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21
The History of Life on Earth
21 The History of Life on Earth
• 21.1 How Do Scientists Date Ancient
Events?
• 21.2 How Have Earth’s Continents and
Climates Changed over Time?
• 21.3 What Are the Major Events in Life’s
History?
• 21.4 Why Do Evolutionary Rates Differ
among Groups of Organisms?
21.1 How Do Scientists Date Ancient Events?
Many evolutionary changes take place
over long periods of time.
To study long-term evolutionary change,
we must think in time frames spanning
millions of years; and imagine
conditions very different from today’s.
21.1 How Do Scientists Date Ancient Events?
Fossils are preserved remains of ancient
organisms, they tell us about body form
or morphology, and where and how they
lived.
Earth’s history is recorded in rocks.
Layers of rocks are called strata.
21.1 How Do Scientists Date Ancient Events?
Relative ages of rocks can be determined
by looking at strata of undisturbed
sedimentary rock. The oldest layers are
at the bottom, youngest at the top.
First observed in the 17th century by
Nicolaus Steno.
Chapter Opener 2 Younger Rocks Lie on Top of Older Rocks
21.1 How Do Scientists Date Ancient Events?
In the eighteenth century, geologists
realized that fossils could also be used
to age rocks.
Certain fossils were always found in
younger rocks, others were found in
older rocks.
Fossils in more recent strata were more
similar to modern organisms.
21.1 How Do Scientists Date Ancient Events?
Radioisotopes can be used to determine
the actual age of rocks.
Radioisotopes decay in a predictable
pattern.
Half-life is the time interval over which
one half of the remaining radioisotope
decays, changing into another element.
Figure 21.1 Radioactive Isotopes Allow Us to Date Ancient Rocks
Table 21.1
Each radioisotope has a characteristic half-life.
21.1 How Do Scientists Date Ancient Events?
To date an event, we must know (or be
able to estimate) the concentration of
the radioisotope at the start of the
event.
For 14C, production in the upper
atmosphere is about equal to its natural
decay.
In an organism, the ratio of 14C to 12C
stays constant during its lifetime.
21.1 How Do Scientists Date Ancient Events?
When an organism dies, it is no longer
incorporating 14C from the environment.
The 14C that was present in the body
decays with no replacement and the
ratio of 14C to 12C decreases.
This ratio can then be used to date
fossils, up to about 50,000 years old.
21.1 How Do Scientists Date Ancient Events?
Sedimentary rocks can not be dated
accurately; the materials that form the
rocks existed for varying lengths of time
before being transported and converted
to rock.
But igneous rocks (e.g., lava or volcanic
ash), that have intruded into layers of
sedimentary rock can be dated.
21.1 How Do Scientists Date Ancient Events?
Other radioisotopes are used to date
older rocks.
Decay of potassium-40 to argon-40 is
used for the most ancient rocks.
Radioisotope dating is combined with
fossil analysis.
21.1 How Do Scientists Date Ancient Events?
Other dating methods include
paleomagnetic dating:
Movement and reversals of Earth’s
magnetic poles are recorded in igneous
and sedimentary rocks at the time they
were formed, by alignment of mineral
grains and other characteristics.
21.1 How Do Scientists Date Ancient Events?
The history of life is divided into geologic
eras, which are subdivided into periods.
Boundaries are based on changes in
fossils.
The eras were established before actual
ages of rocks were known.
Table 21.2 (Part 1)
Table 21.2 (Part 2)
21.2 How Have Earth’s Continents and Climates
Changed over Time?
The idea that land masses have moved
over time was first suggested by Alfred
Wegener in 1912.
By the 1960s, evidence of plate tectonics
convinced geologists that he was right.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Earth’s crust is divided into solid plates
about 40 km thick—collectively, the
lithosphere.
The plates float on a fluid layer of liquid
rock or magma.
Heat from radioactive decay in Earth’s
core causes the magma to circulate,
setting up convection currents.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
The movement of plates is called
continental drift.
Where plates are pushed together, they
move sideways past one another, or
one is pushed underneath the other.
Mountain ranges are pushed up, and
deep rift valleys or trenches are formed.
