Download 1 Evidence of the Past

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CHAPTER 8
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The History of Life on Earth
SECTION
1 Evidence of the Past
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
LS 1a, 3d, 5b, 5c
• What are fossils?
• What is the geologic time scale?
• How have conditions on Earth changed with time?
What Are Fossils?
In 1995, geologist Paul Sereno found a dinosaur skull in
a desert in Africa. The skull was 1.5 m long. The dinosaur
it came from may have been the largest land predator that
ever existed!
Scientists like Paul Sereno look for clues to help
them learn what happened on Earth in the past. These
scientists are called paleontologists. Paleo means “old,”
and onto means “life.” Therefore, paleontologists study
“old life,” or life that existed in the past. One of the most
important ways paleontologists learn about the past is by
studying fossils.
A fossil is the traces or remains of an organism that have
been preserved over thousands of years. Some fossils form
when an organism’s body parts, such as shells or bones, are
preserved in rock. Other fossils are signs, such as imprints,
that an organism once existed. The figure below shows one
way that this kind of fossil can form.
STUDY TIP
Organize After you read this
section, make a Concept Map
using the vocabulary terms
from the section.
READING CHECK
1. Define What is a
paleontologist?
1 An organism dies and
is buried by sediment,
such as mud or clay.
TAKE A LOOK
2 After the organism dies, its body rots
away. An imprint, or mold, is left
in the sediment. The mold has the
same shape as the organism’s body.
2. Infer If a paleontologist
found the cast of this fossil,
would the paleontologist be
able to tell what the inside
of the organism looked like?
Explain your answer.
3 Over time, the mold fills with a different kind of sediment. The sediment
forms a cast of the organism. The
sediment can turn into rock, preserving the cast for millions of years.
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Evidence of the Past continued
How Do Scientists Know the Ages of Fossils?
3. Define What is relative
dating?
Critical Thinking
4. Compare How is absolute
dating different from relative
dating? Give two ways.
Math Focus
5. Read a Graph How does
the ratio of unstable to stable
atoms change over time?
Ratio of unstable atoms to stable atoms
READING CHECK
There are two main methods that scientists use to
learn how old a fossil is: relative dating and absolute dating. During relative dating, a scientist estimates how
old a rock or fossil is compared to other rocks or fossils.
Almost all fossils are found in a kind of rock called
sedimentary rock. Sedimentary rocks form in layers. In
most bodies of sedimentary rock, the oldest layers are
found at the bottom. The newest layers are found at the
top. Scientists can use this information to estimate the
relative ages of fossils in the layers. Fossils in the bottom
layers are usually older than fossils in the top layers.
The second way that scientists learn the age of fossils
is by absolute dating. During absolute dating, scientists
learn the age of a fossil in years. Absolute dating is more
precise than relative dating.
One kind of absolute dating uses atoms. Atoms are
tiny particles that make up all matter. Some atoms are
unstable and can decay, or break down. When an atom
decays, it becomes a different kind of atom. It may also
give off smaller particles, energy, or both. The new atom
that forms is stable—it does not decay.
Each kind of unstable atom decays at a certain rate.
This rate is called the atom’s half-life. Scientists know the
half-lives of many different kinds of unstable atoms. They
can measure the ratio of these unstable atoms to stable
atoms in a rock. They can use this ratio to calculate the
age of the rock and any fossils in it.
Rock
forms
One
Two
half-life half-lives
Time
Three
half-lives
This figure shows how scientists can use half-lives to date
rocks. After each half-life, the ratio of unstable to stable atoms in the rock decreases. Scientists can learn the age of the
rock by measuring the ratio of unstable to stable atoms in it.
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What Is the Geologic Time Scale?
Think about some important things that have happened
to you. You probably know the day, month, and year in
which many of these events happened. The divisions of the
calendar make it easy for you to remember these things.
They also make it easy for you to talk to someone else
about things that happened in the past.
In a similar way, geologists use a type of calendar to
divide the Earth’s history. This calendar is called the
geologic time scale. The Earth is very old. Therefore, the
divisions of time in the geologic time scale are very long.
Geologists use the geologic time scale to help make
sense of how life on Earth has changed. When a new fossil
is discovered, geologists figure out how old it is. Then, they
place it in the geologic time scale at the correct time. In this
way, they can construct a history of life on Earth. The figure
below shows part of the geologic time scale.
