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The Environment and Change Over Time
Fossil Evidence of Evolution
Key Concepts
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind. Before Statement
1. Originaltissues can be preserved as fossils. 2. Organisms become extinct only in mass

After

• How do fossils form? • How do scientists date fossils? • How are fossils evidence of biological evolution?

extinction events.

The Fossil Record
An oak tree changes a little when it loses its leaves. A robin changes when it loses some of its feathers. Yet, these living organisms change little from day today. It might seem as if oak trees and robins have been on Earth forever. If you were to go back a few million years in time, you would not see oak trees or robins. You would see different species of trees and birds. That is because species change over time. You might already know that fossils are the remains or evidence of once-living organisms. The fossil record is made up of all the fossils everdiscovered on Earth. It has millions of fossils that come from many thousands of species. Most of these species are no longer alive on Earth. The fossil record provides evidence that species have changed over time. Fossils help scientists picture what species looked like. Based on fossil evidence, scientists can re-create the physical appearance of species that are no longer alive. The fossilrecord is huge, but it still has many missing parts. Scientists are still looking for more fossils to fill these missing parts. Scientists hypothesize that the fossil record represents only a small fraction of all the organisms that ever lived on Earth.
Reading Essentials

Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Vocabulary Quiz Write a question about eachvocabulary term in this lesson. Exchange quizzes with a partner. After completing the quizzes, discuss the answers with your partner.

1. Explain Why don’t
scientists know the exact number of species that have lived on Earth?

The Environment and Change Over Time

83

SCIENCE USE V. COMMON USE
tissue
Science Use similar cells that work together and perform a function Common Use a pieceof soft, absorbent paper

Fossil Formation
When animals eat a dead animal, they usually leave little behind. Any soft tissues that are not eaten are broken down by bacteria. Only the hard parts—bones, shells, and teeth— remain. Usually, these parts also break down over time. Sometimes they become fossils. Very rarely, the soft tissues of animals or plants—skin, muscles, or leaves—can alsobecome fossils.

Reading Check

2. Analyze Why is it rare
for soft tissue to become a fossil?

Mineralization
After an organism dies, its body might be buried under mud or sand in a river. Minerals in the water can take the place of the organism’s original material and harden into rock. When this happens, a fossil forms. This process is called mineralization. Minerals in water also can fill thesmall spaces of a dead organism’s tissues and become rock. Shells and bones are the most common mineralized fossils. Wood can also become a fossil in this way.

Carbonization
In carbonization, a fossil forms when a dead plant or animal is subjected to pressure over time. Pressure drives off the organism’s liquids and gases. Only the carbon outline, or film, of the organism is left behind.Molds and Casts
Sometimes the shell or bone of a dead animal makes an impression, the outline of its shape, in mud or sand. When the mud or sand hardens, so does the impression. The impression of an organism in a rock is called a mold. Sand or mud can later fill in the mold and harden to form a cast. A cast is a fossil copy of an organism in a rock. Molds and casts show only the outside parts of...