Download When hawk-sized dragonflies ruled the air

25
When hawk-sized dragonflies ruled the air
A
lmost anyone who has spent time around fresh
water ponds is familiar with dragonflies. Their
hovering flight, bright colors, and transparent
wings stimulate our visual senses on bright summer afternoons as they fly about their business of devouring
mosquitoes, mating, and laying their eggs. The largest
dragonflies alive today have wingspans that can be covered by a human hand. Three hundred million years ago,
however, dragonflies such as Meganeuropsis permiana
had wingspans of more than 70 centimeters—well over
2 feet, matching or exceeding the wingspans of many
modern birds of prey—and were the largest flying
predators on Earth.
No flying insects alive today are anywhere near this
size. But during the Carboniferous and Permian geological
periods, between 350 and 250 million years ago, many
groups of flying insects contained gigantic members.
Meganeuropsis probably ate huge mayflies and other
giant flying insects that shared their home in the Permian
swamps. These enormous insects were themselves eaten
by giant amphibians.
None of the giant flying insects or amphibians of that
time would be able to survive on Earth today. The oxygen
concentrations in Earth’s atmosphere were about 50 percent higher then than they are now, and those high oxygen levels are thought to have been necessary to support
giant insects and their huge amphibian predators.
Paleontologists have uncovered fossils of Meganeuropsis permiana in the rocks of Kansas. How do we know
the age of these fossils, and how can we know how much
oxygen that long-vanished atmosphere contained? The
stratigraphic layering of the rocks allows us to tell their
ages relative to each other, but it does not by itself indicate a given layer’s absolute age.
One of the remarkable achievements of twentiethcentury scientists was to develop sophisticated techniques
that use the decay rates of various radioisotopes, changes in Earth’s magnetic field,
and the ratios of certain molecules to infer
conditions and events in the remote past and
to date them accurately. It is those methods
that allow us to age the fossils of Meganeuropsis and to calculate the concentration of
oxygen in Earth’s atmosphere at the time.
The development of the science of
biology is intimately linked to
changing concepts of
Giant Dragonflies Meganeuropsis permiana,
shown here in a reconstruction from fossils,
dwarfed modern dragonflies (shown in the inset
at the same scale) in size. Otherwise, however,
the Permian giant was quite similar to modern
dragonflies in general appearance.
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CHAPTER OUTLINE
25.1 How Do Scientists Date Ancient Events?
25.2 How Have Earth’s Continents and Climates
Changed over Time?
25.3 What Are the Major Events in Life’s History?
Do Scientists Date
Ancient Events?
25.1 How
Younger Rocks Lie on Top of Older Rocks In the Grand
Canyon, the Colorado River cut through and exposed many
strata of ancient rocks. The oldest rocks visible here formed
about 540 million years ago. The youngest, at the top, are
about 500 million years old. Knowing the ages of rock strata
allows scientists to date the fossils found in each stratum.
time, especially of the age of Earth. About 150 years ago,
geologists first provided solid evidence that Earth is ancient; before 1850, most people believed it was no more
than a few thousand years old. For many more years,
physicists continued to underestimate Earth’s age, until
an understanding of radioactive decay was developed.
Today we know that Earth is about 4.5 billion years old
and that life has existed on it for about 3.8 billion of
those years. That means human civilizations have occupied Earth for less than 0.0003 percent of the history of
life. Discovering what happened before humans were
around is an ongoing and exciting area of science.
Many evolutionary changes happen rapidly enough to be
studied directly and manipulated experimentally. Plant and
animal breeding by agriculturalists and insects’ evolution of
resistance to pesticides are examples of rapid, short-term evolution. Other changes, such as the appearance of new species
and evolutionary lineages, usually take place over much
longer time frames.
To understand the long-term patterns of evolutionary
change, we must think in time frames spanning many millions
of years, and consider events and conditions very different from
those we observe today. Earth of the distant past was so unlike
the present that it seams like a foreign planet inhabited by
strange organisms. The continents were not where they are today, and climates were sometimes dramatically different from
those of today.
Fossils—the preserved remains of ancient organisms—can
tell us a great deal about the body form, or morphology, of organisms that lived long ago, as well as how and where they
lived. Fossils provide a direct record of evolution. But to understand patterns of evolutionary change, we must also understand
how Earth has changed over time.
Earth’s history is largely recorded in its rocks. We cannot tell
the ages of rocks just by looking at them, but we can determine
the ages of rocks relative to one another. The first person to formally recognize that this could be done was the seventeenthcentury Danish physician Nicolaus Steno. Steno realized that
in undisturbed sedimentary rocks (rocks formed by the accumulation of grains on the bottom of bodies of water), the oldest layers of rock, or strata (singular stratum), lie at the bottom;
thus successively higher strata are progressively younger.
Geologists, particularly the eighteenth-century English scientist William Smith, subsequently combined Steno’s insight
with their observations of fossils contained in sedimentary
rocks. They concluded that:
• Fossils of similar organisms are found in widely separated
places on Earth.
IN THIS CHAPTER we will examine how biologists assign dates to events in the distant evolutionary past, and how
such dating allows us to review the major changes in physical conditions on Earth during the past 4 billion years. We
will then look at how these changes in physical conditions
have influenced the major patterns in the evolution of life,
and describe how scientists organize our knowledge of biological diversity based on the relationships among species.
• Certain fossils are always found in younger rocks, and certain other fossils in older rocks.
• Organisms found in higher, more recent strata are more
similar to modern organisms than are those found in lower,
more ancient strata.
These patterns revealed much about the relative ages of sedimentary rocks as well as patterns in the evolution of life. But the
geologists still could not tell how old the rocks were. A method
of dating rocks did not become available until after radioactivity was discovered at the beginning of the twentieth century.
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CHAPTER 25
520
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HISTORY OF LIFE ON EARTH
14C
5.7
Radioisotope remaining
in sample (%)
Radioactive isotopes of atoms (see Section 2.1) decay in a predictable pattern over long time periods. During each successive
time interval, known as a half-life, half of the remaining radioactive material of the radioisotope decays to become a different,
stable isotope (Figure 25.1A).
To use a radioisotope to date a past event, we must know or
estimate the concentration of the isotope at the time of that event.
In the case of carbon, the production of new carbon-14 (14C) in
the upper atmosphere (by the reaction of neutrons with nitrogen-14) just balances the natural radioactive decay of 14C to 14N.
Therefore, the ratio of 14C to its stable isotope, carbon-12 (12C),
is relatively constant in living organisms and their environment.
As soon as an organism dies, however, it ceases to exchange carbon compounds with its environment. Its decaying 14C is no
longer replenished, and the ratio of 14C to 12C in its remains decreases through time. Paleontologists can use the ratio of 14C to
12C in fossil material to date fossils that are less than 50,000 years
old (and thus the sedimentary rocks that contain those fossils).
If fossils are older than that, so little 14C remains that the limits
of detection using this particular isotope are reached.
22.8
100
(A)
Radioisotopes provide a way to date rocks
half-lives (thousands of years)
11.4
17.1
1/
2
50
1/
4
1/
8
0
1
2
3
Number of half-lives
1/
16
4
(B)
Radioisotope
Half-life
(years)
Decay
product
Carbon-14 (14C)
5,700
Nitrogen-14 (14N)
Potassium-40
Uranium-238
(40K)
(238U)
1.3 billion
4.5 billion
Useful dating
range (years)
100 – 50,000
Argon-40
(40Ar)
10 million – 4.5 billion
Lead 206
(206Pb)
10 million – 4.5 billion
25.1 Radioactive Isotopes Allow Us to Date Ancient Rocks The
decay of radioactive “parent” atoms into stable “daughter” isotopes happens at a steady rate known as a half-life. (A) The graph demonstrates the
principle of half-life using carbon-14 (14C) as an example. (B) Radioisotopes
have different characteristic half-lives that allow us to measure how much
time has elapsed since the rocks containing them were laid down.
