Lesson Overview
The Fossil Record
Lesson Overview
19.1 The Fossil Record
Lesson Overview
The Fossil Record
Fossils and Ancient Life
What do fossils reveal about ancient life?
From the fossil record, paleontologists learn about the structure of ancient
organisms, their environment, and the ways in which they lived.
Lesson Overview
The Fossil Record
Fossils and Ancient Life
Fossils are the most important source of information about extinct species,
ones that have died out.
Fossils vary enormously in size, type, and degree of preservation. They
form only under certain conditions.
For every organism preserved as a fossil, many died without leaving a
trace, so the fossil record is not complete.
Lesson Overview
The Fossil Record
Types of Fossils
• Fossils can be as large and perfectly preserved as an entire animal,
complete with skin, hair, scales, or feathers.
• They can also be as tiny as bacteria, developing embryos, or pollen
grains.
• Many fossils are just fragments of an organism—teeth, pieces of a
jawbone, or bits of leaf.
• Sometimes an organism leaves behind trace fossils—casts of
footprints, burrows, tracks, or even droppings.
• Although most fossils are preserved in sedimentary rocks, some are
preserved in other ways, like in amber.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
Most fossils are preserved in sedimentary rock.
Sedimentary rock usually forms when small particles of sand, silt, clay,
or lime muds settle to the bottom of a body of water.
As sediments build up, they bury dead organisms that have sunk to the
bottom.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
As layers of sediment continue to build up over time, the remains are
buried deeper and deeper.
Over many years, water pressure gradually compresses the lower
layers and turns the sediments into rock.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
The preserved remains may later be discovered and studied.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
Usually, soft body structures decay quickly after death, so usually
only hard parts like wood, shells, bones, or teeth remain. These
hard structures can be preserved if they are saturated or replaced
with mineral compounds.
Lesson Overview
The Fossil Record
Fossils in Sedimentary Rock
Sometimes, however, organisms are buried so quickly that soft
tissues are protected from aerobic decay. When this happens,
fossils may preserve imprints of soft-bodied animals and
structures like skin or feathers.
Lesson Overview
The Fossil Record
What Fossils Can Reveal
The fossil record contains an enormous amount of information for
paleontologists, researchers who study fossils to learn about ancient
life.
By comparing body structures in fossils to body structures in living
organisms, researchers can infer evolutionary relationships and form
hypotheses about how body structures and species have evolved.
Bone structure and trace fossils, like footprints, indicate how animals
moved.
Lesson Overview
The Fossil Record
What Fossils Can Reveal
Fossilized plant leaves and pollen suggest whether the area was a
swamp, a lake, a forest, or a desert.
When different kinds of fossils are found together, researchers can
sometimes reconstruct entire ancient ecosystems.
Lesson Overview
The Fossil Record
Dating Earth’s History
How do we date events in Earth’s history?
Relative dating allows paleontologists to determine whether a fossil is older
or younger than other fossils.
Radiometric dating uses the proportion of radioactive to nonreactive
isotopes to calculate the age of a sample.
Lesson Overview
The Fossil Record
Relative Dating
Lower layers of sedimentary rock, and fossils they contain, are generally
older than upper layers.
Relative dating places rock layers and their fossils into a temporal
sequence.
Lesson Overview
The Fossil Record
Relative Dating
To help establish the relative ages of rock layers and their fossils,
scientists use index fossils. Index fossils are distinctive fossils used to
establish and compare the relative ages of rock layers and the fossils
they contain.
If the same index fossil is found in two widely separated rock layers, the
rock layers are probably similar in age.
Lesson Overview
The Fossil Record
Relative Dating
A good index fossil species must be easily recognized and will occur in
only a few rock layers (meaning the organism lived only for a short
time). These layers, however, will be found in many places (meaning
the organism was widely distributed).
Trilobites, a large group of distinctive marine organisms, are often
useful as index fossils.
Lesson Overview
The Fossil Record
Radiometric Dating
Relative dating is important, but provides no information about a fossil’s
absolute age in years.
One way to date rocks and fossils is radiometric dating.
Radiometric dating relies on radioactive isotopes, which decay, or
break down, into nonradioactive isotopes at a steady rate.
Radiometric dating compares the amount of radioactive to nonreactive
isotopes in a sample to determine its age.
Lesson Overview
The Fossil Record
Radiometric Dating
A half-life is the time required for half of the radioactive atoms in a
sample to decay.
After one half-life, half of the original radioactive atoms have decayed.
After another half-life, another half of the remaining radioactive atoms
will have decayed.
Lesson Overview
The Fossil Record
Radiometric Dating
Different radioactive elements
have different half-lives, so they
decay at different rates.
• The half-life of potassium-40 is
1.26 billion years.
Lesson Overview
The Fossil Record
Radiometric Dating
Carbon-14, which has a short half-life, can be used to directly date very
young fossils.
Elements with long half-lives can be used to indirectly date older fossils
by dating nearby rock layers, or the rock layers in which they are found.
Lesson Overview
The Fossil Record
Radiometric Dating
Carbon-14 is a radioactive form of carbon naturally found in the
atmosphere. It is taken up by living organisms along with “regular”
carbon, so it can be used to date material that was once alive, such as
bones or wood.
After an organism dies, carbon-14 in its body begins to decay to
nitrogen-14, which escapes into the air.
Researchers compare the amount of carbon-14 in a fossil to the amount
of carbon-14 in the atmosphere, which is generally constant. This
comparison reveals how long ago the organism lived.
Carbon-14 has a half-life of only about 5730 years, so it’s only useful for
dating fossils no older than about 60,000 years.
Lesson Overview
The Fossil Record
Radiometric Dating
For fossils older than 60,00 years, researchers estimate the age of rock
layers close to fossil-bearing layers and infer that the fossils are roughly
same age as the dated rock layers.
