The Earth has a timeline of it's life. In a sense, we are looking at the Earth's scars to piece together the Earth's story. 

People have searched for clues looking for major events in the Earth's lifetime. Scientists use evidence to determine these major events. 

We place these events on a timeline called the Geologic Time Scale:

Image from WikiMedia Commons

You will see Geologic Time Scales in many different forms through this unit.