TABLE 25.1
Earth’s Geological History
RELATIVE TIME SPAN
ERA
PERIOD
Cenozoic
Precambrian
Mesozoic
Paleozoic
ONSET
MAJOR PHYSICAL CHANGES ON EARTH
Quaternary
2.6 mya
Cold/dry climate; repeated glaciations
Tertiary
65 mya
Continents near current positions; climate cools
Cretaceous
145 mya
Northern continents attached; Gondwana begins to drift apart; meteorite
strikes Yucatán Peninsula
Jurassic
200 mya
Two large continents form: Laurasia (north) and Gondwana (south); climate
warm
Triassic
251 mya
Pangaea begins to slowly drift apart; hot/humid climate
Permian
297 mya
Extensive lowland swamps; O2 levels 50% higher than present; by end of period
continents aggregate to form Pangaea, and O2 levels begin to drop rapidly
Carboniferous
359 mya
Climate cools; marked latitudinal climate gradients
Devonian
416 mya
Continents collide at end of period; meteorite probably strikes Earth
Silurian
444 mya
Sea levels rise; two large land masses emerge; hot/humid climate
Ordovician
488 mya
Massive glaciation, sea level drops 50 meters
Cambrian
542 mya
O2 levels approach current levels
Precambrian
900 mya
O2 level at >5% of current level
1.5 bya
O2 level at >1% of current level
3.8 bya
O2 first appears in atmosphere
4.5 bya
Note: mya, million years ago; bya, billion years ago.
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25.2
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HOW HAVE EARTH’S CONTINENTS AND CLIMATES CHANGED OVER TIME?
Radioisotope dating methods have been
expanded and refined
Sedimentary rocks are formed from materials that existed for
varying lengths of time before being transported, sometimes
over long distances, to the site of their deposition. Therefore,
the inorganic isotopes in a sedimentary rock do not contain reliable information about the date of its formation. Dating rocks
more ancient than 50,000 years requires estimating isotope concentrations in igneous rocks—rocks formed when molten material cools. To date older sedimentary rocks, geologists search for
places where sedimentary rocks show igneous intrusions of volcanic ash or lava flows.
A preliminary estimate of the age of an igneous rock determines
which isotope is used to date it (Figure 25.1B). The decay of potassium-40 (which has a half-life of 1.3 billion years) to argon-40 has
been used to date many of the ancient events in the evolution of
life. Fossils in the adjacent sedimentary rock that are similar to those
in other rocks of known ages provide additional clues.
Radioisotope dating of rocks, combined with fossil analysis,
is the most powerful method of determining geological age. But
in places where sedimentary rocks do not contain suitable ig-
521
neous intrusions and few fossils are present, paleontologists
turn to other methods.
One method, known as paleomagnetic dating, relates the
ages of rocks to patterns in Earth’s magnetism, which change
over time. Earth’s magnetic poles move and occasionally reverse themselves. Because both sedimentary and igneous rocks
preserve a record of Earth’s magnetic field at the time they were
formed, paleomagnetism helps determine the ages of those
rocks. Other dating methods use information about continental drift, sea level changes, and molecular clocks (the last of
which is described in Section 22.3).
Using these methods, geologists divided the history of life
into eras, which in turn are subdivided into periods (Table 25.1).
The boundaries between these time frames are based on striking differences scientists have observed in the assemblages of
fossil organisms contained in successive layers of rocks. Geologists defined and named these divisions before they were able
to establish the ages of fossils, adding and refining the time
scales as new methods for geological dating were developed.
25.1 RECAP
MAJOR EVENTS IN THE HISTORY OF LIFE
Fossils in sedimentary rocks enabled geologists to
determine the relative ages of organisms, but absolute dating was not possible until the discovery of
radioactivity. Geologists divide the history of life into
eras and periods, based on assemblages of fossil
organisms found in successive layers of rocks.
Humans evolve; many large mammals become extinct
• What observations about fossils suggested to geolo-
Diversification of birds, mammals, flowering plants, and insects
Dinosaurs continue to diversify; mass extinction at end of period
(≈76% of species disappear)
Diverse dinosaurs; radiation of ray-finned fishes; first fossils of
flowering plants
Early dinosaurs; first mammals; marine invertebrates diversify;
mass extinction at end of period (≈65% of species disappear)
Reptiles diversify; giant amphibians and flying insects present;
mass extinction at end of period (≈96% of species disappear)
Extensive “fern” forests; first reptiles; insects diversify
Fishes diversify; first insects and amphibians; mass extinction at end
of period (≈75% of species disappear)
Jawless fishes diversify; first ray-finned fishes; plants and animals
colonize land
Mass extinction at end of period (≈75% of species disappear)
Rapid diversification of multicellular animals; diverse photosynthetic
protists
Ediacaran fauna; earliest fossils of multicellular animals
Eukaryotes evolve
Origin of life; prokaryotes flourish
gists that they could be used to determine the relative
ages of rocks? See p. 519
• How is the rate of decay of radioisotopes used to estimate the absolute ages of rocks? See p. 520 and
Figure 25.1
The scale at the left of Table 25.1 gives a relative sense of geological time, especially the vast expanse of the Precambrian era, during which early life evolved amid stupendous physical changes
of Earth and its atmosphere. During the Precambrian to Cambrian transition, an “explosion” of new life forms took place as
representatives of many of the major multicellular groups of life
evolved. Earth continued to undergo massive physical changes
that influenced the evolution of life, and these events and important milestones are listed in the table. In the next two sections
we’ll discuss the most important of these changes.
25.2
How Have Earth’s Continents and
Climates Changed over Time?
The globes and maps that adorn our walls, shelves, and books
give an impression of a static Earth. It would be easy for us to
assume that the continents have always been where they are,
but we would be wrong. The idea that Earth’s land masses have
changed position over the millennia, and that they continue to
do so, was first put forth in 1912 by the German meteorologist
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CHAPTER 25
522
25.2 Plate Tectonics and
Continental Drift The heat
of Earth’s core generates convection currents (arrows) in the
magma that push the lithospheric plates, along with the
land masses lying on them,
together or apart. When
lithospheric plates collide, one
often slides under the other.
The resulting seismic activity
can create mountains and
deep rift valleys (the latter
known as trenches when they
occur under ocean basins).
|
HISTORY OF LIFE ON EARTH
Cooling magma
forms crust.
Melting lithosphere
provides magma
that fuels volcanoes.
Lithospheric plate
Magma
Convection currents in liquid magma
generate pressure that pushes the
plates apart, forming ocean basins.
and geophysicist Alfred Wegener. His book The Origin of Continents and Oceans was initially met with skepticism and resistance. By the 1960s, however, physical evidence and increased
understanding of the geophysics of plate tectonics—the study
of movement of major land masses—had convinced virtually
all geologists of the reality of Wegener’s vision.
Earth’s crust consists of several solid plates approximately
40 kilometers thick, which collectively make up the lithosphere.
