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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint®
Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 25
The History of Life on Earth

Overview: Lost Worlds
• Past organisms were very different from those
now alive
• The fossil record shows macroevolutionary
changes over large time scales including
– The emergence of terrestrial vertebrates
– The origin of photosynthesis
– Long-term impacts of mass extinctions

Concept 25.1: Conditions on early Earth made the
origin of life possible
• Chemical and physical processes on early
Earth may have produced very simple cells
through a sequence of stages:
1. Abiotic synthesis of small organic molecules
2. Joining of these small molecules into
macromolecules
3. Packaging of molecules into “protobionts”
4. Origin of self-replicating molecules

Synthesis of Organic Compounds on Early Earth
• Earth formed about 4.6 billion years ago, along
with the rest of the solar system
• Earth’s early atmosphere likely contained water
vapor and chemicals released by volcanic
eruptions (nitrogen, nitrogen oxides, carbon
dioxide, methane, ammonia, hydrogen,
hydrogen sulfide)

• A. I. Oparin and J. B. S. Haldane hypothesized
that the early atmosphere was a reducing
environment
• Stanley Miller and Harold Urey conducted lab
experiments that showed that the abiotic
synthesis of organic molecules in a reducing
atmosphere is possible

• However, the evidence is not yet convincing
that the early atmosphere was in fact reducing
• Instead of forming in the atmosphere, the first
organic compounds may have been
synthesized near submerged volcanoes and
deep-sea vents
Video: Hydrothermal VentVideo: Hydrothermal Vent
Video: TubewormsVideo: Tubeworms

• Amino acids have also been found in
meteorites

Abiotic Synthesis of Macromolecules
• Small organic molecules polymerize when they
are concentrated on hot sand, clay, or rock

Protobionts
• Replication and metabolism are key properties
of life
• Protobionts are aggregates of abiotically
produced molecules surrounded by a
membrane or membrane-like structure
• Protobionts exhibit simple reproduction and
metabolism and maintain an internal chemical
environment

• Experiments demonstrate that protobionts
could have formed spontaneously from
abiotically produced organic compounds
• For example, small membrane-bounded
droplets called liposomes can form when lipids
or other organic molecules are added to water

Fig. 25-3
(a) Simple reproduction by
liposomes (b) Simple metabolism
Phosphate
Maltose
Phosphatase
Maltose
Amylase
Starch
Glucose-phosphate
Glucose-phosphate
20 µm

Fig. 25-3a
(a) Simple reproduction by
liposomes
20 µm

Fig. 25-3b
(b) Simple metabolism
Phosphate
Maltose
Phosphatase
Maltose
Amylase
Starch
Glucose-phosphate
Glucose-phosphate

Self-Replicating RNA and the Dawn of Natural
Selection
• The first genetic material was probably RNA,
not DNA
• RNA molecules called ribozymes have been
found to catalyze many different reactions
– For example, ribozymes can make
complementary copies of short stretches of
their own sequence or other short pieces of
RNA

• Early protobionts with self-replicating, catalytic
RNA would have been more effective at using
resources and would have increased in number
through natural selection
• The early genetic material might have formed
an “RNA world”

Concept 25.2: The fossil record documents the
history of life
• The fossil record reveals changes in the history
of life on earth

The Fossil Record
• Sedimentary rocks are deposited into layers
called strata and are the richest source of
fossils
Video: Grand CanyonVideo: Grand Canyon

Fig. 25-4
Present
Dimetrodon
Coccosteus cuspidatus
Fossilized
stromatolite
Stromatolites
Tappania, a
unicellular
eukaryote
Dickinsonia
costata
Hallucigenia
Casts of
ammonites
Rhomaleosaurus victor,
a plesiosaur
100millionyearsago2001753002704003755005255656003,5001,500
2.5cm
4.5 cm
1 cm

Fig. 25-4-1
Fossilized
stromatolite
Stromatolites
Tappania, a
unicellular
eukaryote
Dickinsonia
costata
Hallucigenia
5005255656003,5001,500
2.5cm
4.5 cm
1 cm

Fig. 25-4a-2
Present
Dimetrodon
Casts of
ammonites
Rhomaleosaurus victor,
a plesiosaur
100millionyearsago200175300270400375
4.5 cm