A number of elements with long half-lives are used for dating very old
fossils, but the most common are potassium-40 (half-life: 1.26 billion
years) and uranium-238 (half-life: 4.5 billion years).
Lesson Overview
The Fossil Record
Geologic Time Scale
How was the geologic time scale established, and what are its major
divisions?
The geologic time scale is based on both relative and absolute dating. The
major divisions of the geologic time scale are eons, eras, and periods.
Lesson Overview
The Fossil Record
Geologic Time Scale
Geologists and paleontologists
have built a time line of Earth’s
history called the geologic time
scale.
The basic divisions of the geologic
time scale are eons, eras, and
periods.
Lesson Overview
The Fossil Record
Establishing the Time Scale
By studying rock layers and index fossils, early paleontologists placed
Earth’s rocks and fossils in order according to their relative age.
They noticed major changes in the fossil record at boundaries between
certain rock layers.
Lesson Overview
The Fossil Record
Establishing the Time Scale
Geologists used these
boundaries to determine where
one division of geologic time
ended and the next began.
Years later, radiometric dating
techniques were used to assign
specific ages to the various rock
layers.
Lesson Overview
The Fossil Record
Divisions of the Geologic Time Scale
The time scale is based on
events that did not follow a
regular pattern.
The Cambrian Period, for
example, began 542 million
years ago and continued until
488 million years ago, which
makes it 54 million years long.
The Cretaceous Period was 80
million years long.
Lesson Overview
The Fossil Record
Divisions of the Geologic Time Scale
Geologists now recognize four eons of
unequal length.
• The Hadean Eon, during which the first
rocks formed, began about 4.6 billion
years ago.
• The Archean Eon, when life first
appeared, began about 4 billion years
ago.
• The Proterozoic Eon began 2.5 billion
years ago and lasted until 542 million
years ago.
• The Phanerozoic Eon began at the
end of the Proterozoic and continues to
the present.
Lesson Overview
The Fossil Record
Divisions of the Geologic Time Scale
Eons are divided into eras.
The Phanerozoic Eon, for
example, is divided into the
Paleozoic, Mesozoic, and
Cenozoic Eras.
Eras are subdivided into
periods, which range in
length from nearly 100
millions of years to just under
2 million years. The Paleozoic
Era, for example, is divided
into six periods.
Lesson Overview
The Fossil Record
Naming the Divisions
Geologists started to name
divisions of the time scale
before any rocks older than the
Cambrian Period had been
identified. For this reason, all of
geologic time before the
Cambrian is simply called
Precambrian Time.
Lesson Overview
The Fossil Record
Naming the Divisions
The Precambrian actually covers about 90 percent of Earth’s history.
In this figure, the history of Earth is depicted as a 24-hour clock. Notice
the relative length of Precambrian Time—almost 22 hours.
Lesson Overview
The Fossil Record
Life on a Changing Planet
How have our planet’s environment and living things affected each other to
shape the history of life on Earth?
Building mountains, opening coastlines, changing climates, and geological
forces have altered habitats of living organisms repeatedly throughout
Earth’s history. In turn, the actions of living organisms over time have
changed conditions in the land, water, and atmosphere of planet Earth.
Lesson Overview
The Fossil Record
Life on a Changing Planet
Earth and its climate has been constantly changing, and organisms have
evolved in ways that responded to those new conditions.
The fossil record shows evolutionary histories for major groups of
organisms as they have both responded to changes on Earth and how they
have changed Earth.
Lesson Overview
The Fossil Record
Physical Forces
Climate is one of the most important aspects of Earth’s physical
environment.
Earth’s climate has undergone dramatic changes over time. Many of
these changes were triggered by fairly small shifts in global
temperature.
During the global “heat wave” of the Mesozoic Era, Earth’s average
temperatures were only 6°C to 12°C higher than they were during the
twentieth century.
During the ice ages, world temperatures were only about 5°C cooler
than they are now.
These relatively small temperature shifts changed the shape of life on
Earth.
Lesson Overview
The Fossil Record
Physical Forces
Geological forces have transformed life on Earth, producing new
mountain ranges and moving continents.
Volcanic forces have altered landscapes and even formed entire
islands.
Local climates are shaped by the interaction of wind and ocean currents
with geological features such as mountains and islands.
Lesson Overview
The Fossil Record
Physical Forces
The theory of plate tectonics explains how solid continental “plates”
move slowly above Earth’s molten core—a process called continental
drift.
Over the long term, continents have collided to form “supercontinents.”
Later, these supercontinents have split apart and reformed.
Lesson Overview
The Fossil Record
Geological Cycles and Events
Continental drift has affected the
distribution of fossils and living
organisms worldwide. As continents
drifted apart, they carried organisms
with them.
For example, the continents of South
America and Africa are now widely
separated. But fossils of Mesosaurus, a
semiaquatic reptile, have been found in
both South America and Africa.
The presence of these fossils on both
continents, along with other evidence,
indicates that South America and Africa
were joined at one time.
Lesson Overview
The Fossil Record
Physical Forces
Evidence indicates that over millions of years, giant asteroids have
crashed into Earth.
Many scientists agree that these kinds of collisions would toss up so
much dust that it would blanket Earth, possibly blocking out enough
sunlight to cause global cooling. This could have contributed to, or even
caused, worldwide extinctions.
Lesson Overview
The Fossil Record
Biological Forces
The activities of organisms have affected global environments.
For example, Earth’s early oceans contained large amounts of soluble
iron and little oxygen.
During the Proterozoic Eon, however, photosynthetic organisms
produced oxygen gas and also removed large amounts of carbon
dioxide from the atmosphere.
The removal of carbon dioxide reduced the greenhouse effect and
cooled the globe. The iron content of the oceans fell as iron ions reacted
with oxygen to form solid deposits.