The lithospheric plates float on a fluid layer of molten rock, or
magma (Figure 25.2). Heat produced by radioactive decay deep
in Earth’s core sets up convection currents in the fluid magma,
which then rises and exerts tremendous pressure on the solid
plates. When the pressure of the rising magma pushes plates
apart, ocean basins may form between them. When plates are
pushed together, they either move sideways past each other or
one plate slides under the other, pushing up mountain ranges
and carving deep rift valleys. When they occur under the water of ocean basins, rift valleys are known as trenches. The
Mantle
Where two plates collide,
one is pushed under the
other, generating seismic
activity, mountains, rift
valleys, and oceanic
trenches.
movement of the lithospheric plates and the continents they
contain is known as continental drift.
We now know that at times the drifting of the plates has
brought continents together and at other times has pushed them
apart (these movements are depicted in Figure 25.12). The positions and sizes of the continents influence oceanic circulation
patterns, global climates, and sea levels. Major drops in sea level
have usually been accompanied by massive extinctions—particularly of marine organisms, which could not survive the exposure of vast areas of the continental shelves and the disappearance of the shallow seas that covered them (Figure 25.3).
yo u r B i oPort al.com
GO TO
Animated Tutorial 25.1 • Evolution of the Continents
25.3 Sea Levels Have Changed Repeatedly Most mass extinctions
of marine organisms (indicated by asterisks) have coincided with periods
of low sea levels.
High
Sea level
*
*
Asterisks indicate times of mass extinctions
of marine organisms, most of which occurred
when sea levels dropped.
*
*
*
Low
Cambrian Ordovician
Silurian
Devonian
Carboniferous Permian
Triassic
Jurassic
Cretaceous
Quaternary
Tertiary
Precambrian
P a l e o z o i c
500
400
300
Millions of years ago (mya)
M e s o z o i c
200
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Cenozoic
100
Present
(A)
523
The layers are
formed as biofilms of
cyanobacteria die and
others take their place.
stromatolites, which are abundantly preserved in the fossil
record. Cyanobacteria are still forming stromatolites today in
a few very salty places on Earth (Figure 25.4). Cyanobacteria
liberated enough O2 to open the way for the evolution of oxidation reactions as the energy source for the synthesis of ATP
(see Section 9.1).
The evolution of life thus irrevocably changed the physical
nature of Earth. Those physical changes, in turn, influenced the
evolution of life. When it first appeared in the atmosphere, O2
was poisonous to the anaerobic prokaryotes that inhabited Earth
at the time. Over millennia, however, prokaryotes that evolved
the ability to metabolize O2 not only survived but gained several advantages. Aerobic metabolism proceeds more rapidly
and harvests energy more efficiently than anaerobic metabolism (see Section 9.4), and organisms with aerobic metabolism
replaced anaerobes in most of Earth’s environments.
An atmosphere rich in O2 also made possible larger cells and
more complex organisms. Small unicellular aquatic organisms
can obtain enough O2 by simple diffusion even when O2 concentrations are very low. Larger unicellular organisms have
lower surface area-to-volume ratios (see Figure 5.2); to obtain
enough O2 by simple diffusion, they must live in an environment with a relatively high oxygen concentration. Bacteria can
thrive on 1 percent of the current atmospheric O2 levels; eukaryotic cells require levels that are at least 2–3 percent of current
concentrations. (For concentrations of dissolved O2 in the oceans
to reach these levels, much higher atmospheric concentrations
were needed.)
Probably because it took many millions of years for Earth
to develop an oxygenated atmosphere, only unicellular prokaryotes lived on Earth for more than 2 billion years. About 1.5 bya,
atmospheric O2 concentrations became high enough for large
eukaryotic cells to flourish (Figure 25.5). Further increases in
atmospheric O2 levels 750 to 570 million years ago (mya) enabled several groups of multicellular organisms to evolve.
12 cm
(B)
Living cyanobacteria
are found in the upper
parts of these structures.
30 cm
25.4 Stromatolites (A) A vertical section through a fossil stromatolite.
(B) These rocklike structures are living stromatolites that thrive in the very
salty waters of Shark Bay in western Australia. Layers of cyanobacteria
are found in the uppermost parts of the structures.
35
Oxygen concentrations in Earth’s
atmosphere have changed over time
30
O2 in atmosphere (%)
As the continents have moved over Earth’s surface, the world has experienced other physical
changes, including large increases and decreases
in atmospheric oxygen. The atmosphere of early
Earth probably contained little or no free oxygen
gas (O2). The increase in atmospheric O2 came in
two big steps more than a billion years apart.
The first step occurred at least 2.4 billion years
ago (bya), when certain bacteria evolved the ability to use water as the source of hydrogen ions
for photosynthesis. By chemically splitting H2O,
these bacteria generated atmospheric O2 as a
waste product. They also made electrons available for reducing CO2 to form organic compounds (see Section 10.3).
One group of O2-generating bacteria, the
cyanobacteria, formed rocklike structures called
O2 levels almost 50%
higher than present.
First
chordates
25
First
photosynthetic First
bacteria eukaryotes
10
5
Invasion
of land
First
flowering
plants
First
multicellular
organisms
20
15
Giant flying insects
First
life
First
aerobic
bacteria
Rapid drop of
O2 levels at end
of the Permian
O2 levels 25–40%
lower than present.
0
4,000 3,000 2,000
1,000
500
Millions of years ago (mya)
250
100 Present
25.5 Larger Cells, Larger Organisms Need More Oxygen Changes in oxygen concentrations have strongly influenced, and been influenced by, the evolution of life. (Note that the
horizontal axis of the graph is on a logarithmic scale.)
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524
CHAPTER 25
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HISTORY OF LIFE ON EARTH
INVESTIGATING LIFE
25.6 Rising Oxygen Levels and Body Size in Insects
In this experiment, flies were raised under hyperbaric conditions
(increased atmospheric pressure), thus increasing the partial
pressure of O2 in a manner that simulated the greater levels of
atmospheric O2 characteristic of the Carboniferous and
Permian. Robert Dudley asked if flies raised in hyperbaric
conditions would grow larger than their normal counterparts.
HYPOTHESIS Under increased atmospheric pressure, the
increased partial pressure of O2 will allow
directional selection for increased body size in
flying insects.
METHOD
1. Divide a population of fruit flies (Drosophila
melanogaster) into two lines.
2. Raise one line (the control) at current atmospheric
oxygen conditions. Raise the experimental line in
hyperbaric conditions (increased partial pressure
of O2, simulating increased atmospheric oxygen
concentrations). Continue for 5 generations.
3. Raise the F6 offspring of both lines under identical
environmental conditions.
4. Weigh all the F6 individuals and test for statistical
differences in the average body mass of the flies
in each population.
RESULTS
The average body mass of F6 individuals of both
sexes in the experimental line was significantly
(p < 0.0001) greater than that of insects in the
control line.
Body mass (mg)
1.2
1.1
1.0
0.9
Normal atmosphere
(control)
0.8
Hyperbaric conditions
0.7
0.6
Males
CONCLUSION
Females
In at least some flying insects, increased
concentrations of oxygen could lead to a
long-term evolutionary trend toward increased
body size.
FURTHER INVESTIGATION: How would you confirm that the
change in average body size is related
to increased partial pressure of O2,
and not to other aspects of overall
increased atmospheric pressure?
Go to yourBioPortal.com for original citations, discussions,
and relevant links for all INVESTIGATING LIFE figures.
O2 concentrations increased again during the Carboniferous
and Permian periods because of the evolution of large vascular
plants in the expansive lowland swamps that existed then (see
Table 25.1). These swamps resulted in extensive burial of plant
debris from vascular plants, which led to the formation of Earth’s
vast coal deposits. As the buried organic material was not subject to oxidation, and the living plants were producing large
quantities of O2, atmospheric O2 increased to concentrations that
have not been reached again in Earth’s history (see Figure 25.5).
