals
Multicellular eukaryotes
Single-celled eukaryotes
1
Billions of years
ago
Origin of solar system and Earth
Prokaryotes
Atmospheric oxygen
Archaean
4
3
Proterozoic
2
Animals
Multicellular eukaryotes
Single-celled eukaryotes
Colonization of land
Humans
CenozoicMeso-
zoic
Paleozoic
1
Billions of years
ago
First Single-Celled Organisms
• Oldest known fossils are stromatolites, rocks formed by accumulation of sedimentary layers on bacterial mats
• Date back 3.5 Byz
• Prokaryotes were Earth’s sole inhabitants from 3.5 to 2.1 Bya
Prokaryotes
1
2 3
4
i
y arag
oi
o
ll o
s
B
e
sn
f
Photosynthesis & Oxygen Revolution
• Most atmospheric oxygen (O2) is of biological origin
• O2 produced by oxygenic photosynthesis reacted with dissolved iron & precipitated out to form banded formations
• By ~ 2.7 Bya., O2 began accumulating in atmosphere and rusting iron-rich terrestrial rocks
• “Oxygen revolution” from 2.7 to 2.3 Bya caused extinction of many prokaryotic groups
• Some groups survived and adapted using cellular respiration to harvest energy
“Oxygen revolution”
Time (billions of years ago)
4 3 2 1 0
1,000
100
10
1
0.1
0.01
0.0001
Atm
osph
eric
O2
(per
cent
of p
rese
nt-d
ay le
vels
; log
sca
le)
0.001
Atmospheric oxygen
1
2 3
4
i
y ar
agoi
o
ll o
s
B
e
snf
Cyanobacteria
• Early rise in O2 likely caused by ancient cyanobacteria
• Later increase in O2 may have been caused by evolution of eukaryotic cells containing chloroplasts
First Eukaryotes
• Oldest fossils of eukaryotic cells date back 2.1 billion years
• Endosymbiont theory: mitochondria and plastids were formerly small prokaryotes living within larger host cells
• Prokaryotic ancestors of mitochondria and plastids probably gained entry to host cell as undigested prey or internal parasites
• In process of becoming more interdependent, host and endosymbionts became single organism
• Serial endosymbiosis: mitochondria evolved before plastids through sequence of endosymbiotic events
Single-celledeukaryotes
1
2 3
4
i
y ar
agoi
o
ll o
sB
e
sn
f
Plasma membrane
DNA
Cytoplasm
Ancestralprokaryote
Nuclear envelope
Nucleus Endoplasmic reticulum
Serial Endosymbiosis
Plasma membrane
DNA
Cytoplasm
Ancestralprokaryote
Nuclear envelope
Nucleus Endoplasmic reticulum
Aerobic heterotrophicprokaryote
Mitochondrion
Ancestralheterotrophic eukaryote
Plasma membrane
DNA
Cytoplasm
Ancestralprokaryote
Nuclear envelope
Nucleus Endoplasmic reticulum
Aerobic heterotrophicprokaryote
Mitochondrion
Ancestralheterotrophic eukaryote
Photosyntheticprokaryote
Mitochondrion
Plastid
Ancestral photosyntheticeukaryote
Key evidence supporting endosymbiotic origin of mitochondria and plastids:
• Inner membranes similar to plasma membranes of prokaryotes
• Division similar in these organelles and some prokaryotes
• Organelles transcribe and translate their own DNA
• Ribosomes are more similar to prokaryotic than eukaryotic ribosomes
© 2011 Pearson Education, Inc.
Origin of Multicellularity
• Evolution of eukaryotic cells allowed for greater range of unicellular forms
• Second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals
• Comparisons of DNA sequences date common ancestor of multicellular eukaryotes to 1.5 Bya
• Oldest known fossils of multicellular eukaryotes are small algae that lived about 1.2 Bya
© 2011 Pearson Education, Inc.
Multicellulareukaryotes
1
2 3
4
i
y ar
agoi
o
ll o
sB
e
sn
f
• “Snowball Earth” hypothesis: periods of extreme glaciation confined life to equatorial region or deep-sea vents 750 - 580 Mya.
