Evolution has occurred over billions of years of Earth's history.

As you watch the seconds tick by on a pocket watch, it seems impossible to visualize the billions of years that the Earth has existed. Most of us never need to think beyond a few tens of years, so a history of billions of years is hard to imagine.

How old do you think the Earth is? (Click on the best answer.)

1. 1 billion years old

2. 2 billion years old

3. Over 4 billion years old