Era
Period
Critical Thinking
6. Predict Consequences
All geologists use the same
basic geologic time scale.
What might happen if every
geologist used a different
geologic time scale to study
the Earth’s history? Explain
your answer.
Millions of years ago
Cenozoic era
Quaternary
1.8
Tertiary
65.5
Cretaceous
146
Mesozoic era
Jurassic
200
Triassic
251
Permian
299
Carboniferous
359
Devonian
416
Silurian
444
Ordovician
488
Cambrian
542
Paleozoic era
Say It
Investigate Many geologic
time-scale divisions are named
after certain places on Earth.
Research one of the periods in
the Paleozoic era to find out
where its name comes from.
Share your findings with a
small group.
TAKE A LOOK
7. List Give the three
periods in the Mesozoic era
in order from oldest to most
recent.
Precambrian time
4,600
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STANDARDS CHECK
LS 5c Extinction of a species
occurs when the environment
changes and the adaptive
characteristics of a species are
insufficient to allow its survival.
Fossils indicate that many organisms that lived long ago are
extinct. Extinction of species is
common; most of the species
that have lived on Earth no
longer exist.
Word Help: occur
to happen
Word Help: environment
the surrounding natural
conditions that affect an
organism
Word Help: insufficient
not enough
Word Help: survival
the continuing to live or
exist; the act of continuing
to live
Word Help: indicate
to be or give a sign of; to
show
8. Infer How could global
cooling cause a species to
go extinct?
How Do Geologists Divide the Geologic
Time Scale?
Many of the divisions of the geologic time scale are
based on information from fossils. As scientists discover new fossils, they may add information to the geologic time scale. For example, most fossils that have
been found are from organisms that have lived since
Precambrian time. Therefore, little is known about life
on Earth before this time. As scientists find more fossils
from Precambrian time, more information may be added
to the time scale.
Most of the divisions in the geologic time scale mark
times when life on Earth changed. During many of these
times, species died out completely, or became extinct. A
mass extinction happens when many species go extinct
at the same time.
Scientists do not know what causes all mass extinctions. The table below gives some events that may cause
mass extinctions.
Event
How it causes mass extinction
Comet or meteorite impact
can throw dust and ash into
the atmosphere, causing global
cooling
Many volcanic eruptions
can throw dust and ash into
the atmosphere, causing global
cooling
Changing ocean currents
can cause climate change by
changing how heat is distributed
on Earth’s surface
How Has the Earth’s Surface Changed?
Geologists have found fossils of tropical plants in
Antarctica. These kinds of plants cannot live in Antarctica
today because the climate is too cold. The fossils show
that Antarctica’s climate must once have been warmer.
In the early 1900s, German scientist Alfred Wegener
studied the shapes of the continents and the fossils found
on them. From his observations, he proposed a hypothesis that all the continents were once joined together. They
formed a large land mass called Pangaea. Since then, the
continents have moved over the Earth’s surface to their
present locations.
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Pangaea
Panthalassa
TAKE A LOOK
About 245 million years ago, all of the continents
were joined into one large land mass called
Pangaea. It was surrounded by a huge sea called
Panthalassa, which is Greek for “all seas.”
9. Infer On the map, circle
the part of Pangaea that
probably became the
continent of Africa.
MOVING CONTINENTS
In the 1960s and 1970s, new technology allowed geologists to learn more about the Earth’s crust. They found
that the crust is not one solid chunk of rock. Instead, it
is broken into many pieces called tectonic plates. These
plates move slowly over the Earth’s surface. Some of
the plates have continents on them. As the plates move,
the continents are carried along. The theory of how the
plates move is called the theory of plate tectonics.
READING CHECK
10. Explain Why do
continents move?
African
plate
South
American
plate
The continents ride on tectonic
plates. The plates move slowly
over the Earth’s surface.
The theory of plate tectonics explains why fossils of tropical plants have been found in Antarctica. Millions of years
ago, Antarctica was located near the equator. Over time, the
plate containing Antarctica has moved to its current location.