Where plates are pushed apart, ocean
basins form.
Figure 21.2 Plate Tectonics and Continental Drift
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Position of the continents has changed
dramatically over time.
Influences of ocean circulation patterns,
sea level, and global climate
Mass extinctions of marine animals have
occurred when sea level dropped,
exposing the continental shelves.
Figure 21.3 Sea Levels Have Changed Repeatedly
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Earth’s atmosphere has also changed.
Early atmosphere probably contained
little or no free oxygen (O2).
O2 began to increase when certain
bacteria evolved the ability to use H2O
as a source of H+ ions in
photosynthesis. O2 was a waste
product.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Cyanobacteria formed rock-like
structures called stromatolites which are
abundant in the fossil record.
Enough O2 was liberated to allow
evolution of oxidation reactions to
synthesize ATP.
Figure 21.4 Stromatolites (Part 1)
Figure 21.4 Stromatolites (Part 2)
21.2 How Have Earth’s Continents and Climates
Changed over Time?
The evolution of life changed the physical
nature of Earth.
These changes in turn influenced the
evolution of life.
When O2 first appeared in the
atmosphere it was poisonous to the
anaerobic prokaryotes.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Some evolved the ability to metabolize
the O2.
Advantages: aerobic metabolism is faster
and more energy is harvested.
Aerobes replaced anaerobes in most
environments.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Atmospheric O2 also made possible
larger and more complex cells.
About 1 billion years ago, eukaryote cells
appeared.
Figure 21.5 Larger Cells Need More Oxygen
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Change in atmospheric O2
concentrations was unidirectional.
Most physical conditions have oscillated
over time in response to drifting
continents, volcanic activity, and even
extraterrestrial events such as meteorite
impacts.
Sometimes these events caused mass
extinctions.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Earth’s climate has changed over time.
Sometimes Earth was considerably
hotter than today; sometimes colder,
with extensive glaciation.
Figure 21.6 Hot/Humid and Cold/Dry Conditions Have Alternated over Earth’s History
21.2 How Have Earth’s Continents and Climates
Changed over Time?
For Earth to be cold and dry, atmospheric
CO2 must have been much lower, but it
is unclear what would cause low
concentrations.
Some climate changes have been very
rapid. Extinctions caused by them
appear to be “instantaneous” in the
fossil record.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Today we are in a period of rapid climate
change due to increasing CO2
concentrations, mostly from burning
fossil fuels.
Current CO2 concentration is greater than
it has been for several thousand years.
If CO2 concentration doubles, average
Earth temperature will increase, causing
droughts, sea increase, melting ice caps,
and other major changes.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Volcanic eruptions can trigger major
climate change.
When continents came together to form
Pangaea in the Permian period, many
volcanic eruptions reduced sunlight
penetration and thus photosynthesis.
Massive glaciation resulted.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
Collisions with large meteorites are
probably the cause of several mass
extinctions.
Evidence of impacts include large craters
and disfigured rocks; molecules with
helium and argon isotope ratios
characteristic of meteorites.
21.2 How Have Earth’s Continents and Climates
Changed over Time?
A meteorite is thought to have caused or
contributed to the mass extinction at the
end of the Cretaceous period, 65 million
years ago.
First evidence was from a thin layer
containing the element iridium. This
element is very rare on Earth but
abundant in some meteorites.
Figure 21.7 Evidence of a Meteorite Impact
21.2 How Have Earth’s Continents and Climates
Changed over Time?
A large crater has been located beneath
the northern coast of the Yucatán
Peninsula, Mexico.
A massive plume of debris from the
impact heated the atmosphere, ignited
fires, and blocked the sunlight.
Settling debris formed the iridium-rich
layer.
An Artist’s Conception of the Presumed Meteorite Impact of 65 Million Years Ago
21.3 What Are the Major Events in Life’s History?
Life first evolved about 3.8 billion years
ago.
Eukaryotic organisms had evolved by
about 1.5 billion years ago.
The number of individuals and species
increased dramatically in the late
Precambrian.