Organisms today shape the landscape by building soil from rock, and
sand and cycle nutrients through the biosphere.
Lesson Overview
The Fossil Record
Speciation and Extinction
What processes influence whether species and clades survive or become
extinct?
If the rate of speciation in a clade is equal to or greater than the rate of
extinction, the clade will continue to exist. If the rate of extinction in a clade
is greater than the rate of speciation, the clade will eventually become
extinct.
Lesson Overview
The Fossil Record
Speciation and Extinction
Grand transformations in anatomy, phylogeny, ecology, and behavior—
which usually take place in clades larger than a single species—are known
as macroevolutionary patterns.
The ways new species emerge through speciation, and the ways species
disappear through extinction, are both examples of macroevolutionary
patterns.
The emergence, growth, and extinction of larger clades, such as mammals
or dinosaurs, are also macroevolutionary patterns.
Lesson Overview
The Fossil Record
Macroevolution and Cladistics
Paleontologists study fossils to learn about patterns of macroevolution
and the history of life.
Fossils are classified using the same cladistic techniques, based on
shared derived characters that are used to classify living species.
In some cases, fossils are placed in clades that contain only extinct
organisms. In other cases, fossils are classified into clades that include
living organisms.
Lesson Overview
The Fossil Record
Adaptation and Extinction
Throughout the history of life, organisms have faced changing
environments. Some species can adapt to new conditions and thrive.
Other species fail to adapt and become extinct.
The rates at which species appear, adapt, and become extinct vary
among clades and from one geologic time to another.
Lesson Overview
The Fossil Record
Adaptation and Extinction
The emergence of new species with different characteristics can serve
as the “raw material” for macroevolutionary change within a clade.
If the “birth” rate of new species in a clade is equal to the “death” rate, or
extinction, the clade will survive.
If the “death” rate of the species exceeds the birth rate, the clade will die
out.
In some cases, the more varied the species in a particular clade are, the
more likely the clade is to survive environmental change.
The clade Reptilia is one example of a highly successful clade. It
includes living organisms like crocodiles, but also dinosaurs and the
surviving members of the dinosaur clade—birds.
Lesson Overview
The Fossil Record
Patterns of Extinction
Species are always evolving and competing—and some species
become extinct because of the slow but steady process of natural
selection, referred to as background extinction.
In contrast, a mass extinction affects many species over a relatively
short period of time.
• In a mass extinction, entire ecosystems vanish and whole food webs
collapse.
•
Species become extinct because their environment breaks down and
the ordinary process of natural selection can’t compensate quickly
enough.
Lesson Overview
The Fossil Record
This graph shows how the rate of extinction has changed over time.
Lesson Overview
The Fossil Record
Patterns of Extinction
Until recently, researchers looked for a single cause for each mass
extinction.
For example, geologic evidence shows that at the end of the
Cretaceous Period, a huge asteroid crashed into Earth and caused
global climate change. At about the same time, dinosaurs and many
other species became extinct.
It is reasonable to infer, then, that the asteroid played a significant role
in this mass extinction.
Many mass extinctions, however, were probably caused by several
factors working in combination: volcanic eruptions, moving continents,
and changing sea levels, for example.
Lesson Overview
The Fossil Record
Patterns of Extinction
After a mass extinction, biodiversity is dramatically reduced.
Extinction, however, offers new opportunities to survivors. As speciation
and adaptation produce new species to fill empty niches, biodiversity
recovers.
This recovery takes a long time—typically between 5 and 10 million
years. Some groups of organisms survive a mass extinction, while other
groups do not.
Lesson Overview
The Fossil Record
Rate of Evolution
How fast does evolution take place?
Evidence shows that evolution has often proceeded at different rates for
different organisms at different times over the long history of life on Earth.
Lesson Overview
The Fossil Record
Gradualism
Gradualism involves a slow, steady
change in a particular line of descent.
The fossil record shows that many
organisms have indeed changed
gradually over time.
Lesson Overview
The Fossil Record
Punctuated Equilibrium
The pattern of slow, steady change does not always hold.
Horseshoe crabs, for example, have changed little in structure from the
time they first appeared in the fossil record.
This species is said to be in a state of equilibrium, which means that the
crab’s structure has not changed much over a very long stretch of time.
Lesson Overview
The Fossil Record
Punctuated Equilibrium
Punctuated equilibrium is the term
used to describe equilibrium that is
interrupted by brief periods of more
rapid change.
The fossil record reveals periods of
relatively rapid change in many
groups of organisms.
Some biologists suggest that most
new species are produced during
periods of rapid change.
Lesson Overview
The Fossil Record
Rapid Evolution After Equilibrium
Rapid evolution may occur after a small population becomes isolated
from the main population. This small population can evolve faster than
the larger one because genetic changes spread more quickly among
fewer individuals.
Rapid evolution may also occur when a small group of organisms
migrates to a new environment. That’s what happened with the
Galápagos finches.
Lesson Overview
The Fossil Record
Rapid Evolution After Equilibrium
Mass extinctions open many ecological niches, creating new
opportunities for those organisms that survive. Groups of organisms that
survive mass extinctions evolve rapidly in the several million years after
the extinction.
Lesson Overview
The Fossil Record
Adaptive Radiation and Convergent
Evolution
What are two patterns of macroevolution?
Two important patterns of macroevolution are adaptive radiation and
convergent evolution.
Lesson Overview
The Fossil Record
Adaptive Radiation
Studies often show that a single species or a small group of species has
diversified over time into a clade containing many species.
These species display variations on the group’s ancestral body plan.
They often occupy different ecological niches.
These differences are the product of adaptive radiation, an
evolutionary process by which a single species or a small group of
species evolves over a relatively short time into several different forms
that live in different ways.