As mentioned in the opening of this chapter, high concentrations
of atmospheric O2 allowed the evolution of giant flying insects
and amphibians that could not survive in today’s atmosphere.
The drying of the lowland swamps at the end of the Permian reduced global organic burial, and also the production of atmospheric O2, so O2 concentrations dropped rapidly. Over the past
200 million years, with the diversification of flowering plants,
O2 concentrations have again increased, but not to the levels that
characterized the Carboniferous and Permian periods.
Biologists have conducted experiments that demonstrate the
changing selective pressures that can accompany changes in
O2 levels. In experimental conditions, an increase in O2 concentration can be simulated by increasing atmospheric pressure in
a hyperbaric chamber. Increasing atmospheric pressure increases
the partial pressure of oxygen (see Chapter 49) in a manner that
simulates an increase in O2 concentration at normal atmospheric
pressure. When lines of fruit flies (Drosophila) are raised in artificial hyperbaric atmospheres (which have higher partial pressure of O2), they quickly evolve larger body sizes over just a few
generations (Figure 25.6). The current levels of atmospheric O2
appear to constrain body size evolution of these flying insects;
increases in O2 appear to relax these constraints. This demonstrates that the stabilizing selection on body size at present O2 concentrations can quickly switch to directional selection (see Section
21.3) for a change in body size in response to a change in O2 levels. Directional selection over a period of millions of years would
be sufficient to account for giant insects such as Meganeuropsis,
described at the beginning of this chapter.
Many physical conditions on Earth have oscillated in response to the planet’s internal processes, such as volcanic activity and continental drift. Extraterrestrial events, such as collisions with meteorites, have also left their mark. In some cases,
as we saw earlier and will see again in this chapter, changing
physical parameters caused mass extinctions, during which a
large proportion of the species living at the time disappeared.
After each mass extinction, the diversity of life rebounded, but
recovery took millions of years.
Earth’s climate has shifted between hot/humid
and cold/dry conditions
Through much of its history, Earth’s climate was considerably
warmer than it is today, and temperatures decreased more gradually toward the poles. At other times, Earth was colder than
it is today. Large areas were covered with glaciers near the end
of the Precambrian and Ordovician, and during parts of the Carboniferous and Permian periods. These cold periods were separated by long periods of milder climates (Figure 25.7). Because
we are living in one of the colder periods in Earth’s history, it is
difficult for us to imagine the mild climates that were found at
high latitudes during much of the history of life. During the
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25.2
|
525
HOW HAVE EARTH’S CONTINENTS AND CLIMATES CHANGED OVER TIME?
High
Cambrian Ordovician
Silurian
Devonian
Carboniferous Permian
Triassic
Jurassic
Cold/dry
Hot/humid
Pangaea
Cold/dry
Hot/humid
Cold/dry
Hot/humid
Low
Cold/dry
Earth’s mean temperature
Large areas of Earth’s surface were
covered by glaciers during these periods.
Cretaceous
Quaternary
Tertiary
Precambrian
P a l e o z o i c
500
400
M e s o z o i c
300
Millions of years ago (mya)
25.7 Hot/Humid and Cold/Dry Conditions Have
Alternated over Earth’s History Throughout Earth’s history, periods of cold climates and glaciations (white depressions)
have been separated by long periods of milder climates.
Quaternary period there has been a series of glacial advances,
interspersed with warmer interglacial intervals during which
the glaciers retreated.
“Weather” refers to daily events, such as individual storms.
“Climate” refers to long-term average expectations of the various seasons at a given location. Weather often changes rapidly;
climates typically change slowly. Major climatic shifts have taken
place over periods as short as 5,000 to 10,000 years, primarily
as a result of changes in Earth’s orbit around the sun. A few climatic shifts have been even more rapid. For example, during one
Quaternary interglacial period, the ice-locked Antarctic Ocean
became nearly ice-free in less than 100 years. Such rapid changes
are usually caused by sudden shifts in ocean currents. Some
climate changes have been so rapid that the extinctions caused
by them appear to be nearly “instantaneous” in the fossil record.
We are currently living in a time of rapid climate change
thought to be caused by a buildup of atmospheric CO2, primarily
from the burning of fossil fuels. We are reversing the process of
organic burial that occurred (especially) in the Carboniferous and
Permian, but we are doing so over a few hundred years rather
than the many millions of years over which these deposits accumulated. The current rate of increase of atmospheric CO2 is unprecedented in Earth’s history. A doubling of the atmospheric CO2
concentration—which may happen during the current century—
is expected to increase the average temperature of Earth, change
rainfall patterns, melt glaciers and ice caps, and raise sea level. The
possible consequences of such climate changes are discussed in
Chapters 58 and 59.
Volcanoes have occasionally changed the history of life
Most volcanic eruptions produce only local or short-lived effects, but a few large volcanic eruptions have had major consequences for life. When Krakatoa erupted in Indonesia in 1883,
200
Cenozoic
100
Present
it ejected more than 25 cubic kilometers of ash and rock, as well
as large quantities of sulphur dioxide gas (SO2). The SO2 was
ejected into the stratosphere and then moved by high-level
winds around the planet. This led to high concentrations of sulphurous acid (H2SO3) in high-level clouds, which meant less
sunlight got through to Earth’s surface. Global temperatures
dropped by 1.2°C in the year following the eruption, and global
weather patterns showed strong effects for another 5 years. This
was all the result of a single volcanic eruption. The collision of
continents during the Permian period (about 275 mya) formed
a single, gigantic land mass (Pangaea) and caused many massive volcanic eruptions. These eruptions resulted in considerable blockage of sunlight, contributing to the glaciations of that
time (see Figure 25.7). Massive volcanic eruptions occurred
again as the continents drifted apart during the late Triassic and
at the end of the Cretaceous.
Extraterrestrial events have triggered changes on Earth
At least 30 meteorites between the sizes of baseballs and soccer
balls hit Earth each year. Collisions with large meteorites or
comets are rare, but such collisions have probably been responsible for several mass extinctions. Several types of evidence tell
us about these collisions. Their craters, and the dramatically disfigured rocks that resulted from their impact, are found in many
places. Geologists have also discovered compounds in these
rocks that contain helium and argon with isotope ratios characteristic of meteorites, which are very different from the ratios
found elsewhere on Earth.
A meteorite caused or contributed to a mass extinction at the
end of the Cretaceous period (about 65 mya). The first clue that
a meteorite was responsible came from the abnormally high
concentrations of the element iridium in a thin layer separating
rocks deposited during the Cretaceous from those deposited
during the Tertiary (Figure 25.8). Iridium is abundant in some
meteorites, but it is exceedingly rare on Earth’s surface. Scientists discovered a circular crater 180 kilometers in diameter
buried beneath the northern coast of the Yucatán Peninsula of
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526
CHAPTER 25
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HISTORY OF LIFE ON EARTH
Are the Major Events
in Life’s History?
25.3 What
Iridium-rich layer at the
Cretaceous-Tertiary
(K/T) boundary
25.8 Evidence of a Meteorite Impact The white layers of rock are
Cretaceous in age; the layers at the upper left were deposited in the
Tertiary. Between the two is a thin, dark layer of clay that contains large
amounts of iridium, a metal common in some meteorites but rare on
Earth. Its high concentration in sediments deposited about 65 million
years ago suggests the impact of a large meteorite.