21.3 What Are the Major Events in Life’s History?
The assemblage of all kinds of organisms
alive at one time (or in one place) is
called the biota.
All the plants are the flora and all the
animals are the fauna.
21.3 What Are the Major Events in Life’s History?
Although about 300,000 species of
fossils have been described, they are
only a tiny fraction of species that have
existed on Earth.
Only a tiny fraction of organisms become
fossils, and only a fraction of those are
studied by paleontologists.
21.3 What Are the Major Events in Life’s History?
Most organisms are decomposed quickly
after death.
If they are transported to sites with no
oxygen, where decomposition is very
slow, fossilization could occur.
Many geologic processes transform
rocks and destroy the fossils they
contain.
21.3 What Are the Major Events in Life’s History?
A large number of fossil species are
marine organisms that had hard shells
or skeletons that resist decomposition.
Insects and spiders are also well
represented in the fossil record.
Figure 21.8 Insect Fossils
21.3 What Are the Major Events in Life’s History?
The Precambrian Era
For most of this era, life was microscopic,
prokaryote cells living in oceans.
Eukaryotes evolved about 2/3 through
the Precambrian.
By the late Precambrian, soft-bodied
multicellular animals had evolved.
Figure 21.9 Ediacaran Animals
21.3 What Are the Major Events in Life’s History?
Cambrian Period
Beginning of the Paleozoic Era
O2 concentration was approaching
modern levels.
Continents formed large land masses,
the largest called Gondwana.
Figure 21.10 Cambrian Continents and Fauna (Part 1)
Figure 21.10 Cambrian Continents and Fauna (Part 2)
21.3 What Are the Major Events in Life’s History?
Rapid diversification of life took place—
called the Cambrian explosion.
Most of the major groups of animals
living today appeared in the Cambrian.
Three different Cambrian fossil beds
have preserved the soft parts of many
animals—the Burgess Shale, Sirius
Passet, and Chengjiang site.
21.3 What Are the Major Events in Life’s History?
Ordovician Period
A great radiation of marine organisms
occurred, especially among the
brachiopods and mollusks.
At the end of the period, massive glaciers
formed over Gondwana, sea levels
were lowered, and a mass extinction
occurred.
21.3 What Are the Major Events in Life’s History?
Silurian Period
Marine life rebounded from the late
Ordovician extinction.
The first vascular plants appeared in the
late Silurian, as well as some terrestrial
arthropods—scorpions and millipedes.
Figure 21.11 Cooksonia, the Earliest Known Vascular Plant
21.3 What Are the Major Events in Life’s History?
Devonian Period
The northern landmass (Laurasia) and
southern landmass (Gondwana) moved
towards each other.
There were evolutionary radiations of
corals and squid-like cephalopods.
Jawed fishes replaced jawless forms.
Figure 21.12 Devonian Continents and Marine Communities (Part 1)
Figure 21.12 Devonian Continents and Marine Communities (Part 2)
21.3 What Are the Major Events in Life’s History?
Club mosses, horsetails, and tree ferns
became common in terrestrial habitats.
Their roots accelerated weathering of
rocks and soil formation.
Ancestors of gymnosperms appeared.
First known fossils of centipedes, spiders,
mites, and insects.
Fish-like amphibians began to occupy
land.
21.3 What Are the Major Events in Life’s History?
An extinction at the end of the Devonian
resulted in loss of 75 percent of marine
animals.
Two meteorite impacts may have
contributed to this extinction. The craters
are in Nevada and western Australia.
21.3 What Are the Major Events in Life’s History?
Carboniferous Period
Large glaciers formed over high latitudes
but great swamp forests of horsetails and
tree ferns grew on the tropical
continents.
These swamp plants became fossilized as
coal.
21.3 What Are the Major Events in Life’s History?
Diversity of terrestrial animals increased.
Snails, centipedes, scorpions, and insects
were abundant.
Insects developed wings. Flight gave them
access to tall plants.
Amphibians became larger; their lineage
split from the amniotes—vertebrates with
well-protected eggs.
In the oceans, crinoids reached their
greatest diversity.