Lesson Overview
The Fossil Record
This diagram shows part of the adaptive
radiation of mammals.
This diagram shows part of the adaptive radiation of mammals.
Lesson Overview
The Fossil Record
Adaptive Radiation
An adaptive radiation may occur when species migrate to a new
environment or when extinction clears an environment of a large
number of inhabitants.
A species may also evolve a new feature that enables it to take
advantage of a previously unused environment.
Lesson Overview
The Fossil Record
Adaptive Radiations in the Fossil Record
Dinosaurs flourished for about 150 million years during the Mesozoic
Era. The fossil record documents that during this time, mammals
diversified but remained small.
After most dinosaurs became extinct, however, an adaptive radiation
began and produced the great diversity of mammals of the Cenozoic
Era.
Lesson Overview
The Fossil Record
Modern Adaptive Radiations
Both Galápagos finches and Hawaiian honeycreepers evolved from a
single bird species.
Both finches and honeycreepers evolved different beaks and different
behaviors that enable each of them to eat different kinds of food.
Lesson Overview
The Fossil Record
Convergent Evolution
Sometimes groups of organisms evolve in different places or at different
times, but in similar environments. These organisms start out with
different structures, but they face similar selection pressures.
In these situations, natural selection may mold different body structures
in ways that perform similar functions. Because they perform similar
functions, these body structures may look similar.
Evolution produces similar structures and characteristics in distantlyrelated organisms through the process of convergent evolution.
Convergent evolution has occurred often in both plants and animals.
Lesson Overview
The Fossil Record
Convergent Evolution
For example, mammals that feed on ants and termites evolved five
times in five different regions as shown in the figure below.
They all developed the powerful front claws, long hairless snout, and
tongue covered with sticky saliva that are necessary adaptations for
hunting and eating insects.
Lesson Overview
The Fossil Record
Coevolution
What evolutionary characteristics are typical of coevolving species?
The relationship between two coevolving organisms often becomes so
specific that neither organism can survive without the other. Thus, an
evolutionary change in one organism is usually followed by a change in the
other organism.
Lesson Overview
The Fossil Record
Coevolution
Sometimes, the life histories of two or more species are so closely
connected that they evolve together.
The process by which two species evolve in response to changes in each
other over time is called coevolution.
Lesson Overview
The Fossil Record
Flowers and Pollinators
Coevolution of flowers and pollinators is common and can lead to
unusual results.
For example, Darwin discovered an orchid whose flowers had a 40centimeter-long structure called a spur with a supply of nectar at the
bottom. Darwin predicted that some pollinating insect must have some
kind of feeding structure that would allow it to reach the nectar. Darwin
never saw that insect.
About 40 years later, researchers discovered a moth with a 40centimeter-long feeding tube that matched Darwin’s prediction.
Lesson Overview
The Fossil Record
Plants and Herbivorous Insects
Plants and herbivorous insects also demonstrate close
coevolutionary relationships.
Over time, many plants have evolved bad-tasting or poisonous
compounds that discourage insects from eating them.
• Once plants began to produce poisons, natural selection on
herbivorous insects favored any variants that could alter, inactivate,
or eliminate those poisons.
•
Milkweed plants, for example, produce toxic chemicals. But
monarch caterpillars not only can tolerate this toxin, they also can
store it in their body tissues to use as a defense against their
predators.
Lesson Overview
The Fossil Record
The Mysteries of Life’s Origins
What do scientists hypothesize about early Earth and the origin of life?
Earth’s early atmosphere contained little or no oxygen. It was principally
composed of carbon dioxide, water vapor, and nitrogen, with lesser
amounts of carbon monoxide, hydrogen sulfide, and hydrogen cyanide.
Miller and Urey’s experiment suggested how mixtures of the organic
compounds necessary for life could have arisen from simpler compounds
on a primitive Earth.
Lesson Overview
The Fossil Record
The Mysteries of Life’s Origins
“The RNA world” hypothesis proposes that RNA existed by itself before
DNA. From this simple RNA-based system, several steps could have led to
DNA-directed protein synthesis.
Lesson Overview
The Fossil Record
The Mysteries of Life’s Origins
Geological and astronomical evidence suggests that Earth formed as
pieces of cosmic debris collided with one another. While the planet
was young, it was struck by one or more huge objects, and the entire
globe melted.
For millions of years, violent volcanic activity shook Earth’s crust.
Comets and asteroids bombarded its surface.
About 4.2 billion years ago, Earth cooled enough to allow solid rocks
to form and water to condense and fall as rain. Earth’s surface
became stable enough for permanent oceans to form.
Lesson Overview
The Fossil Record
The Mysteries of Life’s Origins
This infant planet was very different from Earth today.
Earth’s early atmosphere contained little or no oxygen. It was principally
composed of carbon dioxide, water vapor, and nitrogen, with lesser
amounts of carbon monoxide, hydrogen sulfide, and hydrogen
cyanide.
Because of the gases in the atmosphere, the sky was probably pinkishorange.
Because they contained lots of dissolved iron, the oceans were probably
brown.
Lesson Overview
The Fossil Record
The First Organic Molecules
In 1953, chemists Stanley Miller
and Harold Urey tried recreating
conditions on early Earth to see if
organic molecules could be
assembled under these
conditions.
They filled a sterile flask with
water, to simulate the oceans, and
boiled it.
Lesson Overview
The Fossil Record
The First Organic Molecules
To the water vapor, they added
methane, ammonia, and
hydrogen, to simulate what they
thought had been the composition
of Earth’s early atmosphere.
They passed the gases through
electrodes, to simulate lightning.
Lesson Overview
The Fossil Record
The First Organic Molecules
Next, they passed the gases
through a condensation chamber,
where cold water cooled them,
causing drops to form. The liquid
continued to circulate through the
experimental apparatus for a
week.
