Evidence About Earth’s Past (Book)

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T-RexDo you recognize this animal from its skeleton? If you guessed it’s Tyrannosaurus rex, you’re right. Like other dinosaurs, T. rex went extinct about 65 million years ago. How do we know what this extinct animal looked like? The answer is right in front of you: from the fossils it left behind. This T. rex isn’t a true fossil. It’s just a copy on display in a museum. But many fossils of T. rex have been found. Fossils not only show us what extinct animals looked like. They also provide evidence about past environments and geological processes. In this chapter, you’ll find out how scientists use clues from fossils to understand Earth’s history.

Image copyright Geoff Hardy, 2014. www.shutterstock.com. Used under license from Shutterstock.com.



For thousands of years, people have discovered fossils. They have wondered about the creatures that left them. In ancient times, fossils inspired myths. Stories were told about monsters and other incredible creatures. For example, dinosaur fossils discovered in China two thousand years ago were thought to be dragon bones.

Do you know what fossils are? Do you know how they form? And do you know what they can tell us about the past?


SCI-MS.ESS1.04 Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth’s 4.6-billion-year-old history.

SCI-MS.ESS2.03 Analyze and interpret data on the distribution of fossils and rocks, continental shapes, and seafloor structures to provide evidence of the past plate motions.


Key Concepts

  • Definition and types of fossils
  • How fossils form
  • What can be learned from fossils

Lesson Objectives

  • Explain what fossils are.
  • Describe how fossils form.
  • State what scientists can learn from fossils.

What Are Fossils?

Fossils are preserved remains or traces of organisms that lived in the past. Most preserved remains are hard parts, such as teeth, bones, or shells. Examples of these kinds of fossils are pictured in Figure below. Preserved traces can include footprints, burrows, or even wastes. Examples of trace fossils are also shown in Figure below.


A variety of fossil types are pictured here. Preserved Remains: (A) teeth of a cow, (B) nearly complete dinosaur skeleton embedded in rock, (C) sea shell preserved in a rock. Preserved Traces: (D) dinosaur tracks in mud, (E) fossil animal burrow in rock, (F) fossil feces from a meat-eating dinosaur in Canada.

How Fossils Form

The process by which remains or traces of living things become fossils is called fossilization. Most fossils are preserved in sedimentary rocks.

- in Sedimentary Rock

Most fossils form when a dead organism is buried in sediment. Layers of sediment slowly build up. The sediment is buried and turns into sedimentary rock. The remains inside the rock also turn to rock. The remains are replaced by minerals. The remains literally turn to stone. Fossilization is illustrated in Figure.


Fossilization. This flowchart shows how most fossils form.

- Other Ways

Fossils may form in other ways. With complete preservation, the organism doesn't change much. As seen below, tree sap may cover an organism and then turn into amber. The original organism is preserved so that scientists might be able to study its DNA. Organisms can also be completely preserved in tar or ice. Molds and casts are another way organisms can be fossilized. A mold is an imprint of an organism left in rock. The organism's remains break down completely. Rock that fills in the mold resembles the original remains. The fossil that forms in the mold is called a cast. Molds and casts usually form in sedimentary rock. With compression, an organism's remains are put under great pressure inside rock layers. This leaves behind a dark stain in the rock.

You can read about them in Figure.

Ways Fossils Form Ways Fossils Form.

(A) Complete Preservation. This spider looks the same as it did the day it died millions of years ago!

(B) Molds and Casts. A mold is a hole left in rock after an organism's remains break. A cast forms from the minerals that fill that hole and solidify.

(C) Compression. A dark stain is left on a rock that was compressed. These ferns were fossilized by compression.

Why Fossilization is Rare

It’s very unlikely that any given organism will become a fossil. The remains of many organisms are consumed. Remains also may be broken down by other living things or by the elements. Hard parts, such as bones, are much more likely to become fossils. But even they rarely last long enough to become fossils. Organisms without hard parts are the least likely to be fossilized. Fossils of soft organisms, from bacteria to jellyfish, are very rare.

Learning from Fossils

Of all the organisms that ever lived, only a tiny number became fossils. Still, scientists learn a lot from fossils. Fossils are our best clues about the history of life on Earth.

- Fossil Clues

Fossils give clues about major geological events. Fossils can also give clues about past climates.

  • Fossils of ocean animals are found at the top of Mt. Everest. Mt. Everest is the highest mountain on Earth. These fossils show that the area was once at the bottom of a sea. The seabed was later uplifted to form the Himalaya mountain range. An example is shown in the Figure.
    fossil clues
    What can we learn from fossil clues like this fish fossil found in the Wyoming desert?
  • Fossils of plants are found in Antarctica. Currently, Antarctica is almost completely covered with ice. The fossil plants show that Antarctica once had a much warmer climate.

