The air that makes up Earth's atmosphere contains 20.9 percent oxygen. This is true up to about 70,000 feet above sea level. However, above 10,000 feet, the atmosphere becomes too thin to support human life, even though its proportions do not change much.
When the Earth was formed, and for the 2 billion years following, there was very little oxygen in the atmosphere, rendering the planet inhospitable to animal life as we know it. Around 600 million years ago, a sharp increase in the oxygen content of the atmosphere occurred, allowing a wider variety of life to develop and evolve.