Earth is the only planet in the universe known to have life. Some sedimentary rocks are formed by the action of living things. This limestone is formed by microscopic marine organisms, that secrete protective shells of calcium carbonate. When the animals die, the shells remain and solidify into limestone.

Life affects Earth in many ways. Life has actually made the atmosphere around us. Plants take in water and carbon dioxide, both of which contain oxygen. They use the carbon in carbon dioxide and the hydrogen in water to make chemicals of many kinds and give off oxygen as a waste product. Animals eat plants to get energy and return water and carbon dioxide back into the environment.