Mexico. When it collided with Earth, the meteorite released energy equivalent to that of 100 million megatons of high explosives, creating great tsunamis. A massive plume of debris
swelled to a diameter of up to 200 kilometers, spread around
Earth, and descended. The descending debris heated the atmosphere to several hundred degrees, ignited massive fires, and
blocked the sun, preventing plants from photosynthesizing. The
settling debris formed the iridium-rich layer. About a billion
tons of soot, which has a composition that matches smoke from
forest fires, was also deposited. Many fossil species (particularly
dinosaurs) that are found in Cretaceous rocks are not found in
the Tertiary rocks of the next layer.
25.2 RECAP
Conditions on Earth have changed dramatically over
time. Changes in atmospheric concentrations of O2
and in Earth’s climate have had major effects on biological evolution. Continental drift, volcanic eruptions, and large meterorite strikes have contributed
to climatic changes during Earth’s history.
• Describe how increases in atmospheric concentrations
of O2 affected the evolution of multicellular organisms.
See pp. 523–524 and Figure 25.5
• How have volcanic eruptions and meteorite strikes influenced the course of life’s evolution? See p. 525
The many dramatic physical events of Earth’s history have influenced the nature and timing of evolutionary changes among
Earth’s living organisms. We now will look more closely at some
of the major events that characterize the history of life on Earth.
Life first evolved on Earth about 3.8 bya. By about 1.5 bya, eukaryotic organisms had evolved (see Table 25.1). The fossil
record of organisms that lived prior to 550 mya is fragmentary,
but it is good enough to show that the total number of species
and individuals increased dramatically in late Precambrian
times. As discussed above, pre-Darwinian geologists divided
geological history into eras and periods based on their distinct
fossil assemblages. Biologists refer to the assemblage of all organisms of all kinds living at a particular time or place as a biota.
All of the plants living at a particular time or place are its flora;
all of the animals are its fauna. Table 25.1 describes some of the
physical and biological changes, such as mass extinctions and
dramatic increases in the diversity of major groups of organisms, associated with each unit of time.
About 300,000 species of fossil organisms have been described, and the number steadily grows. The number of named
species, however, is only a tiny fraction of the species that have
ever lived. We do not know how many species lived in the past,
but we have ways of making reasonable estimates. Of the present-day biota, nearly 1.8 million species have been named. The
actual number of living species is probably well over 10 million,
and possibly much higher, because many species have not yet
been discovered and described by biologists. So the number of
described fossil species is only about 3 percent of the estimated
minimum number of living species. Life has existed on Earth
for about 3.8 billion years. Many species last only a few million years before undergoing speciation or going extinct; therefore, Earth’s biota must have turned over many times during
geological history. So the total number of species that have lived
over evolutionary time must vastly exceed the number living
today. Why have only about 300,000 of these tens of millions
of species been described from fossils to date?
Several processes contribute to the paucity of fossils
Only a tiny fraction of organisms ever become fossils, and only
a tiny fraction of fossils are ever discovered by paleontologists.
Most organisms live and die in oxygen-rich environments in
which they quickly decompose. They are not likely to become
fossils unless they are transported by wind or water to sites that
lack oxygen, where decomposition proceeds slowly or not at
all. Furthermore, geological processes often transform rocks, destroying the fossils they contain, and many fossil-bearing rocks
are deeply buried and inaccessible. Paleontologists have studied only a tiny fraction of the sites that contain fossils, but they
find and describe many new ones every year.
The fossil record is most complete for marine animals that
had hard skeletons (which resist decomposition). Among the
nine major animal groups with hard-shelled members, approximately 200,000 species have been described from fossils—
roughly twice the number of living marine species in these
same groups. Paleontologists lean heavily on these groups in
their interpretations of the evolution of life. Insects and spiders
are also relatively well represented in the fossil record, because
they are numerically abundant and have hard exoskeletons
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25.3
|
527
WHAT ARE THE MAJOR EVENTS IN LIFE’S HISTORY?
gan to teem with life. For most of the Precambrian, life consisted
of microscopic prokaryotes; eukaryotes evolved about twothirds of the way through the era (Figure 25.10). Unicellular eukaryotes and small multicellular animals fed on floating photosynthetic microorganisms. Small floating organisms, known
collectively as plankton, were strained from the water and eaten
by slightly larger filter-feeding animals. Other animals ingested
sediments on the seafloor and digested the remains of organisms within them. By the late Precambrian (630–542 mya), many
kinds of multicellular soft-bodied animals had evolved. Some
of them were very different from any animals living today, and
may be members of groups that have no living descendants
(Figure 25.11).
Life expanded rapidly during the Cambrian period
Solenopsis sp.
25.9 Insect Fossils Chunks of amber—fossilized tree resin—often
contain insects that were preserved when they were trapped in the sticky
resin. This fire ant fossil is some 30 million years old.
(Figure 25.9). The fossil record, though incomplete, is good
enough to document clearly the factual history of the evolution of life.
By combining information about geological changes during Earth’s history with evidence from the fossil record, scientists have composed portraits of what Earth and its inhabitants may have looked like at different times. We know in
general where the continents were and how life changed over
time, but many of the details are poorly known, especially for
events in the more remote past.
The Cambrian period (542–488 mya) marks the beginning of the
Paleozoic era. The oxygen concentration in the Cambrian atmosphere was approaching its current level, and the land
masses had come together to form several large continents. A
geologically rapid diversification of life took place that is sometimes referred to as the Cambrian explosion (although in fact it
began before the Cambrian, and the “explosion” took millions
of years). Several of the major groups of animals that have
species living today first evolved during the Cambrian. An
overview of the continental and biotic shifts that characterized
the Cambrian and subsequent periods is shown in Figure 25.12
on the following pages.
For the most part, fossils tell us only about the hard parts of
organisms, but in three known Cambrian fossil beds—the
Burgess Shale in British Columbia, Sirius Passet in northern
Greenland, and the Chengjiang site in southern China—the soft
parts of many animals were preserved. Crustacean arthropods
(crabs, shrimps, and their relatives) are the most diverse group
Precambrian life was small and aquatic
For most of its history, life was confined to the oceans, and all
organisms were small. Over the long ages of the Precambrian
era—more than 3 billion years—the shallow seas slowly be-
Formation
of the Earth
First
oceans
25.10 A Sense of Life’s Time The top timeline shows the 4.5 billion
year history of life on Earth. Most of this time is accounted for by the
Precambrian, a 3.4 billion year era that saw the origin of life and the evolution of cells, photosynthesis, and multicellularity. The final 600 million
years are expanded in the second timeline and detailed in Figure 25.12.
Origin of
photosynthesis
Origin
of life
BILLIONS OF
YEARS AGO
First
eukaryotes
First
photosynthetic
eukaryotes
First fossils of
multicellular
animals
P r e c a m b r i a n
4
3
Cambrian Ordovician
MILLIONS OF Precambrian
YEARS AGO
Silurian
2
Devonian
Carboniferous Permian
P a l e o z o i c
500
400
1
Triassic
Jurassic
0
Cretaceous
M e s o z o i c
300
200
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any form without express written permission from the publisher.
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Quaternary
Tertiary
Cenozoic
100
Present
528
CHAPTER 25
Spriggina floundersi
|
HISTORY OF LIFE ON EARTH
Mawsonites spriggi
25.11 Precambrian Life These fossils of soft-bodied invertebrates,
excavated at Ediacara in southern Australia, were formed about 600 million years ago. Very different from later life forms, they illustrate the diversity of life at the end of the Precambrian era.
in the Chinese fauna; some of them were large carnivores. Multicellular diversity was largely or completely aquatic during the
Cambrian. If there was life on land at this time, it was probably
restricted to microbial organisms.