After a week, they had produced
21 amino acids—building blocks
of proteins.
Lesson Overview
The Fossil Record
The First Organic Molecules
Miller and Urey’s experiment
suggested how mixtures of the
organic compounds necessary
for life could have arisen from
simpler compounds on a primitive
Earth.
We now know that Miller and
Urey’s ideas on the composition
of the early atmosphere were
incorrect. But new experiments
based on current ideas of the
early atmosphere have produced
similar results.
Lesson Overview
The Fossil Record
Formation of Microspheres
Geological evidence suggests that during the Archean Eon, 200 to 300
million years after Earth cooled enough to carry liquid water, cells
similar to bacteria were common. How did these cells originate?
Large organic molecules form tiny bubbles called proteinoid
microspheres under certain conditions.
•
Microspheres are not cells, but they have some characteristics of living systems.
•
Like cells, microspheres have selectively permeable membranes through which
water molecules can pass.
•
Microspheres also have a simple means of storing and releasing energy.
•
Several hypotheses suggest that structures similar to proteinoid microspheres
acquired the characteristics of living cells as early as 3.8 billion years ago.
Lesson Overview
The Fossil Record
Evolution of RNA and DNA
Cells are controlled by information stored in DNA, which is transcribed
into RNA and then translated into proteins.
The “RNA world” hypothesis about the origin of life suggests that RNA
evolved before DNA. From this simple RNA-based system, several
steps could have led to DNA-directed protein synthesis.
Lesson Overview
The Fossil Record
Evolution of RNA and DNA
A number of experiments that simulated conditions on early Earth
suggest that small sequences of RNA could have formed from simpler
molecules.
Under the right conditions, some RNA sequences help DNA replicate.
Other RNA sequences process messenger RNA after transcription. Still
other RNA sequences catalyze chemical reactions. Some RNA
molecules even grow and replicate on their own.
Lesson Overview
The Fossil Record
Evolution of RNA and DNA
One hypothesis about the origin of life suggests that RNA evolved
before DNA.
Lesson Overview
The Fossil Record
Production of Free Oxygen
Microscopic fossils, or microfossils, of prokaryotes that resemble
bacteria have been found in Archean rocks more than 3.5 billion years
old.
Those first life forms evolved in the absence of oxygen because at that
time, Earth’s atmosphere contained very little of that highly reactive gas.
Lesson Overview
The Fossil Record
Production of Free Oxygen
During the early Proterozoic Eon, photosynthetic bacteria became
common. By 2.2 billion years ago, these organisms were producing
oxygen.
Lesson Overview
The Fossil Record
Production of Free Oxygen
At first, the oxygen combined with iron in the oceans, producing iron
oxide, or rust.
Iron oxide, which is not soluble in water, sank to the ocean floor and
formed great bands of iron that are the source of most iron ore mined
today.
Without iron, the oceans changed color from brown to blue-green.
Lesson Overview
The Fossil Record
Production of Free Oxygen
Next, oxygen gas began to accumulate in the atmosphere. The ozone
layer began to form, and the skies turned their present shade of blue.
Over several hundred million years, oxygen concentrations rose until
they reached today’s levels
Lesson Overview
The Fossil Record
Production of Free Oxygen
Many scientists think that Earth’s early atmosphere may have been
similar to the gases released by a volcano today.
The graphs show the composition of the atmosphere today and the
composition of gases released by a volcano.
Lesson Overview
The Fossil Record
Production of Free Oxygen
To the first cells, which evolved in the absence of oxygen, this reactive
oxygen gas was a deadly poison that drove this type of early life to
extinction.
Some organisms, however, evolved new metabolic pathways that used
oxygen for respiration and also evolved ways to protect themselves
from oxygen’s powerful reactive abilities.
Lesson Overview
The Fossil Record
Origin of Eukaryotic Cells
What theory explains the origin of eukaryotic cells?
The endosymbiotic theory proposes that a symbiotic relationship evolved
over time, between primitive eukaryotic cells and the prokaryotic cells
within them.
Lesson Overview
The Fossil Record
Origin of Eukaryotic Cells
One of the most important events in the history of life was the evolution of
eukaryotic cells from prokaryotic cells.
Eukaryotic cells have nuclei, but prokaryotic cells do not.
Eukaryotic cells also have complex organelles. Virtually all eukaryotes
have mitochondria, and both plants and algae also have chloroplasts.
Lesson Overview
The Fossil Record
Endosymbiotic Theory
It is believed that about 2 billion years ago, some ancient prokaryotes
began evolving internal cell membranes. These prokaryotes were the
ancestors of eukaryotic organisms.
According to endosymbiotic theory, prokaryotic cells entered those
ancestral eukaryotes. The small prokaryotes began living inside the
larger cells.
Lesson Overview
The Fossil Record
Endosymbiotic Theory
Over time a symbiotic relationship evolved between primitive eukaryotic
cells and prokaryotic cells in them.
Lesson Overview
The Fossil Record
Endosymbiotic Theory
The endosymbiotic theory was proposed more than a century ago.
At that time, microscopists saw that the membranes of mitochondria and
chloroplasts resembled the cell membranes of free-living prokaryotes.
This observation led to two related hypotheses.
Lesson Overview
The Fossil Record
Endosymbiotic Theory
One hypothesis proposes that mitochondria evolved from endosymbiotic
prokaryotes that were able to use oxygen to generate energy-rich ATP
molecules.
Without this ability to metabolize oxygen, cells would have been killed
by the free oxygen in the atmosphere.
Lesson Overview
The Fossil Record
Endosymbiotic Theory
Another hypothesis proposes that chloroplasts evolved from
endosymbiotic prokaryotes that had the ability to photosynthesize.
Over time, these photosynthetic prokaryotes evolved within eukaryotic
cells into the chloroplasts of plants and algae.
