Earth's atmosphere is composed primarily of nitrogen (at 77%), 21% oxygen and much smaller amounts of argon, carbon dioxide and water. The biological processes constantly replenish oxygen into the atmosphere as others consume it. When the Earth was first formed, there was probably a lot of carbon dioxide, but most of went into carbonate rocks and oceans. Now the small amount of carbon dioxide contributes to the regulation of temperature on Earth.
The relatively large amount of oxygen on Earth is remarkable due to the fact that other planets have little or almost no free oxygen. The ecological cycle between plants and animals, living things in general is what makes this possible.
Composition of the Atmosphere 