• Ediacaran biota = assemblage of larger and more diverse soft-bodied organisms that lived 575 - 535 Mya.
Cambrian Explosion• Sudden appearance of fossils resembling modern animal
phyla in Cambrian period (535 - 525 Mya)• Sponges, cnidarians, molluscs appear even earlier• Cambrian explosion provides 1st evidence of predator-prey
interactions• DNA analyses – many animal phyla diverged before Cambrian
explosion, perhaps 700 Mya. to 1 Bya.• Fossils in China provide evidence of modern animal phyla tens
of millions of years before Cambrian explosion• Chinese fossils suggest that Cambrian explosion lasted 40
million years
Sponges
Cnidarians
Echinoderms
Chordates
Brachiopods
Annelids
Molluscs
Arthropods
Ediacaran CambrianPROTEROZOIC PALEOZOIC
Time (millions of years ago)635 605 575 545 515 485 0
Animals
1
2 3
4
i
y ar
agoi
o
ll o
sB
e
sn
f
150 m (b) Later stage 200 m(a) Two-cell stage
Proterozoic fossils that may be animal embryos (SEM).
The Colonization of Land
• Fungi, plants, & animals began to colonize land about 500 Mya
• Vascular tissue in plants appeared ~ 420 Mya• Plants and fungi today form mutually beneficial
associations and likely colonized land together• Arthropods and tetrapods – most widespread and
diverse land animals• Tetrapods evolved from lobe-finned fishes ~ 365 Mya
Colonization of land
1
2 3
4
i
y ar
agoi
o
llo
sB
e
snf
The rise and fall of groups of organisms reflect differences in speciation and
extinction rates
• History of life on Earth has seen rise and fall of many groups of organisms
• Rise and fall of groups depends on speciation and extinction rates
Plate Tectonics
• At three points in time, land masses of Earth have formed a supercontinent: 1.1 Bya., 600 Mya., & 250 Mya.
• According to theory of plate tectonics, Earth’s crust is composed of plates floating on mantle
• Tectonic plates move slowly through process of continental drift
• Oceanic & continental plates can collide, separate, or slide past each other
• Interactions between plates cause formation of mountains and islands, and earthquakes
Crust
Mantle
Outercore
Innercore
Juan de FucaPlate
NorthAmerican Plate
CaribbeanPlate
Cocos Plate
PacificPlate
NazcaPlate
SouthAmericanPlate
Eurasian Plate
Philippine Plate
Indian Plate
African Plate
Antarctic Plate
Australian Plate
Scotia Plate
Arabian Plate
Earth’s major tectonic plates.
Consequences of Continental Drift
• Formation of Pangaea ~ 250 Mya had many effects– Deepening of ocean basins– Reduction in shallow water habitat– Colder and drier climate inland
• Continental drift has many effects on living organisms– Continent’s climate can change as it moves– Separation of masses can lead to allopatric speciation
• Distribution of fossils and living groups reflects historic movement of continents
– Similarity of fossils in South America and Africa consistent with idea that continents were attached
65.5
135
251
Pres
ent
Ceno
zoic
North America
Eurasia
Africa
SouthAmerica
India
Antarctica
Madagascar
Australia
Mes
ozoi
cPa
leoz
oic
Mill
ions
of y
ears
ago
Laurasia
Gondwana
Pangaea
Mass Extinctions
• Fossil record shows that most species that have ever lived are now extinct
• Extinction can be caused by changes to a species’ environment
• At times, rate of extinction has increased dramatically and caused mass extinction– Mass extinction is result of disruptive global environmental
changes
• In each of five mass extinction events, more than 50% of Earth’s species became extinct
25
20
15
10
5
0
542 488 444
EraPeriod
416
E O S D
359 299
C
251
P Tr
200 65.5
J CMesozoic
P NCenozoic
0
0
Q
100
200
300
400
500
600
700
800
900
1,000
1,100To
tal e
xtinc
tion
rate
(fam
ilies
per
mill
ion
year
s):
Num
ber o
f fam
ilies
:
Paleozoic
145
Permian Mass Extinction
• Defines boundary between Paleozoic and Mesozoic eras
• 251 Mya
• Mass extinction occurred in less than 5 million years and caused extinction of about 96% of marine animal species
• Number of factors might have contributed:– Intense volcanism in what is now Siberia
– Global warming resulting from emission of large amounts of CO2 from volcanoes
– Reduced temperature gradient from equator to poles
– Oceanic anoxia from reduced mixing of ocean waters
Cretaceous Mass Extinction
• 65.5 Mya
• Separates Mesozoic from Cenozoic
• Organisms that went extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs
• Presence of iridium in sedimentary rocks suggests a meteorite impact about 65 Mya.