- Index Fossils

Fossils are used to determine the ages of rock layers. Index fossils are the most useful for this. Index fossils are of organisms that lived over a wide area. They lived for a fairly short period of time. An index fossil allows a scientist to determine the age of the rock it is in.

Trilobite fossils, as shown in Figure, are common index fossils. Trilobites were widespread marine animals. They lived between 500 and 600 million years ago. Rock layers containing trilobite fossils must be that age. Different species of trilobite fossils can be used to narrow the age even more.

Trilobites are good index fossils 

Trilobites are good index fossils. Why are trilobite fossils useful as index fossils?

Lesson Summary

  • Fossils are preserved remains or traces of organisms that lived in the past. Most fossils form in sedimentary rock. Fossils can also be preserved in other ways. Fossilization is rare. It’s very unlikely for any given organism to become a fossil.
  • Fossils are the best form of evidence about the history of life on Earth. Fossils also give us clues about major geological events and past climates. Index fossils are useful for determining the ages of rock layers.

Points to Consider

Fossils can help scientists estimate the ages of rocks. Some types of evidence show only that one rock is older or younger than another. Other types of evidence reveal a rock’s actual age in years.

  • What evidence might show that one rock is older or younger than another?
  • What evidence might reveal how long ago rocks formed?


The way things happen now is the same way things happened in the past. Earth processes have not changed over time. Mountains grow and mountains slowly wear away, just as they did billions of years ago. As the environment changes, living creatures adapt. They change over time. Some organisms may not be able to adapt. They become extinct, meaning that they die out completely.

Historical geologists study the Earth’s past. They use clues from rocks and fossils to figure out the order of events. They think about how long it took for those events to happen.


SCI-MS.ESS1.04 Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth’s 4.6-billion-year-old history.


Lesson Objectives

  • Explain how stratigraphy can be used to determine the relative ages of rocks.
  • State how unconformities occur.
  • Identify ways to match rock layers in different areas.
  • Describe how Earth’s history can be represented by the geologic time scale.

Laws of Stratigraphy

The study of rock strata is called stratigraphy. The laws of stratigraphy can help scientists understand Earth’s past. The laws of stratigraphy are usually credited to a geologist from Denmark named Nicolas Steno. He lived in the 1600s. The laws are illustrated in Figure. Refer to the figure as you read about the laws below

Laws of Stratigraphy

Laws of Stratigraphy.

This diagram illustrates the laws of stratigraphy.

A = Law of Superposition

B = Law of Lateral Continuity

C = Law of Original Horizontality

D = Law of Cross-Cutting Relationships

- Superposition

Superposition refers to the position of rock layers and their relative ages. Relative age means age in comparison with other rocks, either younger or older. The relative ages of rocks are important for understanding Earth’s history. New rock layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. This is the law of superposition. You can see an example in Figure.


Superposition. The rock layers at the bottom of this cliff are much older than those at the top. What force eroded the rocks and exposed the layers?

- Lateral Continuity

Rock layers extend laterally, or out to the sides. They may cover very broad areas, especially if they formed at the bottom of ancient seas. Erosion may have worn away some of the rock, but layers on either side of eroded areas will still “match up.”

Look at the Grand Canyon in Figure. It’s a good example of lateral continuity. You can clearly see the same rock layers on opposite sides of the canyon. The matching rock layers were deposited at the same time, so they are the same age.

Lateral Continuity

Lateral Continuity. Layers of the same rock type are found across canyons at the Grand Canyon.

- Original Horizontality

Sediments were deposited in ancient seas in horizontal, or flat, layers. If sedimentary rock layers are tilted, they must have moved after they were deposited.

- Cross-Cutting Relationships

Rock layers may have another rock cutting across them, like the igneous rock in Figure. Which rock is older? To determine this, we use the law of cross-cutting relationships. The cut rock layers are older than the rock that cuts across them.

Cross-cutting relationships in rock layers


Cross-cutting relationships in rock layers.
Rock D is a dike that cuts across all the other rocks. Is it older or younger than the other rocks?

- Unconformities

Geologists can learn a lot about Earth’s history by studying sedimentary rock layers. But in some places, there’s a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity.

Hutton's unconformityLook at the rock layers in Figure.
Hutton's unconformity, in Scotland.

They show a feature called Hutton’s unconformity. The unconformity was discovered by James Hutton in the 1700s. Hutton saw that the lower rock layers are very old. The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited.

Hutton's discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow. He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually.

Matching Rock Layers

When rock layers are in the same place, it’s easy to give them relative ages. But what if rock layers are far apart? What if they are on different continents? What evidence is used to match rock layers in different places?

