Oxygen is one of the primary components that allows air to be breathable to people and it began forming in the atmosphere approximately one billion years ago. Until the ozone layer formed, most life on the planet was restricted to the oceans.
The Earth's atmosphere formed in three stages:
- Hydrogen and helium developed along with the planet
- Ammonia, carbon dioxide, and steam released from volcanoes as the Earth's crust hardened
- Carbon dioxide dissolved into the oceans
As the planet formed during the first 500 million years of its existence, the air was a combination of vapor and gases that escaped from the Earth's interior. The hydrosphere formed about four billion years ago and through a process much like condensation created the first oceans.
Blue-green algae appeared one billion years ago and were the first sea organisms. They fed off the carbon dioxide in the oceans and excreted oxygen as a waste product. The oxygen eventually accumulated in the atmosphere, where it began to interact with the ultraviolet rays of the sun. The interaction formed ozone molecules, which serve to absorb UV rays and partially block them from reaching the ground. The ozone layer is thought to have reached a point at which it can block biologically lethal forms of UV radiation by 600 million years ago. As the ozone layer formed, oxygen levels increased to 10 percent, which is half the concentration found in the air today.