Name
Early Earth
Date
Class
Chapter 7 Lesson 5
On a separate sheet of paper, explain what scientists have theorized about important
events involving early Earth.
Name
Early Earth
Date
Class
Chapter 7 Lesson 5
Understanding Main Ideas
Put the six events or processes in the correct sequence by writing their letters in the
correct order in the numbered blanks below.
A. Volcanic eruptions release carbon dioxide into the atmosphere.
B. Early organisms release oxygen into the air.
C. Oceans form on Earth’s surface.
D. Hydrogen and helium are captured by gravity to form an atmosphere.
E. A dense, iron core forms at Earth’s center.
F. Earth is a ball of dust, rock, and ice in space.
1. ____
2. ____
3. ____
4. ____
5. ____
6. ____
Answer the following questions on a separate sheet of paper.
7. What were the earliest life forms on Earth like?
8. What hypothesis explains why Earth and the moon are about the same age?
9. How did photosynthesis lead to the development of animals living on land?
10. How have scientists used radioactive dating to show that Earth is about 4.6 billion years old?
Building Vocabulary
Fill in the blank to complete each statement.
11. A(n) _____________________ is a ball of dust and ice that orbits the sun.
12. The process of ________________ involves organisms using energy from the sun to make food.
Name
Early Earth
Date
Class
Chapter 7 Lesson 5
Read the passage below and then answer the questions that follow in the spaces
provided.
Life and Earth’s Atmosphere
More than 3 billion years ago, an amazing thing happened on Earth. Certain types of bacteria developed
the ability to make their own food. These bacteria made food by using a process called photosynthesis.
Using energy from the sun, the bacteria combined carbon dioxide and water to make food and oxygen.
The oxygen that was given off had a big effect on Earth’s atmosphere.
At first, the oxygen was taken up by iron that was in the ocean water. This caused the iron and oxygen,
or iron oxides, to fall to the ocean bottom and form deposits of iron-rich sedimentary rock. Most of the
iron that is used today to make cars and buildings comes from these deposits. Eventually most of the iron
in the oceans had combined with oxygen and fallen to the ocean floor. Now, oxygen that was produced
by microbes was free to enter the atmosphere. As a result, the oxygen level in earth’s atmosphere began
to increase significantly about 2.5 billion years ago. Slowly, over hundreds of millions of years, the oxygen
level increased to the present-day level of about 20 percent.
The buildup of oxygen in earth’s atmosphere allowed the emergence of organisms that cannot exist
without oxygen. Animals, for example, take in oxygen, combine it with the food they eat, and produce
a waste product: carbon dioxide.
1. What advantage does an organism have because of photosynthesis?
2. What gas is given off during photosynthesis?
3. After certain types of bacteria first started producing oxygen, where
did most of the oxygen go?
4. Why was the buildup of oxygen in Earth’s atmosphere important for
animals and other types of organisms?