Fig. 25-4b
Rhomaleosaurus victor, a plesiosaur

Fig. 25-4e
4.5 cm

Fig. 25-4g
Dickinsonia costata 2.5 cm

Fig. 25-4h
Tappania, a unicellular eukaryote

Fig. 25-4j
Fossilized stromatolite

• Few individuals have fossilized, and even
fewer have been discovered
• The fossil record is biased in favor of species
that
– Existed for a long time
– Were abundant and widespread
– Had hard parts
Animation: The Geologic RecordAnimation: The Geologic Record

How Rocks and Fossils Are Dated
• Sedimentary strata reveal the relative ages of
fossils
• The absolute ages of fossils can be determined
by radiometric dating
• A “parent” isotope decays to a “daughter”
isotope at a constant rate
• Each isotope has a known half-life, the time
required for half the parent isotope to decay

Fig. 25-5
Time (half-lives)
Accumulating
“daughter”
isotope
Remaining
“parent”
isotope
Fractionofparent
isotoperemaining
1 2 3 4
1
/2
1
/4
1
/8 1
/16

• Radiocarbon dating can be used to date fossils
up to 75,000 years old
• For older fossils, some isotopes can be used to
date sedimentary rock layers above and below
the fossil

• The magnetism of rocks can provide dating
information
• Reversals of the magnetic poles leave their
record on rocks throughout the world

The Origin of New Groups of Organisms
• Mammals belong to the group of animals called
tetrapods
• The evolution of unique mammalian features
through gradual modifications can be traced
from ancestral synapsids through the present

Fig. 25-6
Very late cynodont (195 mya)
Later cynodont (220 mya)
Early cynodont (260 mya)
Therapsid (280 mya)
Synapsid (300 mya)
Temporal
fenestra
Temporal
fenestra
Temporal
fenestra
EARLY
TETRAPODS
Articular
Key
Quadrate
Dentary
Squamosal
Reptiles
(including
dinosaurs and birds)
Dimetrodon
Very late cynodonts
Mammals
Synapsids
Therapsids
Earliercynodonts

Fig. 25-6-1
Therapsid (280 mya)
Synapsid (300 mya)
Temporal
fenestra
Temporal
fenestra
Articular
Key
Quadrate
Dentary
Squamosal

Fig. 25-6-2
Very late cynodont (195 mya)
Later cynodont (220 mya)
Early cynodont (260 mya)
Temporal
fenestra
Articular
Key
Quadrate
Dentary
Squamosal

• The geologic record is divided into the
Archaean, the Proterozoic, and the
Phanerozoic eons
Concept 25.3: Key events in life’s history include the
origins of single-celled and multicelled organisms and
the colonization of land

• The Phanerozoic encompasses multicellular
eukaryotic life
• The Phanerozoic is divided into three eras: the
Paleozoic, Mesozoic, and Cenozoic
• Major boundaries between geological divisions
correspond to extinction events in the fossil
record

Fig. 25-7
Animals
Colonization
of land
Paleozoic
Meso-
zoic
Humans
Ceno-
zoic
Origin of solar
system and
Earth
Prokaryotes
Proterozoic Archaean
Billions of years ago
1 4
32
Multicellular
eukaryotes
Single-celled
eukaryotes
Atmospheric
oxygen

The First Single-Celled Organisms
• The oldest known fossils are stromatolites,
rock-like structures composed of many layers
of bacteria and sediment
• Stromatolites date back 3.5 billion years ago
• Prokaryotes were Earth’s sole inhabitants from
3.5 to about 2.1 billion years ago

Fig 25-UN2
Prokaryotes
Billions
of
years
ago
4
32
1

Photosynthesis and the Oxygen Revolution
• Most atmospheric oxygen (O2) is of biological
origin
• O2 produced by oxygenic photosynthesis
reacted with dissolved iron and precipitated out
to form banded iron formations
• The source of O2 was likely bacteria similar to
modern cyanobacteria

• By about 2.7 billion years ago, O2 began
accumulating in the atmosphere and rusting
iron-rich terrestrial rocks
• This “oxygen revolution” from 2.7 to 2.2 billion
years ago
– Posed a challenge for life
– Provided opportunity to gain energy from light
– Allowed organisms to exploit new ecosystems

Fig 25-UN3
Atmospheric
oxygen
Billions
of
years
ago
4
32
1

The First Eukaryotes
• The oldest fossils of eukaryotic cells date back
2.1 billion years
• The hypothesis of endosymbiosis proposes
that mitochondria and plastids (chloroplasts
and related organelles) were formerly small
prokaryotes living within larger host cells
• An endosymbiont is a cell that lives within a
host cell

