- Suppose that the Earth had been much cooler when it first formed. How would the
earth’s interior be different than it is today?
Vocabulary
atmosphere The mixture of gases that surrounds the Earth and contains the air we
breathe.
condensed Cooled and changed from water vapor to liquid water.
density The measure of how much mass an object has in a given volume.
molecules The smallest possible amounts of a chemical substance.
radiation Energy given off by the Sun.
species A group of living things that have similar characteristics.
water Vapor water in a gas form.
Points to Consider
- How did life on Earth develop from simple bacteria to more complex organisms?
- When did complex organisms like fish, reptiles, and mammals appear on Earth?
- When did the major features of the Earth that we know today first form?
12.3 History of Earth’s Life Forms
Lesson Objectives
- Describe how adaptations develop.
- Explain how the fossil record shows us that species evolve over time.
- Describe the general development of Earth’s life forms over the last 540 million years.