Lesson Overview
The Fossil Record
Modern Evidence
During the 1960s, Lynn Margulis of Boston University noted that
mitochondria and chloroplasts contain DNA similar to bacterial DNA.
She also noted that mitochondria and chloroplasts have ribosomes
whose size and structure closely resemble those of bacteria.
In addition, she found that mitochondria and chloroplasts, like bacteria,
reproduce by binary fission when cells containing them divide by
mitosis.
These similarities provide strong evidence of a common ancestry
between free-living bacteria and the organelles of living eukaryotic cells.
Lesson Overview
The Fossil Record
Sexual Reproduction and Multicellularity
What is the evolutionary significance of sexual reproduction?
The development of sexual reproduction sped up evolutionary change
because sexual reproduction increases genetic variation.
Lesson Overview
The Fossil Record
Significance of Sexual Reproduction
When prokaryotes reproduce asexually, they duplicate their genetic
material and pass it on to daughter cells.
This process is efficient, but it yields daughter cells whose genomes
duplicate their parent’s genome.
Genetic variation is basically restricted to mutations in DNA.
Lesson Overview
The Fossil Record
Significance of Sexual Reproduction
When eukaryotes reproduce sexually, offspring receive genetic material
from two parents.
Meiosis and fertilization shuffle and reshuffle genes, generating lots of
genetic diversity. The offspring of sexually reproducing organisms are
never identical to either their parents or their siblings (except for
identical twins).
Genetic variation increases the likelihood of a population’s adapting to
new or changing environmental conditions.
Lesson Overview
The Fossil Record
Multicellularity
Multicellular organisms evolved a few hundred million years after the
evolution of sexual reproduction.
Early multicellular organisms likely underwent a series of adaptive
radiations, resulting in great diversity.
Lesson Overview
Patterns and Processes of Evolution
Lesson Overview
19.2 Patterns and
Processes of Evolution
Lesson Overview
Patterns and Processes of Evolution
Adaptation and Extinction
Throughout the history of life, organisms have faced changing
environments. Some species can adapt to new conditions and thrive.
Other species fail to adapt and become extinct.
The rates at which species appear, adapt, and become extinct vary
among clades and from one geologic time to another.
Lesson Overview
Patterns and Processes of Evolution
Patterns of Extinction
Species are always evolving and competing—and some species
become extinct because of the slow but steady process of natural
selection, referred to as background extinction.
In contrast, a mass extinction affects many species over a relatively
short period of time.
Lesson Overview
Patterns and Processes of Evolution
Patterns of Extinction
In a mass extinction, entire ecosystems vanish and whole food webs
collapse.
Species become extinct because their environment breaks down and
the ordinary process of natural selection can’t compensate quickly
enough.
Lesson Overview
Patterns and Processes of Evolution
Patterns of Extinction
Until recently, researchers looked for a single cause for each mass
extinction.
For example, geologic evidence shows that at the end of the
Cretaceous Period, a huge asteroid crashed into Earth and caused
global climate change. At about the same time, dinosaurs and many
other species became extinct.
It is reasonable to infer, then, that the asteroid played a significant role
in this mass extinction.
Many mass extinctions, however, were probably caused by several
factors working in combination: volcanic eruptions, moving continents,
and changing sea levels, for example.
Lesson Overview
Patterns and Processes of Evolution
Patterns of Extinction
After a mass extinction, biodiversity is dramatically reduced.
Extinction, however, offers new opportunities to survivors. As speciation
and adaptation produce new species to fill empty niches, biodiversity
recovers.
This recovery takes a long time—typically between 5 and 10 million
years. Some groups of organisms survive a mass extinction, while other
groups do not.
Lesson Overview
Patterns and Processes of Evolution
Rate of Evolution
How fast does evolution take place?
Evidence shows that evolution has often proceeded at different rates for
different organisms at different times over the long history of life on Earth.
Lesson Overview
Patterns and Processes of Evolution
Gradualism
Gradualism involves a slow, steady
change in a particular line of descent.
The fossil record shows that many
organisms have indeed changed
gradually over time.
Lesson Overview
Patterns and Processes of Evolution
Punctuated Equilibrium
The pattern of slow, steady change does not always hold.
Horseshoe crabs, for example, have changed little in structure from the
time they first appeared in the fossil record.
This species is said to be in a state of equilibrium, which means that the
crab’s structure has not changed much over a very long stretch of time.
Lesson Overview
Patterns and Processes of Evolution
Punctuated Equilibrium
Punctuated equilibrium is the term
used to describe equilibrium that is
interrupted by brief periods of more
rapid change.
The fossil record reveals periods of
relatively rapid change in many
groups of organisms.
Some biologists suggest that most
new species are produced during
periods of rapid change.
Lesson Overview
Patterns and Processes of Evolution
Rapid Evolution After Equilibrium
Rapid evolution may occur after a small population becomes isolated
from the main population. This small population can evolve faster than
the larger one because genetic changes spread more quickly among
fewer individuals.
Rapid evolution may also occur when a small group of organisms
migrates to a new environment. That’s what happened with the
Galápagos finches.
Lesson Overview
Patterns and Processes of Evolution
Rapid Evolution After Equilibrium
Mass extinctions open many ecological niches, creating new
opportunities for those organisms that survive. Groups of organisms that
survive mass extinctions evolve rapidly in the several million years after
the extinction.
Lesson Overview
Patterns and Processes of Evolution
Adaptive Radiation and Convergent
Evolution
What are two patterns of macroevolution?
Two important patterns of macroevolution are adaptive radiation and
convergent evolution.
Lesson Overview
Patterns and Processes of Evolution
Adaptive Radiation
Studies often show that a single species or a small group of species has
diversified over time into a clade containing many species.
These species display variations on the group’s ancestral body plan.