Fig 25-UN4
Single-
celled
eukaryotes
Billions
of years
ago
4
32
1

• The prokaryotic ancestors of mitochondria and
plastids probably gained entry to the host cell
as undigested prey or internal parasites
• In the process of becoming more
interdependent, the host and endosymbionts
would have become a single organism
• Serial endosymbiosis supposes that
mitochondria evolved before plastids through a
sequence of endosymbiotic events

Fig. 25-9-1
Nucleus
Cytoplasm
DNA
Plasma membrane
Endoplasmic reticulum
Nuclear envelope
Ancestral
prokaryote

Fig. 25-9-2
Aerobic
heterotrophic
prokaryote
Mitochondrion
Ancestral
heterotrophic
eukaryote

Fig. 25-9-3
Ancestral photosynthetic
eukaryote
Photosynthetic
prokaryote
Mitochondrio
n
Plastid

Fig. 25-9-4
Ancestral photosynthetic
eukaryote
Photosynthetic
prokaryote
Mitochondrion
Plastid
Nucleus
Cytoplasm
DNA
Plasma membrane
Endoplasmic reticulum
Nuclear envelope
Ancestral
prokaryote
Aerobic
heterotrophic
prokaryote
Mitochondrion
Ancestral
heterotrophic
eukaryote

• Key evidence supporting an endosymbiotic
origin of mitochondria and plastids:
– Similarities in inner membrane structures and
functions
– Division is similar in these organelles and
some prokaryotes
– These organelles transcribe and translate their
own DNA
– Their ribosomes are more similar to
prokaryotic than eukaryotic ribosomes

The Origin of Multicellularity
• The evolution of eukaryotic cells allowed for a
greater range of unicellular forms
• A second wave of diversification occurred
when multicellularity evolved and gave rise to
algae, plants, fungi, and animals

The Earliest Multicellular Eukaryotes
• Comparisons of DNA sequences date the
common ancestor of multicellular eukaryotes to
1.5 billion years ago
• The oldest known fossils of multicellular
eukaryotes are of small algae that lived about
1.2 billion years ago

• The “snowball Earth” hypothesis suggests that
periods of extreme glaciation confined life to
the equatorial region or deep-sea vents from
750 to 580 million years ago
• The Ediacaran biota were an assemblage of
larger and more diverse soft-bodied organisms
that lived from 565 to 535 million years ago

Fig 25-UN5
Multicellular
eukaryotes
Billions
of
years
ago
4
32
1

The Cambrian Explosion
• The Cambrian explosion refers to the sudden
appearance of fossils resembling modern phyla
in the Cambrian period (535 to 525 million
years ago)
• The Cambrian explosion provides the first
evidence of predator-prey interactions

Fig 25-UN6
Animals
Billions
of years
ago
4
32
1

Fig. 25-10
Sponges
Late
Proterozoic
eon
Early
Paleozoic
era
(Cambrian
period)
Cnidarians
Annelids
Brachiopods
Echinoderms
Chordates
Millionsofyearsago
500
542
Arthropods
Molluscs

• DNA analyses suggest that many animal phyla
diverged before the Cambrian explosion,
perhaps as early as 700 million to 1 billion
years ago
• Fossils in China provide evidence of modern
animal phyla tens of millions of years before
the Cambrian explosion
• The Chinese fossils suggest that “the
Cambrian explosion had a long fuse”

Fig. 25-11
(a) Two-cell stage 150 µm 200 µm(b) Later stage

The Colonization of Land
• Fungi, plants, and animals began to colonize
land about 500 million years ago
• Plants and fungi likely colonized land together
by 420 million years ago
• Arthropods and tetrapods are the most
widespread and diverse land animals
• Tetrapods evolved from lobe-finned fishes
around 365 million years ago

Fig 25-UN7
Colonization of land
Billions
of
years
ago
4
32
1

• The history of life on Earth has seen the rise
and fall of many groups of organisms
Concept 25.4: The rise and fall of dominant groups
reflect continental drift, mass extinctions, and
adaptive radiations
Video: Lava FlowVideo: Lava Flow
Video: Volcanic EruptionVideo: Volcanic Eruption

Continental Drift
• At three points in time, the land masses of
Earth have formed a supercontinent: 1.1 billion,
600 million, and 250 million years ago
• Earth’s continents move slowly over the
underlying hot mantle through the process of
continental drift
• Oceanic and continental plates can collide,
separate, or slide past each other
• Interactions between plates cause the
formation of mountains and islands, and
earthquakes