Figure 21.13 Evidence of Insect Diversification
Figure 21.14 A Carboniferous “Crinoid Meadow”
21.3 What Are the Major Events in Life’s History?
Permian Period
Continents came together to form the
supercontinent Pangaea.
Reptiles outnumbered amphibians at the
end of that period.
Ray-finned fishes diversified.
Figure 21.15 Pangaea Formed in the Permian Period
21.3 What Are the Major Events in Life’s History?
Near the end of the Permian, massive
volcanic eruptions poured lava over large
areas of Earth.
Volcanic ash blocked sunlight and caused
climate cooling, resulting in the largest
glaciers in Earth’s history.
21.3 What Are the Major Events in Life’s History?
O2 concentrations dropped to about 12
percent—most animals would have been
unable to survive at elevations above
500 m.
A combination of factors resulted in the
greatest mass extinction in Earth’s
history.
21.3 What Are the Major Events in Life’s History?
At the start of the Mesozoic era, the
surviving organisms inhabited a relatively
empty world.
The continents began to drift apart, sea
levels rose, and flooded the continents
forming large shallow seas.
Three groups of phytoplankton became
ecologically important: dinoflagellates,
coccolithophores, and diatoms.
21.3 What Are the Major Events in Life’s History?
New seed plants replaced the trees of the
Permian forests.
Earth’s biota became increasingly
provincialized—distinct biota’s evolved
on each continent.
21.3 What Are the Major Events in Life’s History?
Triassic Period
Pangaea began to break apart.
On land, conifers and pteridosperms became
dominant.
A great radiation of reptiles began, which
gave rise to crocodilians, dinosaurs, and
birds.
A mass extinction at the end may have been
caused by a meteorite impact in presentday Quebec.
21.3 What Are the Major Events in Life’s History?
Jurassic Period
Land once again in two continents,
Laurasia and Gondwana.
Ray-finned fishes began a great radiation.
First salamanders, lizards, and flying
reptiles (pterosaurs).
21.3 What Are the Major Events in Life’s History?
Dinosaur lineages evolved into predators
on two legs, and large herbivores on four
legs.
Several groups of mammals appeared.
Flowering plants appeared.
Figure 21.16 Jurassic Parkland
21.3 What Are the Major Events in Life’s History?
Cretaceous Period
A continuous sea encircled the tropics.
Earth was warm and humid.
Dinosaurs continued to diversify.
Flowering plants began the radiation that
led to their current dominance.
Figure 21.17 Positions of the Continents during the Cretaceous Period
Figure 21.18 Flowering Plants of the Cretaceous
21.3 What Are the Major Events in Life’s History?
By the end of the period, many mammal
groups had evolved.
Another mass extinction at the end of the
Cretaceous was caused by a meteorite.
On land, all animals larger than about 25
kg became extinct.
Many insects went extinct, perhaps
because of lack of food plants.
21.3 What Are the Major Events in Life’s History?
The Cenozoic Era
Characterized by an extensive radiation of
mammals.
Flowering plants came to dominate forests
except in cool regions.
Mutations in one group of plants allowed
them to form symbiotic associations with
N-fixing bacteria. This dramatically
increased N available for terrestrial plants.
Table 21.3
21.3 What Are the Major Events in Life’s History?
Tertiary Period
Climate was hot and humid at the
beginning, but became cooler and drier
about half way through.
Many flowering plants evolved
herbaceous forms. Grasslands spread.
Snakes, lizards, birds, and mammals
underwent extensive radiations.
21.3 What Are the Major Events in Life’s History?
Three waves of mammals dispersed from
Asia to North America across the Bering
land bridge.
Rodents, marsupials, primates, and
hoofed mammals appeared in North
America for the first time.
21.3 What Are the Major Events in Life’s History?
Quaternary Period
Divided into Pleistocene and Holocene
epochs.
Pleistocene was a time of drastic cooling
and climate fluctuation.
During four major and 20 minor “ice ages,”
continental glaciers spread, shifting the
ranges of plants and animals towards the
equator.
21.3 What Are the Major Events in Life’s History?
The last glaciers retreated from temperate
latitudes about 15,000 years ago.