Many groups of organisms that arose during the
Cambrian later diversified
Geologists divide the remainder of the Paleozoic era into the
Ordovician, Silurian, Devonian, Carboniferous, and Permian
periods. Each period is characterized by the diversification of
specific groups of organisms. Mass extinctions marked the ends
of the Ordovician, Devonian, and Permian.
THE ORDOVICIAN (488–444 MYA) During the Ordovician period,
the continents, which were located primarily in the Southern
Hemisphere, still lacked multicellular plants. Evolutionary radiation of marine organisms was spectacular during the early
Ordovician, especially among animals, such as brachiopods and
mollusks, that lived on the seafloor and filtered small prey from
the water. At the end of the Ordovician, as massive glaciers
formed over the southern continents, sea levels dropped about
50 meters and ocean temperatures dropped. About 75 percent
of the animal species became extinct, probably because of these
major environmental changes.
THE SILURIAN (444–416 MYA) During the Silurian period, the continents began to merge together. Marine life rebounded from
the mass extinction at the end of the Ordovician. Animals able
to swim in open water and feed above the ocean bottom appeared for the first time. Jawless fishes diversified, and the first
ray-finned fishes evolved. The tropical sea was uninterrupted
by land barriers, and most marine organisms were widely distributed. On land, the first vascular plants evolved late in the
Silurian (about 420 mya). The first terrestrial arthropods—scorpions and millipedes—evolved at about the same time.
Dickinsonia costata
THE DEVONIAN (416–359 MYA) Rates of evolutionary change accelerated in many groups of organisms during the Devonian
period. The major land masses continued to move slowly toward each other. In the oceans there were great evolutionary radiations of corals and of shelled, squidlike cephalopod mollusks. Fishes diversified as jawed forms replaced jawless ones
and as heavy armor gave way to the less rigid outer coverings
of modern fishes.
Terrestrial communities changed dramatically during the Devonian. Club mosses, horsetails, and tree ferns became common;
some attained the size of large trees. Their roots accelerated the
weathering of rocks, resulting in the development of the first
forest soils. The ancestors of gymnosperms—the first plants to
produce seeds—appeared in the Devonian. The first known fossils of centipedes, spiders, mites, and insects date to this period,
and fishlike amphibians began to occupy the land.
A massive extinction of about 75 percent of all marine species
marked the end of the Devonian. Paleontologists are uncertain
about its cause, but two large meteorites that collided with Earth
at that time (one in present-day Nevada and the other in western Australia) may have been responsible, or at least a contributing factor. The continued coalescence of the continents, with the
corresponding reduction in continental shelves, may have also
contributed to this mass extinction event.
THE CARBONIFEROUS (359–297 MYA) Large glaciers formed over
high-latitude portions of the southern land masses during the
Carboniferous period, but extensive swamp forests grew on the
tropical continents. These forests were not made up of the kinds
of trees we know today, but were dominated by giant tree ferns
and horsetails with small leaves. Fossilized remains of those
forests formed the coal we now mine for energy. In the seas,
25.12 A Brief History of Life on Earth The geologically rapid “explosion” of life during the Cambrian saw the rise of several animal groups that
have representatives surviving today. The following three pages depict life’s
history from the Cambrian forward. Movements of the major continents
during the past half-billion years are shown in the maps of Earth, and
associated biotas for each time period are depicted. The artists’ reconstructions are based on fossils such as those shown in the photographs.
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© 2010 Sinauer Associates, Inc.
Rapid increase of multicellular
organisms (Cambrian “explosion”)
Major radiation of
several marine groups
Cambrian
MILLIONS OF
YEARS AGO
First jawed fishes;
First vascular plants many animal groups
and terrestrial
radiate; forests appear
arthropods evolve
on land
Ordovician
Silurian
Devonian
Precambrian
P a l e o z o i c
500
400
75% of all animals go extinct as
sea levels drop by 50 meters
Cambrian
75% of marine
species go extinct
Devonian
Phacops ferdinandi
Ottoia sp.
Orthoconic
nautiloid
Marrella splendens
Anomalocaris canadensis (claw only)
Codiacrinus schultzei
Eridophyllum sp.
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any form without express written permission from the publisher.
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Extensive swamp
forests produce coal;
origin of amniotes;
great increase in
terrestrial animal diversity
Giant amphibians
and flying insects;
ray-finned fishes
abundant in freshwater
Carboniferous
On land, conifers
become dominant
plants; frogs and reptiles
begin to diversify
Permian
Dinosaurs, pterosaurs,
ray-finned fishes diversify;
first mammals appear
Triassic
First known
flowering
plant fossils
Jurassic
P a l e o z o i c
M e s o z o i c
300
200
Extinction of 96% of Earth’s species;
oxygen levels drop rapidly
L A U R A S I
A
P A
N
G O N D
W
G
A
E
A
Permian
A
N
A
Triassic
Coelophysis bauri
Phlebopteris
smithii
Walchia piniformis
Cacops sp.
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25.3
Flowering
plants diversify
Many radiations of
animal groups, on
both land and sea
Flowering plants
dominate on land;
rapid radiation of
mammals
Cretaceous
|
Grasslands
spread as
climates cool
WHAT ARE THE MAJOR EVENTS IN LIFE’S HISTORY?
Four major ice
ages; evolution
of Homo
Tertiary
M e s o z o i c
531
Quaternary
C e n o z o i c
100
Present
Mass extinction event, including
loss of most dinosaurs
Cretaceous
Tertiary
Hyracotherium leporinum
Gryposaurus sp.
Magnolia sp.
Plesiadapis fodinatus (jaw)
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any form without express written permission from the publisher.
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532
CHAPTER 25
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HISTORY OF LIFE ON EARTH
25.13 Evidence of Insect
Diversification The margins of this fossil fern leaf
from the Carboniferous have
been chewed by insects.
crinoids (sea lilies and feather stars) reached their greatest diversity, forming “meadows” on the seafloor.
The diversity of terrestrial animals increased greatly during
the Carboniferous. Snails, scorpions, centipedes, and insects were
abundant and diverse. Insects evolved wings, becoming the first
animals to fly. Flight gave herbivorous insects easy access to tall
plants; plant fossils from this period show evidence of chewing
by insects (Figure 25.13). The terrestrial vertebrate lineage split,
and amphibians became larger and better adapted to terrestrial
existence, while the sister lineage led to the amniotes, vertebrates
with well-protected eggs that can be laid in dry places.
THE PERMIAN (297–251 MYA) During the Permian period, the continents coalesced completely into the supercontinent Pangaea.
Permian rocks contain representatives of many major groups of
insects we know today. By the end of the period, the reptiles
split from a second amniote lineage (which would lead to the
mammals). Ray-finned fishes became common in the fresh waters of Pangaea.
Conditions for life deteriorated toward the end of the Permian. Massive volcanic eruptions resulted in outpourings of
lava that covered large areas of Earth. The ash and gases produced by the volcanoes blocked sunlight and cooled the climate,
resulting in the largest glaciers in Earth’s history. Atmospheric
oxygen concentrations gradually dropped from about 30 to 15
percent. At such low concentrations, most animals would have
been unable to survive at elevations above 500 meters; thus
about half of the land area would have been uninhabitable at
the end of the Permian. The combination of these changes resulted in the most drastic mass extinction event in Earth’s history. Scientists estimate that about 96 percent of all species became extinct at the end of the Permian.