• Dust clouds caused by impact would have blocked sunlight and disturbed global climate
• Chicxulub crater off coast of Mexico is evidence of a meteorite that dates to same time
NORTH AMERICA
YucatánPeninsula
Chicxulubcrater
Is a Sixth Mass Extinction Under Way?
• Scientists estimate that current rate of extinction is 100 to 1,000 times typical background rate
• Extinction rates tend to increase when global temperatures increase
• Data suggest that sixth, human-caused mass extinction is likely to occur unless dramatic action is taken
Mass extinctions
Cooler Warmer
Rela
tive
extin
ction
rate
of m
arin
e an
imal
gen
era
3
2
1
0
1
23 2 1 0 1 2 3 4
Relative temperature
Consequences of Mass Extinctions
• Mass extinction can alter ecological communities and niches available to organisms
• It can take from 5 - 100 million years for diversity to recover following mass extinction
• Percentage of marine organisms that were predators increased after Permian & Cretaceous mass extinctions
• Mass extinction can pave way for adaptive radiations
Pred
ator
gen
era
(per
cent
age
of m
arin
e ge
nera
) 50
40
30
20
10
0EraPeriod
542 488 444 416
E O S D
359 299
C
251
P Tr
200 65.5
J C
MesozoicP N
Cenozoic
0
Paleozoic
145 Q
Cretaceous massextinction
Permian massextinction
Time (millions of years ago)
Adaptive Radiations
• Adaptive radiation = evolution of diversely adapted species from common ancestor
• Adaptive radiations may follow– Mass extinctions– Evolution of novel characteristics– Colonization of new regions
Worldwide Adaptive Radiations
• Mammals underwent adaptive radiation after extinction of terrestrial dinosaurs
• Disappearance of dinosaurs (except birds) allowed for expansion of mammals in diversity and size
• Other notable radiations include photosynthetic prokaryotes, large predators in Cambrian, land plants, insects, and tetrapods
Ancestralmammal
ANCESTRALCYNODONT
250 200 150 100 50 0Time (millions of years ago)
Monotremes(5 species)
Marsupials(324 species)
Eutherians(5,010 species)
Regional Adaptive Radiations
• Adaptive radiations can occur when organisms colonize new environments with little competition
• Hawaiian Islands are one of world’s great showcases of adaptive radiation
Close North American relative,the tarweed Carlquistia muirii
KAUAI5.1
million years OAHU
3.7million years
1.3millionyears
MOLOKAI
LANAI MAUI
HAWAII0.4
millionyears
N
Argyroxiphium sandwicense
Dubautia laxa
Dubautia scabraDubautia linearis
Dubautia waialealae
Major changes in body form can result from changes in developmental genes
• Studying genetic mechanisms of change can provide insight into large-scale evolutionary change
• Genes that program development control rate, timing, & spatial pattern of changes in organism’s form as it develops into adult
Changes in Rate and Timing
• Heterochrony: evolutionary change in rate or timing of developmental events
• Can have significant impact on body shape
• Contrasting shapes of human & chimpanzee skulls are result of small changes in relative growth rates
© 2011 Pearson Education, Inc.