They often occupy different ecological niches.
These differences are the product of adaptive radiation, an
evolutionary process by which a single species or a small group of
species evolves over a relatively short time into several different forms
that live in different ways.
Lesson Overview
Patterns and Processes of Evolution
This diagram shows part of the adaptive
radiation of mammals.
This diagram shows part of the adaptive radiation of mammals.
Lesson Overview
Patterns and Processes of Evolution
Adaptive Radiation
An adaptive radiation may occur when species migrate to a new
environment or when extinction clears an environment of a large
number of inhabitants.
A species may also evolve a new feature that enables it to take
advantage of a previously unused environment.
Lesson Overview
Patterns and Processes of Evolution
Convergent Evolution
Sometimes groups of organisms evolve in different places or at different
times, but in similar environments. These organisms start out with
different structures, but they face similar selection pressures.
In these situations, natural selection may mold different body structures
in ways that perform similar functions. Because they perform similar
functions, these body structures may look similar.
Evolution produces similar structures and characteristics in distantlyrelated organisms through the process of convergent evolution.
Convergent evolution has occurred often in both plants and animals.
Lesson Overview
Patterns and Processes of Evolution
Convergent Evolution
For example, mammals that feed on ants and termites evolved five
times in five different regions as shown in the figure below.
They all developed the powerful front claws, long hairless snout, and
tongue covered with sticky saliva that are necessary adaptations for
hunting and eating insects.
Lesson Overview
Patterns and Processes of Evolution
Coevolution
What evolutionary characteristics are typical of coevolving species?
The relationship between two coevolving organisms often becomes so
specific that neither organism can survive without the other. Thus, an
evolutionary change in one organism is usually followed by a change in the
other organism.
Lesson Overview
Patterns and Processes of Evolution
Coevolution
Sometimes, the life histories of two or more species are so closely
connected that they evolve together.
The process by which two species evolve in response to changes in each
other over time is called coevolution.
Lesson Overview
Patterns and Processes of Evolution
Plants and Herbivorous Insects
Plants and herbivorous insects also demonstrate close
coevolutionary relationships.
Over time, many plants have evolved bad-tasting or poisonous
compounds that discourage insects from eating them.
Lesson Overview
Earth’s Early History
Lesson Overview
19.3 Earth’s Early History
Lesson Overview
Earth’s Early History
The Mysteries of Life’s Origins
What do scientists hypothesize about early Earth and
the origin of life?
Earth’s early atmosphere contained little or no oxygen. It
was principally composed of carbon dioxide, water
vapor, and nitrogen, with lesser amounts of carbon
monoxide, hydrogen sulfide, and hydrogen cyanide.
Miller and Urey’s experiment suggested how mixtures of
the organic compounds necessary for life could have
arisen from simpler compounds on a primitive Earth.
Lesson Overview
Earth’s Early History
The Mysteries of Life’s Origins
What do scientists hypothesize about early Earth and
the origin of life?
“The RNA world” hypothesis proposes that RNA existed
by itself before DNA. From this simple RNA-based
system, several steps could have led to DNA-directed
protein synthesis.
Lesson Overview
Earth’s Early History
The Mysteries of Life’s Origins
Geological and astronomical evidence suggests that
Earth formed as pieces of cosmic debris collided
with one another. While the planet was young, it
was struck by one or more huge objects, and the
entire globe melted.
Lesson Overview
Earth’s Early History
The Mysteries of Life’s Origins
For millions of years, violent volcanic activity shook
Earth’s crust. Comets and asteroids bombarded its
surface.
About 4.2 billion years ago, Earth cooled enough to
allow solid rocks to form and water to condense and
fall as rain. Earth’s surface became stable enough
for permanent oceans to form.
Lesson Overview
Earth’s Early History
The Mysteries of Life’s Origins
This infant planet was very different from Earth today.
Earth’s early atmosphere contained little or no oxygen. It
was principally composed of carbon dioxide, water
vapor, and nitrogen, with lesser amounts of carbon
monoxide, hydrogen sulfide, and hydrogen cyanide.
Because of the gases in the atmosphere, the sky was
probably pinkish-orange.
Because they contained lots of dissolved iron, the
oceans were probably brown.
Lesson Overview
Earth’s Early History
The First Organic Molecules
In 1953, chemists Stanley
Miller and Harold Urey
tried recreating
conditions on early Earth
to see if organic
molecules could be
assembled under these
conditions.
They filled a sterile flask
with water, to simulate
the oceans, and boiled it.
Lesson Overview
Earth’s Early History
The First Organic Molecules
To the water vapor, they
added methane,
ammonia, and hydrogen,
to simulate what they
thought had been the
composition of Earth’s
early atmosphere.
They passed the gases
through electrodes, to
simulate lightning.
Lesson Overview
Earth’s Early History
The First Organic Molecules
Next, they passed the
gases through a
condensation chamber,
where cold water cooled
them, causing drops to
form. The liquid
continued to circulate
through the experimental
apparatus for a week.
After a week, they had
produced 21 amino
acids—building blocks of
proteins.
Lesson Overview
Earth’s Early History
The First Organic Molecules
Miller and Urey’s experiment
suggested how mixtures of the
organic compounds necessary
for life could have arisen from
simpler compounds on a
primitive Earth.
We now know that Miller and
Urey’s ideas on the composition
of the early atmosphere were
incorrect. But new experiments
based on current ideas of the
early atmosphere have produced
similar results.
Lesson Overview
Earth’s Early History
Formation of Microspheres
Geological evidence suggests that during the Archean
Eon, 200 to 300 million years after Earth cooled
enough to carry liquid water, cells similar to bacteria
were common. How did these cells originate?
Large organic molecules form tiny bubbles called
proteinoid microspheres under certain conditions.
Microspheres are not cells, but they have some
characteristics of living systems.