Fig. 25-12
(a) Cutaway view of Earth (b) Major continental plates
Inner
core
Outer
core
Crust
Mantle
Pacific
Plate
Nazca
Plate
Juan de Fuca
Plate
Cocos Plate
Caribbean
Plate
Arabian
Plate
African
Plate
Scotia Plate
North
American
Plate
South
American
Plate
Antarctic
Plate
Australian
Plate
Philippine
Plate
Indian
Plate
Eurasian Plate

Fig. 25-12a
(a) Cutaway view of Earth
Inner
core
Outer
core
Crust
Mantle

Fig. 25-12b
(b) Major continental plates
Pacific
Plate
Nazca
Plate
Juan de Fuca
Plate
Cocos Plate
Caribbean
Plate
Arabian
Plate
African
Plate
Scotia Plate
North
American
Plate
South
American
Plate
Antarctic
Plate
Australian
Plate
Philippine
Plate
Indian
Plate
Eurasian Plate

Consequences of Continental Drift
• Formation of the supercontinent Pangaea
about 250 million years ago had many effects
– A reduction in shallow water habitat
– A colder and drier climate inland
– Changes in climate as continents moved
toward and away from the poles
– Changes in ocean circulation patterns leading
to global cooling

Fig. 25-13
South
America
Pangaea
Millionsofyearsago
65.5
135
Mesozoic
251
Paleozoic
Gondwana
Laurasia
Eurasia
India
Africa
Antarctica
Australia
North
Am
erica
Madagascar
Cenozoic
Present

Fig. 25-13a
South
America
Millionsofyearsago
65.5
Eurasia
India
Africa
Antarctica
Australia
North America
Madagascar
Cenozoic
Present

Fig. 25-13b
Pangaea
Millionsofyearsago
135
Mesozoic
251
Paleozoic
Gondwana
Laurasia

• The break-up of Pangaea lead to allopatric
speciation
• The current distribution of fossils reflects the
movement of continental drift
• For example, the similarity of fossils in parts of
South America and Africa is consistent with the
idea that these continents were formerly
attached

Mass Extinctions
• The fossil record shows that most species that
have ever lived are now extinct
• At times, the rate of extinction has increased
dramatically and caused a mass extinction

The “Big Five” Mass Extinction Events
• In each of the five mass extinction events,
more than 50% of Earth’s species became
extinct

Fig. 25-14
Totalextinctionrate
(familiespermillionyears):
Time (millions of years ago)
Numberoffamilies:
CenozoicMesozoicPaleozoic
E O S D C P Tr J
542
0
488 444 416 359 299 251 200 145
Era
Period
5
C P N
65.5
0
0
200
100
300
400
500
600
700
800
15
10
20

• The Permian extinction defines the boundary
between the Paleozoic and Mesozoic eras
• This mass extinction occurred in less than 5
million years and caused the extinction of
about 96% of marine animal species
• This event might have been caused by
volcanism, which lead to global warming, and a
decrease in oceanic oxygen

• The Cretaceous mass extinction 65.5 million
years ago separates the Mesozoic from the
Cenozoic
• Organisms that went extinct include about half
of all marine species and many terrestrial
plants and animals, including most dinosaurs

Fig. 25-15
NORTH
AMERICA
Chicxulub
craterYucatán
Peninsula

• The presence of iridium in sedimentary rocks
suggests a meteorite impact about 65 million
years ago
• The Chicxulub crater off the coast of Mexico is
evidence of a meteorite that dates to the same
time

Is a Sixth Mass Extinction Under Way?
• Scientists estimate that the current rate of
extinction is 100 to 1,000 times the typical
background rate
• Data suggest that a sixth human-caused mass
extinction is likely to occur unless dramatic
action is taken

Consequences of Mass Extinctions
• Mass extinction can alter ecological
communities and the niches available to
organisms
• It can take from 5 to 100 million years for
diversity to recover following a mass extinction
• Mass extinction can pave the way for adaptive
radiations

Fig. 25-16
Predatorgenera
(percentageofmarinegenera)
Time (millions of years ago)
CenozoicMesozoicPaleozoic
E O S D C P Tr J
542
0
488 444 416 359 299 251 200 145
Era
Period C P N
65.5 0
10
20
30
40
50

Adaptive Radiations
• Adaptive radiation is the evolution of diversely
adapted species from a common ancestor
upon introduction to new environmental
opportunities