Mesozoic era (251 mya). As Pangaea slowly began to break
apart, the oceans rose and once again flooded the continental
shelves, forming huge, shallow inland seas. Atmospheric oxygen concentrations gradually rose. Life once again proliferated and diversified, but different groups of organisms came to
the fore. The three groups of phytoplankton (floating photosynthetic organisms) that dominate today’s oceans—dinoflagellates, coccolithophores, and diatoms—became ecologically important at this time; their remains are the primary origin of the
world’s oil deposits. Seed-bearing plants replaced the trees that
had ruled the Permian forests.
The Mesozoic era is divided into three periods: the Triassic, Jurassic, and Cretaceous. The Triassic and Cretaceous were
terminated by mass extinctions, probably caused by meteorite
impacts.
Pangaea began to break apart during the Triassic period. Many invertebrate groups became more
species-rich, and many burrowing animals evolved from groups
living on the surfaces of seafloor sediments. On land, conifers
and seed ferns were the dominant trees. The first frogs and turtles appeared. A great radiation of reptiles began, which eventually gave rise to crocodilians, dinosaurs, and birds. The end
of the Triassic was marked by a mass extinction that eliminated
about 65 percent of the species on Earth.
THE TRIASSIC (251–200 MYA)
Geographic differentiation increased during
the Mesozoic era
THE JURASSIC (200–145 MYA) During the Jurassic period, Pangaea became fully divided into two large continents: Laurasia
drifted northward and Gondwana drifted south. Ray-finned
fishes rapidly diversified in the oceans. The first lizards appeared, and flying reptiles (pterosaurs) evolved. Most of the
large terrestrial predators and herbivores of the period were
dinosaurs. Several groups of mammals made their first appearance, and the earliest known fossils of flowering plants are from
late in this period.
The few organisms that survived the Permian mass extinction
found themselves in a relatively empty world at the start of the
THE CRETACEOUS (145–65 MYA) By the early Cretaceous period,
Laurasia and Gondwana had begun to break apart into the con-
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© 2010 Sinauer Associates, Inc.
25.3
tinents we know today. A continuous sea encircled the tropics.
Sea levels were high, and Earth was warm and humid. Life proliferated both on land and in the oceans. Marine invertebrates
increased in diversity and in number of species. On land, the
reptile radiation continued as dinosaurs diversified further and
the first snakes appeared. Early in the Cretaceous, flowering
plants began the radiation that led to their current dominance
of the land. By the end of the period, many groups of mammals
had evolved. Most early mammals were small, but one species
recently discovered in China, Repenomamus giganticus, was large
enough to capture and eat young dinosaurs.
As described in Section 25.2, another meteorite-caused mass
extinction took place at the end of the Cretaceous (the impact
site was near the present day Yucatán Peninsula of Mexico). In
the seas, many planktonic organisms and bottom-dwelling invertebrates became extinct. On land, almost all animals larger
than about 25 kilograms in body weight became extinct. Many
species of insects died out, perhaps because the growth of their
food plants was greatly reduced following the impact. Some
species in northern North America and Eurasia survived in areas that were not subjected to the devastating fires that engulfed
most low-latitude regions.
Modern biota evolved during the Cenozoic era
By the early Cenozoic era (65 mya), the positions of the continents resembled those of today, but Australia was still attached
to Antarctica, and the Atlantic Ocean was much narrower. The
Cenozoic was characterized by an extensive radiation of mammals, but other groups were also undergoing important changes.
Flowering plants diversified extensively and came to dominate world forests, except in the coolest regions, where the
forests were composed primarily of gymnosperms. Mutations
of two genes in one group of plants (the legumes) allowed them
to use atmospheric nitrogen directly by forming symbioses with
a few species of nitrogen-fixing bacteria (see Section 36.4). The
evolution of this symbiosis between certain early Cenozoic
plants and these specialized bacteria was the first “green revolution” and dramatically increased the amount of nitrogen available for terrestrial plant growth; the symbiosis remains fundamental to the ecological base of life as we know it today.
The Cenozoic era is divided into the Tertiary and the Quaternary periods. Because both the fossil record and our subsequent knowledge of evolutionary history become more extensive as we approach our own time, paleontologists have
subdivided these two periods into epochs (Table 25.2).
During the Tertiary period, Australia
began its northward drift. By 20 mya it had nearly reached its
current position. The early Tertiary was a hot and humid time,
and the ranges of many plants shifted latitudinally. The tropics were probably too hot for rainforests and were clothed in
low-lying vegetation instead. In the middle of the Tertiary, however, Earth’s climate became considerably cooler and drier.
Many lineages of flowering plants evolved herbaceous (nonwoody) forms, and grasslands spread over much of Earth.
THE TERTIARY (65–2.6 MYA)
|
WHAT ARE THE MAJOR EVENTS IN LIFE’S HISTORY?
533
TABLE 25.2
Subdivisions of the Cenozoic Era
PERIOD
EPOCH
Quaternary
Holocenea
0.01 (~10,000 years ago)
Pleistocene
2.6
Pliocene
5.3
Tertiary
aThe
ONSET (MYA)
Miocene
23
Oligocene
34
Eocene
55.8
Paleocene
65
Holocene is also known as the Recent.
By the start of the Cenozoic era, invertebrate faunas had already come to resemble those of today. It is among the terrestrial vertebrates that evolutionary changes during the Tertiary
were most rapid. Frogs, snakes, lizards, birds, and mammals all
underwent extensive radiations during this period. Three waves
of mammals dispersed from Asia to North America across one
of the several land bridges that have intermittently connected
the two continents during the past 55 million years. Rodents,
marsupials, primates, and hoofed mammals appeared in North
America for the first time.
We are living in the
Quaternary period. It is subdivided into two epochs, the Pleistocene and the Holocene (the Holocene also being known as the
Recent).
The Pleistocene was a time of drastic cooling and climate
fluctuations. During 4 major and about 20 minor “ice ages,”
massive glaciers spread across the continents, and the ranges of
animal and plant populations shifted toward the equator. The
last of these glaciers retreated from temperate latitudes less than
15,000 years ago. Organisms are still adjusting to these changes.
Many high-latitude ecological communities have occupied their
current locations for no more than a few thousand years.
It was during the Pleistocene that divergence within one
group of mammals, the primates, resulted in the evolution of
the hominoid lineage. Subsequent hominoid radiation eventually led to the species Homo sapiens—modern humans (see Section 33.5). Many large bird and mammal species became extinct
in Australia and in the Americas when H. sapiens arrived on
those continents about 45,000 and 15,000 years ago, respectively.
Many paleontologists believe these extinctions were probably
the result of hunting and other influences of Homo sapiens.
THE QUATERNARY (2.6 MYA TO PRESENT)
The tree of life is used to reconstruct evolutionary events
The fossil record reveals broad patterns in life’s evolution. To
reconstruct major events in the history of life, biologists also rely
on the phylogenetic information in the tree of life (see Chapter
22 and the Tree of Life Appendix). We can use phylogeny (in
combination with the paleontological record) to reconstruct the
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CHAPTER 25
534
|
HISTORY OF LIFE ON EARTH
timing of such major events as the acquisition of mitochondria
in the ancestral eukaryotic cell, the several independent origins
of multicellularity, and the movement of life onto dry land. We
can also follow major changes in the genomes of organisms, and
even reconstruct many gene sequences of species that are long
extinct (see Chapter 24).