Lesson Overview
Earth’s Early History
Formation of Microspheres
Like cells, microspheres have selectively permeable
membranes through which water molecules can pass.
Microspheres also have a simple means of storing
and releasing energy.
Several hypotheses suggest that structures similar to
proteinoid microspheres acquired the characteristics
of living cells as early as 3.8 billion years ago.
Lesson Overview
Earth’s Early History
Evolution of RNA and DNA
Cells are controlled by information stored in DNA,
which is transcribed into RNA and then translated into
proteins.
The “RNA world” hypothesis about the origin of life
suggests that RNA evolved before DNA. From this
simple RNA-based system, several steps could have
led to DNA-directed protein synthesis.
Lesson Overview
Earth’s Early History
Evolution of RNA and DNA
One hypothesis about the origin of life suggests that RNA evolved
before DNA.
Lesson Overview
Earth’s Early History
Production of Free Oxygen
During the early Proterozoic Eon, photosynthetic
bacteria became common. By 2.2 billion years ago,
these organisms were producing oxygen.
Lesson Overview
Earth’s Early History
Production of Free Oxygen
At first, the oxygen combined with iron in the oceans,
producing iron oxide, or rust.
Iron oxide, which is not soluble in water, sank to the
ocean floor and formed great bands of iron that are
the source of most iron ore mined today.
Without iron, the oceans changed color from brown to
blue-green.
Lesson Overview
Earth’s Early History
Production of Free Oxygen
Next, oxygen gas began to accumulate in the
atmosphere. The ozone layer began to form, and the
skies turned their present shade of blue.
Over several hundred million years, oxygen
concentrations rose until they reached today’s levels
Lesson Overview
Earth’s Early History
Production of Free Oxygen
Many scientists think that Earth’s early atmosphere
may have been similar to the gases released by a
volcano today.
The graphs show the composition of the atmosphere
today and the composition of gases released by a
volcano.
Lesson Overview
Earth’s Early History
Production of Free Oxygen
To the first cells, which evolved in the absence of
oxygen, this reactive oxygen gas was a deadly poison
that drove this type of early life to extinction.
Some organisms, however, evolved new metabolic
pathways that used oxygen for respiration and also
evolved ways to protect themselves from oxygen’s
powerful reactive abilities.
Lesson Overview
Earth’s Early History
Origin of Eukaryotic Cells
What theory explains the origin of eukaryotic cells?
The endosymbiotic theory proposes that a symbiotic
relationship evolved over time, between primitive
eukaryotic cells and the prokaryotic cells within them.
Lesson Overview
Earth’s Early History
Origin of Eukaryotic Cells
One of the most important events in the history of life
was the evolution of eukaryotic cells from prokaryotic
cells.
Eukaryotic cells have nuclei, but prokaryotic cells do
not.
Eukaryotic cells also have complex organelles. Virtually
all eukaryotes have mitochondria, and both plants and
algae also have chloroplasts.
Lesson Overview
Earth’s Early History
Endosymbiotic Theory
It is believed that about 2 billion years ago, some
ancient prokaryotes began evolving internal cell
membranes. These prokaryotes were the ancestors of
eukaryotic organisms.
According to endosymbiotic theory, prokaryotic cells
entered those ancestral eukaryotes. The small
prokaryotes began living inside the larger cells.
Lesson Overview
Earth’s Early History
Endosymbiotic Theory
Over time a symbiotic relationship evolved between
primitive eukaryotic cells and prokaryotic cells in
them.
Lesson Overview
Earth’s Early History
Endosymbiotic Theory
One hypothesis proposes that mitochondria evolved
from endosymbiotic prokaryotes that were able to use
oxygen to generate energy-rich ATP molecules.
Without this ability to metabolize oxygen, cells would
have been killed by the free oxygen in the
atmosphere.
Lesson Overview
Earth’s Early History
Endosymbiotic Theory
Another hypothesis proposes that chloroplasts
evolved from endosymbiotic prokaryotes that had the
ability to photosynthesize.
Over time, these photosynthetic prokaryotes evolved
within eukaryotic cells into the chloroplasts of plants
and algae.
Lesson Overview
Earth’s Early History
Modern Evidence
During the 1960s, Lynn Margulis of Boston University
noted that mitochondria and chloroplasts:
1) contain DNA similar to bacterial DNA.
2) have ribosomes whose size and structure closely
resemble those of bacteria.
3) reproduce by binary fission when cells containing
them divide by mitosis.
These similarities provide strong evidence of a
common ancestry between free-living bacteria and
the organelles of living eukaryotic cells.
Lesson Overview
Earth’s Early History
Sexual Reproduction and Multicellularity
What is the evolutionary significance of sexual
reproduction?
The development of sexual reproduction sped up
evolutionary change because sexual reproduction
increases genetic variation.
Lesson Overview
Earth’s Early History
Significance of Sexual Reproduction
When prokaryotes reproduce asexually, they
duplicate their genetic material and pass it on to
daughter cells.
This process is efficient, but it yields daughter cells
whose genomes duplicate their parent’s genome.
Genetic variation is basically restricted to mutations in
DNA.
Lesson Overview
Earth’s Early History
Significance of Sexual Reproduction
When eukaryotes reproduce sexually, offspring
receive genetic material from two parents.
Meiosis and fertilization shuffle and reshuffle genes,
generating lots of genetic diversity. The offspring of
sexually reproducing organisms are never identical to
either their parents or their siblings (except for
identical twins).
Genetic variation increases the likelihood of a
population’s adapting to new or changing
environmental conditions.
Lesson Overview
Earth’s Early History
Multicellularity
Multicellular organisms evolved a few hundred million
years after the evolution of sexual reproduction.
Early multicellular organisms likely underwent a
series of adaptive radiations, resulting in great
diversity.