- Widespread Rock Layers

Some rock layers extend over a very wide area. They may be found on more than one continent or in more than one country. For example, the famous White Cliffs of Dover are on the coast of southeastern England. These distinctive rocks are matched by similar white cliffs in France, Belgium, Holland, Germany, and Denmark (see Figure). It is important that this chalk layer goes across the English Channel. The rock is so soft that the Channel Tunnel connecting England and France was carved into it!

Chalk CliffsChalk Cliffs.

(A) Matching chalk cliffs in Denmark and

(B) in Dover, U.K.

- Key Beds

Like index fossils, key beds are used to match rock layers. A key bed is a thin layer of rock. The rock must be unique and widespread. For example, a key bed from around the time that the dinosaurs went extinct is very important. A thin layer of clay was deposited over much of Earth’s surface. The clay has large amount of the element iridium. Iridium is rare on Earth but common in asteroids. This unusual clay layer has been used to match rock up layers all over the world. It also led to the hypothesis that a giant asteroid struck Earth and caused the dinosaurs to go extinct.

- Using Index Fossils

Index fossils are commonly used to match rock layers in different places. You can see how this works in Figure. If two rock layers have the same index fossils, then they’re probably about the same age.

Using Index Fossils to Match Rock Layers

Using Index Fossils to Match Rock Layers. Rock layers with the same index fossils must have formed at about the same time. The presence of more than one type of index fossil provides stronger evidence that rock layers are the same age.

The Geologic Time Scale

Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history.

- Dividing Geologic Time

Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example.

- Putting Events in Order

To create the geologic time scale, geologists correlated rock layers. Steno's laws were used to determine the relative ages of rocks. Older rocks are at the bottom and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years.

- Divisions of the Geologic Time Scale

The largest blocks of time on the geologic time scale are called “eons.” Eons are split into “eras.” Each era is divided into “periods.” Periods may be further divided into “epochs.” Geologists may just use “early” or “late.” An example is “late Jurassic,” or “early Cretaceous.” Figure shows you what the geologic time scale looks like.

The Geologic Time Scale

The Geologic Time Scale.

- Life and the Geologic Time Scale

The geologic time scale may include illustrations of how life on Earth has changed. Major events on Earth may also be shown. These include the formation of the major mountains or the extinction of the dinosaurs. Figure is a different kind of the geologic time scale. It shows how Earth’s environment and life forms have changed.

The evolution of life is shown on this spiral

The evolution of life is shown on this spiral.

- Your Place in Geologic Time

We now live in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene Epoch. “Phanerozoic” means visible life. During this eon, rocks contain visible fossils. Before the Phanerozoic, life was microscopic. The Cenozoic Era means new life. It encompasses the most recent forms of life on Earth. The Cenozoic is sometimes called the Age of Mammals. Before the Cenozoic came the Mesozoic and Paleozoic. The Mesozoic means middle life. This is the age of reptiles, when dinosaurs ruled the planet. The Paleozoic is old life. Organisms like invertebrates and fish were the most common lifeforms.

Lesson Summary

  • The study of rock layers is called stratigraphy. Laws of stratigraphy help scientists determine the relative ages of rocks. The main law is the law of superposition. This law states that deeper rock layers are older than layers closer to the surface.
  • An unconformity is a gap in rock layers. They occur where older rock layers eroded away completely before new rock layers were deposited.
  • Other clues help determine the relative ages of rocks in different places. They include key beds and index fossils.
  • Scientists use the geologic time scale to illustrate the order in which events on Earth have happened.
  • The geologic time scale was developed after scientists observed changes in the fossils going from oldest to youngest sedimentary rocks. They used relative dating to divide Earth’s past in several chunks of time when similar organisms were on Earth.
  • The geologic time scale is divided into eons, eras, periods, and epochs.

Points to Consider

In this lesson, you read how scientists determine the relative ages of sedimentary rock layers. The law of superposition determines which rock layers are younger or older than others.

  • What about the actual ages of rocks? Is there a way to estimate their ages in years?
  • And what about other kinds of rocks? For example, is there a way to estimate the ages of igneous rocks?


The age of a rock in years is its absolute age. Absolute ages are much different from relative ages. The way of determining them is different, too. Absolute ages are determined by radiometric methods, such as carbon-14 dating. These methods depend on radioactive decay.


SCI-MS.ESS1.04 Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth’s 4.6-billion-year-old history.


Lesson Objectives

  • Describe radioactive decay.
  • Explain radiometric dating.

Radioactive Decay

Radioactive decay is the breakdown of unstable elements into stable elements. To understand this process, recall that the atoms of all elements contain the particles protons, neutrons, and electrons.