Worldwide Adaptive Radiations
• Mammals underwent an adaptive radiation
after the extinction of terrestrial dinosaurs
• The disappearance of dinosaurs (except birds)
allowed for the expansion of mammals in
diversity and size
• Other notable radiations include photosynthetic
prokaryotes, large predators in the Cambrian,
land plants, insects, and tetrapods

Fig. 25-17
Millions of years ago
Monotremes
(5 species)
250 150 100200 50
ANCESTRAL
CYNODONT
0
Marsupials
(324 species)
Eutherians
(placental
mammals;
5,010 species)
Ancestral
mammal

Regional Adaptive Radiations
• Adaptive radiations can occur when organisms
colonize new environments with little
competition
• The Hawaiian Islands are one of the world’s
great showcases of adaptive radiation

Fig. 25-18
Close North American relative,
the tarweed Carlquistia muirii
Argyroxiphium sandwicense
Dubautia linearis
Dubautia scabra
Dubautia waialealae
Dubautia laxa
HAWAII
0.4
million
years
OAHU
3.7
million
years
KAUAI
5.1
million
years
1.3
million
years
MOLOKAI
MAUI
LANAI

Fig. 25-18a
HAWAII
0.4
million
years
OAHU
3.7
million
years
KAUAI
5.1
million
years
1.3
million
years
MOLOKAI
MAUI
LANAI

Fig. 25-18b
Close North American relative,
the tarweed Carlquistia muirii

Fig. 25-18c
Dubautia waialealae

Fig. 25-18f
Argyroxiphium sandwicense

• Studying genetic mechanisms of change can
provide insight into large-scale evolutionary
change
Concept 25.5: Major changes in body form can result
from changes in the sequences and regulation of
developmental genes

Evolutionary Effects of Development Genes
• Genes that program development control the
rate, timing, and spatial pattern of changes in
an organism’s form as it develops into an adult

Changes in Rate and Timing
• Heterochrony is an evolutionary change in the
rate or timing of developmental events
• It can have a significant impact on body shape
• The contrasting shapes of human and
chimpanzee skulls are the result of small
changes in relative growth rates
Animation: Allometric GrowthAnimation: Allometric Growth

Fig. 25-19
(a) Differential growth rates in a human
(b) Comparison of chimpanzee and human skull growth
Newborn
Age (years)
Adult1552
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult

Fig. 25-19a
(a) Differential growth rates in a human
Newborn
Age (years)
Adult1552

Fig. 25-19b
(b) Comparison of chimpanzee and human skull growth
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult

• Heterochrony can alter the timing of
reproductive development relative to the
development of nonreproductive organs
• In paedomorphosis, the rate of reproductive
development accelerates compared with
somatic development
• The sexually mature species may retain body
features that were juvenile structures in an
ancestral species

Changes in Spatial Pattern
• Substantial evolutionary change can also result
from alterations in genes that control the
placement and organization of body parts
• Homeotic genes determine such basic
features as where wings and legs will develop
on a bird or how a flower’s parts are arranged

• Hox genes are a class of homeotic genes that
provide positional information during
development
• If Hox genes are expressed in the wrong
location, body parts can be produced in the
wrong location
• For example, in crustaceans, a swimming
appendage can be produced instead of a
feeding appendage

• Evolution of vertebrates from invertebrate
animals was associated with alterations in Hox
genes
• Two duplications of Hox genes have occurred
in the vertebrate lineage
• These duplications may have been important in
the evolution of new vertebrate characteristics

Fig. 25-21
Vertebrates (with jaws)
with four Hox clusters
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
Second Hox
duplication
First Hox
duplication

The Evolution of Development
• The tremendous increase in diversity during
the Cambrian explosion is a puzzle
• Developmental genes may play an 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
• A possible mechanism for the evolution of six-
legged insects from a many-legged crustacean
ancestor has been demonstrated in lab
experiments
• Specific changes in the Ubx gene have been
identified that can “turn off” leg development

Fig. 25-22
Hox gene 6 Hox gene 7 Hox gene 8
About 400 mya
Drosophila Artemia
Ubx

Changes in Gene Regulation
• Changes in the form of organisms may be
caused more often by changes in the
regulation of developmental genes instead of
changes in their sequence
• For example three-spine sticklebacks in lakes
have fewer spines than their marine relatives
• The gene sequence remains the same, but the
regulation of gene expression is different in the
two groups of fish