EFFECTS OF PLATE MOTIONS ON LIVING THINGS
Sudden changes on Earth’s surface, such as a meteorite
impact, may cause mass extinctions. Slower changes, such
as tectonic plate movements, give populations of organisms time to adapt. Geologists use fossils of organisms to
study how populations have adapted to changes on Earth.
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Name
Class
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Section 1 Review
NSES
LS 1a, 3d, 5b, 5c
SECTION VOCABULARY
absolute dating any method of measuring the
age of an event or object in years
extinct describes a species that has died out
completely
fossil the trace or remains of an organism that
lived long ago, most commonly preserved in
sedimentary rock
geologic time scale the standard method used
to divide the Earth’s long natural history into
manageable parts
plate tectonics the theory that explains how
large pieces of the Earth’s outermost layer,
called tectonic plates, move and change shape
relative dating any method of determining
whether an event or object is older or younger
than other events or objects
1. Define Write your own definition for geologic time scale.
2. List Give three things that can cause mass extinctions.
3. Compare Which fossil in the rock layers below is probably the oldest? Explain
your answer.
Fossil A
Fossil B
Fossil C
4. Explain Describe one way that a fossil can form.
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The History of Life on Earth
Life Science Answer Key continued
Review
1. Lyell’s book gave evidence that Earth is very
2.
3.
4.
5.
2. No, because only the shape of the shell is pre3.
old. Because Earth is very old, there has been
enough time for organisms to change slowly
over time.
The finches have differently shaped beaks
based on the type of food they primarily eat.
During natural selection, certain individuals
survive because they have traits that give them
a survival advantage over other individuals.
During selective breeding, people breed individuals for traits that people find appealing or useful, even if the traits have no particular survival
advantage.
Darwin realized that Malthus’s ideas apply to
all species. Most individuals produce more offspring than can survive. The size of a population is limited by the availability of resources,
starvation, disease, and predation.
overproduction, inherited variation, struggle to
survive, successful reproduction
4.
5.
6.
7.
8.
9.
10.
served. The “inside” of the cast is just sediment.
determining the age of a rock or fossil
compared with other rocks or fossils
Absolute dating finds the age of an object in
years, but relative dating finds its age compared
with other objects. Absolute dating is more
precise than relative dating.
It decreases.
It would be more difficult for them to share
information about Earth’s history, because their
time scales would have different divisions.
Triassic, Jurassic, Cretaceous
If the species cannot adapt to cooler
temperatures, all of its members will die.
The part of Pangaea that looks like Africa
should be circled.
They are carried along as the plates move.
Review
1. Possible answer: The geologic time scale is
a calendar that geologists use to study the
Earth’s history.
2. impacts, volcanic eruptions, changing ocean
currents
3. Fossil C is probably the oldest, because it is
the closest to the bottom.
4. An organism dies and is buried in sediment.
The body of the organism rots away, leaving
an impression in the sediment. Then, different
sediment fills the impression. This sediment
then hardens into a fossil called a cast.
SECTION 3 NATURAL SELECTION IN ACTION
1.
2.
3.
4.
5.
People hunt tusked elephants.
something that kills insects
They would all die.
It will probably decrease.
Speciation happens when new species form
because of evolution.
6. Different environmental factors may affect the
populations, leading to different adaptations.
7. South America
SECTION 2 ERAS OF THE GEOLOGIC TIME
SCALE
Review
1. separation, adaptation, division
1. about 4.1 billion years
2. Energy from radiation or storms caused
2. salty water, changing water level, high risk of
being crushed between or against rocks
3. mountains, canyons, lakes
4. Insects have many offspring, and they have a
short generation time.
5. Organism B will probably be able to adapt
more quickly to environmental changes
because it has a short generation time and
produces many offspring per generation.
simple chemicals to react.
3. a process in which a living thing converts
4.
5.
Chapter 8 The History of
Life on Earth
6.
7.
8.
SECTION 1 EVIDENCE OF THE PAST
1. a scientist who studies organisms that lived
9.
in the past
sunlight, water, and carbon dioxide into
food and oxygen
Cyanobacteria released oxygen during
photosynthesis.
Cambrian, Ordovician, Silurian, Devonian,
Carboniferous, Permian
in the oceans
crawling insects
Many species of reptiles evolved during the
Mesozoic.
Scientists think that birds may have evolved
from some dinosaurs.
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Interactive Textbook Answer Key
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Life Science