- Isotopes

An element is defined by the number of protons it contains. All atoms of a given element contain the same number of protons. The number of neutrons in an element may vary. Atoms of an element with different numbers of neutrons are called isotopes.

Consider carbon as an example. Two isotopes of carbon are shown in Figure. Compare their protons and neutrons. Both contain 6 protons. But carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.


Isotopes are named for their number of protons plus neutrons. If a carbon atom had 7 neutrons, what would it be named?

Almost all carbon atoms are carbon-12. This is a stable isotope of carbon. Only a tiny percentage of carbon atoms are carbon-14. Carbon-14 is unstable. Figure below shows carbon dioxide, which forms in the atmosphere from carbon-14 and oxygen. Neutrons in cosmic rays strike nitrogen atoms in the atmosphere. The nitrogen forms carbon-14. Carbon in the atmosphere combines with oxygen to form carbon dioxide. Plants take in carbon dioxide during photosynthesis. In this way, carbon-14 enters food chains.


Carbon-14 forms in the atmosphere. It combines with oxygen and forms carbon dioxide. How does carbon-14 end up in fossils?

- Decay of Unstable Isotopes

Like other unstable isotopes, carbon-14 breaks down, or decays. For carbon-14 decay, each carbon-14 atom loses an alpha particle. It changes to a stable atom of nitrogen-14. This is illustrated in Figure

Unstable isotopes

Unstable isotopes, such as carbon-14, decay by losing atomic particles. They form different, stable elements when they decay. In this case, the daughter is nitrogen-14.

The decay of an unstable isotope to a stable element occurs at a constant rate. This rate is different for each isotope pair. The decay rate is measured in a unit called the half-life. The half-life is the time it takes for half of a given amount of an isotope to decay. For example, the half-life of carbon-14 is 5730 years. Imagine that you start out with 100 grams of carbon-14. In 5730 years, half of it decays. This leaves 50 grams of carbon-14. Over the next 5730 years, half of the remaining amount will decay. Now there are 25 grams of carbon-14. How many grams will there be in another 5730 years? Figure graphs the rate of decay of carbon-14.

The rate of decay

The rate of decay of carbon-14 is stable over time.

Radiometric Dating

The rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and rocks. This type of dating is called radiometric dating.

- Carbon-14 Dating

The best-known method of radiometric dating is carbon-14 dating. A living thing takes in carbon-14 (along with stable carbon-12). As the carbon-14 decays, it is replaced with more carbon-14. After the organism dies, it stops taking in carbon. That includes carbon-14. The carbon-14 that is in its body continues to decay. So the organism contains less and less carbon-14 as time goes on. We can estimate the amount of carbon-14 that has decayed by measuring the amount of carbon-14 to carbon-12. We know how fast carbon-14 decays. With this information, we can tell how long ago the organism died.

Carbon-14 has a relatively short half-life. It decays quickly compared to some other unstable isotopes. So carbon-14 dating is useful for specimens younger than 50,000 years old. That’s a blink of an eye in geologic time. But radiocarbon dating is very useful for more recent events. One important use of radiocarbon is early human sites. Carbon-14 dating is also limited to the remains of once-living things. To date rocks, scientists use other radioactive isotopes.

- Other Radioactive Isotopes

The isotopes in Table below are used to date igneous rocks. These isotopes have much longer half-lives than carbon-14. Because they decay more slowly, they can be used to date much older specimens. Which of these isotopes could be used to date a rock that formed half a million years ago?

Unstable Isotope Decays to At a Half-Life of (years) Dates Rocks Aged (years old)
Potassium-40 Argon-40 1.3 billion 100 thousand – 1 billion
Uranium-235 Lead-207 700 million 1 million – 4.5 billion
Uranium-238 Lead-206 4.5 billion 1 million – 4.5 billion

Lesson Summary

  • The age of a rock in years is its absolute age. The main evidence for absolute age comes from radiometric dating methods, such as carbon-14 dating. These methods depend on radioactive decay.
  • Radioactive decay is the breakdown of unstable isotopes into stable elements. For example, carbon-14 is an unstable isotope of carbon that decays to the stable element nitrogen-14. The rate of decay of an isotope is measured in half-lives. A half-life is the time it takes for half a given amount of an isotope to decay.
  • Radiometric dating uses the rate of decay of unstable isotopes to estimate the absolute ages of fossils and rocks. Carbon-14 can be used to date recent organic remains. Other isotopes can be used to date igneous rocks that are much older.

Points to Consider

Scientists estimate the ages of rock layers in order to better understand Earth’s history and the history of life.

  • What do you already know about Earth’s history? For example, do you know how Earth formed?
  • How old is Earth? When did the planet first form? And when did life first appear?