Animation: Allometric Growth Right-click slide / select “Play”
Chimpanzee infant Chimpanzee adult
Chimpanzee adult
Human adultHuman fetus
Chimpanzee fetus
• Heterochrony can alter timing of reproductive development relative to development of nonreproductive organs
• Paedomorphosis – rate of reproductive development accelerates compared with somatic development
• Sexually mature species may retain body features that were juvenile structures in ancestral species
Figure 25.22
Gills
Changes in Spatial Pattern
• Substantial evolutionary change can result from alterations in genes that control placement and organization of body parts
• Homeotic genes determine such basic features as where wings and legs will develop or how flower parts are arranged
• Hox genes = class of homeotic genes that provide positional information
• If Hox genes are expressed in wrong location, body parts can be produced in wrong location
• In crustaceans, swimming appendage can be produced instead of feeding appendage
Evolution of Development
• Tremendous increase in diversity during Cambrian explosion is puzzling
• Developmental genes may play especially important role
• Changes in developmental genes can result in new morphological forms
Changes in Genes
• New morphological forms likely come from gene duplication events that produce new developmental genes
• Possible mechanism for evolution of six-legged insects from many-legged crustacean ancestor has been demonstrated in lab experiments
• Specific changes in Ubx gene have been identified that can “turn off” leg development
Hox gene 6 Hox gene 7 Hox gene 8
Ubx
About 400 mya
Drosophila Artemia
Changes in Gene Regulation
• Changes in morphology likely result from changes in regulation of developmental genes rather than changes in sequence of developmental genes
– Threespine sticklebacks in lakes have fewer spines than their marine relatives
– Gene sequence remains same, but regulation of gene expression different in the two groups
Threespine stickleback(Gasterosteus aculeatus)
Ventral spines
Test of Hypothesis A:Differences in the codingsequence of the Pitx1 gene?
Marine stickleback embryo
Close-up of mouth
Close-up of ventral surface
Lake stickleback embryo
Test of Hypothesis B:Differences in the regulation of expression of Pitx1?
Result:No
Result:Yes
The 283 amino acids of the Pitx1 protein are identical.
Pitx1 is expressed in the ventral spine and mouth regions of developing marine sticklebacks but only in the mouth region of developing lake sticklebacks.
RESULTS
Evolution is not goal-oriented
• Evolution is like tinkering— new forms arise by slight modification of existing forms
• Most novel biological structures evolve in many stages from previously existing structures
• Complex eyes have evolved from simple photosensitive cells independently many times
• Exaptations are structures that evolve in one context but become co-opted for different function
• Natural selection can only improve structures in context of its current utility
Figure 25.26
(a) Patch of pigmented cells (b) Eyecup
Pigmented cells(photoreceptors)
Pigmented cells
Nerve fibersNerve fibers
Epithelium
CorneaCornea
Lens
RetinaOptic nerve
Optic nerveOptic nerve
(c) Pinhole camera-type eye (d) Eye with primitive lens (e) Complex camera lens-type eye
EpitheliumFluid-filled cavity
Cellularmass(lens)
Pigmented layer (retina)
Evolutionary Trends
• Extracting single evolutionary progression from fossil record can be misleading
• Apparent trends should be examined in broader context
• Species selection model suggests that differential speciation success may determine evolutionary trends
• Evolutionary trends do not imply intrinsic drive toward particular phenotype
Holocene
Pleistocene
Pliocene
0
5
10
Anchitherium
Mio
cene
15
20
25
30 Olig
ocen
e
Mill
ions
of y
ears
ago
35
40
50
45
55
Eoce
ne
Equus
Pliohippus
Merychippus
Sino
hipp
us
Meg
ahip
pus
Hyp
ohip
pus
Arch
aeoh
ippu
s
Para
hipp
us
Mio
hipp
us
Mesohippus
Prop
alae
othe
rium
Pach
ynol
ophu
s
Pala
eoth
eriu
m
Hap
lohi
ppus
Epih
ippu
s
Oro
hipp
us
Hyracotherium relatives
Hyracotherium
KeyGrazersBrowsers
Hip
pario
n
Neo
hipp
ario
n
Nan
nipp
us
Calli
ppus
Hip
pidi
on a
nd
clos
e re
lativ
es
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