Name
Date
Eras of Earth’s History
Class
Chapter 7 Lesson 6
On a separate sheet of paper, explain how several different life forms have developed on
Earth.
Name
Date
Eras of Earth’s History
Class
Chapter 7 Lesson 6
Understanding Main Ideas
Fill in the blank with the correct era or period to complete each statement.
1. Until the ______________________________________, only one-celled organisms lived on land, but
during this period, plants became abundant.
2. At the start of the _____________________________________, many organisms evolved with hard
parts including shells and outer skeletons.
3. During the ___________________________________ animals began to spread widely on land.
4. At the end of the ______________________________ , most species of life on Earth died.
5. Reptiles were so successful that the __________________________________ is often called the Age
of Reptiles.
6. At the close of the _______________________________________, about 65 million years ago, a
second mass extinction occurred.
7. The first dinosaurs appeared during the ______________________________________.
8. During the _______________________________, mammals evolved to live in many different
environments—on land, in water, and even in the air.
9. The fossil record suggests that humans migrated to most continents during the
__________________________________________.
Building Vocabulary
Match each definition with its term by writing the letter of the correct term in the right
column on the line beside the definition in the left column.
10. ____ animal with a backbone
a. reptile
11. ____ animal without a backbone
b. mammal
12. ____ animal that evolved from lungfish
c. invertebrate
13. ____ animal with strong legs and eggs with thick shells
d. mass extinction
14. ____ an event in which many types of living things die out at once
e. amphibian
15. ____ animal that feeds its young with milk
f.
vertebrate
Name
Date
Eras of Earth’s History
Class
Chapter 7 Lesson 6
Read the passages below. Then answer the questions that follow on a separate sheet
of paper.
The End of an Era
What happened at the close of the Mesozoic Era to cause the extinction of dinosaurs and many other
types of organisms? There is more than one theory. Each uses evidence from fossils and from analysis of
rock layers.
Theory 1: An Asteroid Impact
About 65 million years ago, an asteroid about 10 kilometers in diameter collided with earth. The
impact had the force of millions of atomic bombs. Sediment and particles of the asteroid blasted into
the atmosphere. A dust cloud blocked the sun for months or longer. Plants and other organisms that
performed photosynthesis died, and many animals that depended on those plants for food died, too.
Blocking the sun also dropped Earth’s temperature, changing the climate. Animals that couldn’t adapt
became extinct. Evidence for this theory includes the fossil record, which shows that many organisms
died out suddenly at the end of the Cretaceous Period. Scientists also discovered an impact crater in the
Gulf of Mexico. Other evidence includes a thin layer of rock around Earth that is enriched with the rare
element iridium—and that is 65 million years old. Asteroids contain iridium.
Theory 2: Volcanic Eruptions
About 65 millions years ago, huge volcanic eruptions occurred on the Indian subcontinent of south
Asia. Movements of Earth’s continents caused these eruptions. Lava poured over a large area, equal
in size to about half of Europe. Volcanic dust spewed into the atmosphere, and dark clouds encircled
Earth. For months or longer, the volcanic dust blocked the sun’s energy from reaching Earth’s surface,
causing plants, and then animals, to die. Evidence for this theory includes a fossil record, which shows
many organisms died suddenly at the end of the Cretaceous Period. Geologists identified a huge area of
volcanic rock 65 million years old on the Indian subcontinent. Also a thin rock layer enriched with iridium
around Earth dates back 65 million years. Eruptions produce dust-containing iridium.
1. What is the evidence for the asteroid-impact theory?
2. What is the evidence for the volcanic-eruptions theory?
3. In each theory, what causes the extinction of plants and animals?
4. What evidence would be needed to prove one of the theories true?
5. Which theory makes the most sense to you? Give reasons.
Eras of Earth’s History
Answer Key