Humans have always depended on energy to survive.

Early people relied on the sun's heat energy for warmth and to grow plants for food and fuel. They relied on their own muscle power and that of their animals to travel and move objects. Their use of animals, water, wind, and wood had only a slight impact on the environment because the earth's population was so small and the energy resources they used were renewable.

Today, we rely on other forms of energy (see next page) and the world has more than six billion people. Can you imagine how much energy 6,000,000,000 people use and the impact this has on the environment?

Here is a time line that shows important milestones in human energy use. Choose three widely spaced time periods. For each time period, answer two or more of these questions:

• How did people cook their food?
• How did they keep warm?
• How did they travel?
• How did they entertain themselves?
• How did their energy use affect the environment?