The atmosphere is the blanket of gases that surround a planet. Earth's atmosphere has evolved from being one filled with carbon dioxide and water vapor to one containing mainly nitrogen and oxygen.
In the very beginning of Earth's atmosphere, carbon dioxide and water vapor were the main constituents. As temperatures cooled, the water vapor began falling as rain. The carbon dioxide level in the atmosphere was also decreasing because it was being used by photosynthetic organisms to make glucose. These organisms released oxygen gas as a by-product. Part of this oxygen has been broken down and reformed as ozone, three atoms of oxygen bonded together. This layer of ozone protects life on Earth from much of the damaging rays of the sun.
Earth's atmosphere now contains 78 percent nitrogen and 21 percent oxygen. Only trace amounts of carbon dioxide and water vapor remain in the air.