Fig. 25-23
Test of Hypothesis A:
Differences in the coding
sequence of the Pitx1 gene?
Result:
No
Marine stickleback embryo
Close-up of ventral surface
Test of Hypothesis B:
Differences in the regulation
of expression of Pitx1 ?
Pitx1 is expressed in the ventral spine
and mouth regions of developing marine
sticklebacks but only in the mouth region
of developing lake stickbacks.
The 283 amino acids of the Pitx1 protein
are identical.
Result:
Yes
Lake stickleback embryo
Close-up
of mouth
RESULTS

Fig. 25-23a
Marine stickleback embryo
Close-up of ventral surface
Lake stickleback embryo
Close-up
of mouth

Concept 25.6: Evolution is not goal oriented
• Evolution is like tinkering—it is a process in
which new forms arise by the slight
modification of existing forms

Evolutionary Novelties
• 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 a different
function
• Natural selection can only improve a structure
in the context of its current utility

Fig. 25-24
(a) Patch of pigmented cells
Optic
nerve Pigmented
layer (retina)
Pigmented cells
(photoreceptors)
Fluid-filled cavity
Epithelium
Epithelium
(c) Pinhole camera-type eye
Optic nerve
Cornea
Retina
Lens
(e) Complex camera-type eye
(d) Eye with primitive lens
Optic nerve
CorneaCellular
mass
(lens)
(b) Eyecup
Pigmented
cells
Nerve fibers Nerve fibers

Evolutionary Trends
• Extracting a single evolutionary progression
from the fossil record can be misleading
• Apparent trends should be examined in a
broader context

Fig. 25-25
Recent
(11,500 ya)
NeohipparionPliocene
(5.3 mya)
Pleistocene
(1.8 mya)
Hipparion
Nannippus
Equus
Pliohippus
Hippidion and other genera
Callippus
Merychippus
Archaeohippus
Megahippus
Hypohippus
Parahippus
Anchitherium
Sinohippus
Miocene
(23 mya)
Oligocene
(33.9 mya)
Eocene
(55.8 mya)
Miohippus
Paleotherium
Propalaeotherium
Pachynolophus
Hyracotherium
Orohippus
Mesohippus
Epihippus
Browsers
Grazers
Key

Fig. 25-25a
Oligocene
(33.9 mya)
Eocene
(55.8 mya)
Miohippus
Paleotherium
Propalaeotherium
Pachynolophus
Hyracotherium
Orohippus
Mesohippus
Epihippus
Browser
s
Grazers
Key

Fig. 25-25b
Recent
(11,500 ya)
NeohipparionPliocene
(5.3 mya)
Pleistocene
(1.8 mya)
Hipparion
Nannippus
Equus
Pliohippus
Hippidion and other genera
Callippus
Merychippus
Archaeohippus
Megahippus
Hypohippus
Parahippus
Anchitherium
Sinohippus
Miocene
(23 mya)

• According to the species selection model,
trends may result when species with certain
characteristics endure longer and speciate
more often than those with other characteristics
• The appearance of an evolutionary trend does
not imply that there is some intrinsic drive
toward a particular phenotype

Fig 25-UN8
Millions of years ago (mya)
1.2 bya:
First multicellular eukaryotes
2.1 bya:
First eukaryotes (single-celled)
3.5 billion years ago (bya):
First prokaryotes (single-celled)
535–525 mya:
Cambrian explosion
(great increase
in diversity of
animal forms)
500 mya:
Colonization
of land by
fungi, plants
and animals
Present
500
2,000
1,500
1,000
3,000
2,500
3,500
4,000

Fig 25-UN9
Origin of solar system
and Earth
4
32
1
Paleozoic
Meso-
zoic
Ceno-
zoic

Fig 25-UN10
Flies and
fleas
Moths and
butterflies
Caddisflies
Herbivory

Fig 25-UN11
Origin of solar system
and Earth
4
32
1
Paleozoic
Meso-
zoic
Ceno-
zoic

You should now be able to:
1. Define radiometric dating, serial
endosymbiosis, Pangaea, snowball Earth,
exaptation, heterochrony, and
paedomorphosis
2. Describe the contributions made by Oparin,
Haldane, Miller, and Urey toward
understanding the origin of organic molecules
3. Explain why RNA, not DNA, was likely the first
genetic material

4. Describe and suggest evidence for the major
events in the history of life on Earth from
Earth’s origin to 2 billion years ago
5. Briefly describe the Cambrian explosion
6. Explain how continental drift led to Australia’s
unique flora and fauna
7. Describe the mass extinctions that ended the
Permian and Cretaceous periods
8. Explain the function of Hox genes

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