12.3). Huge mountains have formed, been destroyed, and replaced with new mountains. The
oceans have opened up and moved around the globe. The continents have moved around,
split apart from each other, and collided with each other, until finally reaching their present
locations. Life on Earth has also changed tremendously. At first, the Earth was not even able
to support life. There was no oxygen in the atmosphere, and Earth’s surface was extremely
hot. Slowly, over millions of years, the Earth changed so that plants and animals could begin
to grow. Living things then changed the Earth even more.
We often enjoy using our imagination to think about what the Earth was like when dinosaurs
roamed around (Figure12.4). What images come to your mind when you think about the
dinosaurs? Now imagine a time on Earth before even the dinosaurs. Imagine the time before
any living thing was on Earth. What images come to mind now? How do you think the
Earth looked when it was first formed? This lesson will help you understand how the Earth
formed, what it looked like during its earliest years, and how life first developed on Earth.
Figure 12.4: The Earth and its dominant life forms have changed throughout the Earth’s
long history. ( 18 )
Evaluating Prior Knowledge
The following questions are addressed in other chapters and will help you work through this
lesson. Research these before you move on.
What are chemical elements?
What conditions do plants and animals require to live?