Changes to the physical environment on Earth have clearly
influenced the great diversity in living organisms we see on the
planet today. To study the evolution of that diversity, biologists
examine the evolutionary relationships among species. Deciphering these relationships is an important step in understanding how life has diversified on Earth. Part Seven of this book
explores the major groups of life and the different solutions that
have evolved for major functions such as reproduction, energy acquisition, dispersal, and escape from predation.
25.3 RECAP
Life evolved in the Precambrian oceans. It diversified
as atmospheric oxygen approached its current level
and the continents came together to form several
large land masses. Numerous climate changes and
rearrangements of the continents, as well as
meteorite impacts, contributed to five major mass
extinctions.
yo u r B i oPort al.com
GO TO
•
Why have so few of the multitudes of organisms that
have existed over millennia become fossilized? See
pp. 526–527
•
What do we mean when we refer to the “Cambrian
explosion”? See p. 527
•
In what ways has continental drift affected the evolution of life on Earth? See Figure 25.12
The Interactive Tree of Life
CHAPTER SUMMARY
25.1
•
•
How Do Scientists Date Ancient Events?
Review Figure 25.1
•
Geologists divide the history of life into eras and periods, based
on major differences in the fossil assemblages found in successive layers of rocks. Review Table 25.1
25.2
•
•
How Have Earth’s Continents and Climates
Changed over Time?
Earth’s crust consists of solid lithospheric plates that float on
fluid magma. Continental drift caused by convection currents
in the magma moves these plates and the continents that lie on
top of them. Review Figure 25.2, ANIMATED TUTORIAL 25.1
Conditions on Earth have changed dramatically over time.
Increases in atmospheric oxygen and changes in Earth’s climate
have greatly influenced the evolution of life on Earth. Review
Figures 25.5 and 25.7
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The relative ages of organisms can be determined by the dating
of fossils and the strata of sedimentary rocks in which they are
found.
Paleontologists use a variety of radioisotopes with different
half-lives to date events at different times in the remote past.
Oxygen-generating cyanobacteria liberated enough O2 to open
the door to oxidation reactions in metabolic pathways. The aerobic prokaryotes were able to harvest more energy than anaerobic organisms and began to proliferate. Increases in atmospheric O2 levels supported the evolution of large eukaryotic
cells.
Major physical events on Earth, such as the collision of continents that formed the supercontinent Pangaea, have affected
Earth’s surface, climate, and atmosphere. In addition, extraterrestrial events such as meteorite strikes created sudden and
dramatic environmental shifts. All of these changes have affected the history of life.
25.3
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What Are The Major Events in Life’s History?
Paleontologists use fossils and evidence of geological changes
to determine what Earth and its biota may have looked like at
different times.
During most of its history, life was confined to the oceans.
Multicellular life diversified extensively during the Cambrian
explosion. Review Figure 25.11
The periods of the Paleozoic era were each characterized by the
diversification of specific groups of organisms. Amniotes—vertebrates whose eggs can be laid in dry places—first appeared
during the Carboniferous period.
During the Mesozoic era, distinct terrestrial biotas evolved on
each continent.
Five episodes of mass extinction punctuated the history of life
in the Paleozoic and Mesozoic eras.
Earth’s flora has been dominated by flowering plants since the
Cenozoic era.
Phylogenetic trees help reconstruct the timing of evolutionary
events and clarify relationships among modern species.
SEE WEB ACTIVITY 25.1 for a Concept Review of this Chapter.
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© 2010 Sinauer Associates, Inc.
SELF QUIZ
535
SELF-QUIZ
1.Which of the following is not true of the giant flying dragonflies of the Carboniferous and Permian?
a. Some species grew to have wing spans as wide or wider
than many modern birds of prey.
b. They were the largest flying predators of the time.
c. Such large flying insects could exist because of the higher
concentrations of atmospheric oxygen compared to the
present.
d. Their predators were giant reptiles.
e. Fossils of one large species, Meganeurosis permiana, have
been found in the Permian rocks of Kansas.
2. In undisturbed strata of sedimentary rock, the oldest rocks
a. lie at the top.
b. lie at the bottom.
c. are in the middle.
d. are distributed among the strata of younger rocks.
e. none of the above
14
3. C can be used to determine the ages of fossil organisms
because
a. all organisms contain many carbon compounds.
b. 14C has a regular rate of decay to 14N.
c. the ratio of 14C to 12C in living organisms is always the
same as that in the atmosphere.
d. the production of new 14C in the atmosphere just balances the natural radioactive decay of 14C.
e. all of the above
4. The concentration of oxygen in the Earth’s atmosphere
a. has increased steadily through time.
b. has decreased steadily through time.
c. has been both higher and lower in the past than at present.
d. was lower during most of the Permian than at present.
e. was at its highest levels in the Cambrian.
5. The total of all species of organisms in a given region is
known as the region’s
a. biota.
b. flora.
c. fauna.
d. flora and fauna.
e. biogeography.
6. The coal beds we now mine for energy are largely the
remains of
a. plants that grew in swamps during the Carboniferous
period.
b. algae that grew in marshes during the Devonian period.
c. giant insects and amphibians of the Permian period.
d. plants that grew in the oceans during the Carboniferous
period.
e. none of the above
7. The mass extinction at the end of the Ordovician period
was probably caused by
a. the collision of Earth with a large meteorite.
b. massive volcanic eruptions.
c. massive glaciation on the southern continents and
associated climatic changes.
d. the uniting of all continents to form Pangaea.
e. changes in Earth’s orbit.
8. The cause of the mass extinction at the end of the Mesozoic
era probably was
a. continental drift.
b. the collision of Earth with a large meteorite.
c. changes in Earth’s orbit.
d. massive glaciation.
e. changes in the salt concentration of the oceans.
9. Which of the following times was marked by the largest
mass extinction of life in the history of Earth?
a. The end of the Cretaceous
b. The end of the Devonian
c. The end of the Permian
d. The end of the Triassic
e. The end of the Silurian
10. Paleontologists have subdivided the Cenozoic era into
epochs because
a. Homo sapiens evolved at the start of the Cenozoic.
b. the continents had achieved their present positions.
c. the number of species stopped increasing at this time.
d. our knowledge of the evolutionary events of the
Cenozoic is more extensive than for other eras.
e. starting with the Cenozoic, the fossil record is no longer a
necessary source of information about evolutionary
relationships.
FOR DISCUSSION
1. Some groups of organisms have evolved to contain large
numbers of species; other groups have produced only a few
species. Is it meaningful to consider the former groups
more successful than the latter? What does the word “success” mean in evolution?
2. Scientists date ancient events using a variety of methods,
but nobody was present to witness or record those events.
Accepting those dates requires us to understand the accuracy and appropriateness of indirect measurement techniques. What other basic scientific concepts are also based
on the results of indirect measurement techniques?
3. Why is it useful to be able to date past events absolutely as
well as relatively?
4. If we are living during one of the cooler periods in Earth’s
history, why should we be concerned about human activities
that are thought to contribute to global climate warming?
5. What conditions may have favored the evolution of multicellular groups of organisms near the end of the
Precambrian?
6. In what ways do endosymbiotic events (such as the origin
of mitochondria and chloroplasts) complicate the classification of the major groups of life?
A D D I T I O N A L I N V E S T I G AT I O N
The experiment in Figure 25.6 showed that body size of insects
may evolve quickly following changes in atmospheric oxygen
concentrations. What other experiments could you devise to test
the effects of changing atmospheric oxygen?
This material cannot be copied, reproduced, manufactured, or disseminated in
any form without express written permission from the publisher.
© 2010 Sinauer Associates, Inc.