The Pleistocene was also the time of
hominid evolution and radiation.
Many large mammal species became
extinct in Australia and the Americas
when Homo sapiens arrived—possibly
due to hunting pressure.
21.3 What Are the Major Events in Life’s History?
Three great evolutionary radiations
occurred that resulted in major new
faunas.
• The Cambrian explosion
• 60 million years later, the radiation that
resulted in the Paleozoic fauna
• After the Permian extinction, in the
Triassic
Figure 21.19 Evolutionary Faunas
21.4 Why Do Evolutionary Rates Differ among Groups of
Organisms?
The rate of evolutionary change has
varied greatly at different times and in
different lineages.
Changes in the physical and biological
environment are likely to stimulate
evolutionary change.
Climate change can shift ranges of
organisms, bringing them into contact
with previously unknown competitors or
predators.
21.4 Why Do Evolutionary Rates Differ among Groups of
Organisms?
Species whose morphology has changed
little over millions of years are called
“living fossils.”
Examples: horseshoe crabs, chambered
nautilus, Gingko trees
Figure 21.20 “Living Fossils”
21.4 Why Do Evolutionary Rates Differ among Groups of
Organisms?
On average, rates of evolutionary change
are very slow.
There are many series of fossils that show
gradual change.
Example: Eight lineages of trilobites show
gradual change in the number of rear
dorsal ribs on the exoskeleton.
Figure 21.21 Rib Number Evolved Gradually in Trilobites (Part 1)
Figure 21.21 Rib Number Evolved Gradually in Trilobites (Part 2)
21.4 Why Do Evolutionary Rates Differ among Groups of
Organisms?
Gradual change appears to dominate the
fossil record.
One explanation is that climate change is
usually slow.
Ranges of organisms shift accordingly, so
the environment in which an individual
lived actually changed very little.
As the climate warmed after the last
glaciers, plants and animals shifted their
ranges northward.
Figure 21.22 Some Species Expanded Their Ranges as Continental Glaciers Retreated (Part 1)
21.4 Why Do Evolutionary Rates Differ among Groups of
Organisms?
If the environment changes rapidly, some
lineages may change rapidly.
Example: The house finch lived in
semiarid regions of western North
America.
It was released in New York City in 1939
and formed a small population there.
Now birds in the eastern populations have
already evolved distinct differences.
Figure 21.23 House Finches Changed Rapidly as Their Range Expanded (Part 1)
Figure 21.23 House Finches Changed Rapidly as Their Range Expanded (Part 2)
21.4 Why Do Evolutionary Rates Differ
among Groups of Organisms?
Rates of extinction have also varied.
The five major extinction events reduced
the biota, and were followed by high
rates of evolution.
But some groups have had high rates of
extinction while others are proliferating.
21.4 Why Do Evolutionary Rates Differ
among Groups of Organisms?
At the end of the Cretaceous, groups of
related mollusk species with large
geographic ranges survived better than
groups with small ranges, even if
individual species in the group had small
ranges.
21.4 Why Do Evolutionary Rates Differ
among Groups of Organisms?
An organism’s diet can also affect
extinction rates.
Animals with specialized diets are more
vulnerable to loss of their food supply.
Large, specialized carnivores may be
more likely to go extinct than small
carnivores with generalized diets.
This hypothesis has been tested using
canid fossils.
Figure 21.24 Large, Specialized Canids Survived Shorter Times (Part 1)
Figure 21.24 Large, Specialized Canids Survived Shorter Times (Part 2)
21.4 Why Do Evolutionary Rates Differ
among Groups of Organisms?
Although agents of evolutionary change
are operating today as they have in the
past, the dramatic increase in human
population is driving major changes.
Hunting has caused extinction of many
species.
Humans drastically alter the vegetation of
Earth—converting forests and
grasslands to agricultural land.
21.4 Why Do Evolutionary Rates Differ
among Groups of Organisms?
Humans move thousands of species
around the globe, deliberately and
accidentally changing the ranges of
species.
Humans practice artificial selection and
biotechnology that influences the
evolution of some species.
Humans have become a dominant agent
of evolutionary change.