Chapter 12

Earth’s History

12.1 Geologic Time Scale

In this Earth’s history chapter, you will learn about some of the ways that scientists study
the history of Earth and how they use clues from rocks and fossils to piece together pictures
of how the Earth has changed over billions of years. You will learn how the Earth formed
and how life gradually developed on Earth. You will also gain an appreciation for how life
changes on Earth and how living things respond to changes in their environments.

Lesson Objectives

• Discuss how scientists know that the Earth is billions of years old.


• Describe how Earth’s history can be represented by the geologic time scale.

Introduction

How many years is a “long time?” We often express time in hours or days, and 10 or 20
years certainly feels like a long time. Imagine if you needed to think about one million, 100
million, or even several billion years. These exceptional lengths of time seem unbelievable,
but they are exactly the spans of times that scientists use to describe the Earth.

The Earth is 4 1/2 billion years old. That’s 4,500,000,000 years! Have places like the Grand
Canyon and the Mississippi River been around for all of those years, or were they formed
more recently? When did the giant Rocky Mountains form and when did dinosaurs walk
the Earth? To answer these questions, you have to think about times that were millions or
billions of years ago.

Historical geologists are scientists who study the Earth’s past. They study clues left on the