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Earth’s atmosphere was made of gases like hydrogen, methane, ammonia, and water vapor from volcanic eruptions there was no oxygen.

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Volcanoes kept adding gases, but cyanobacteria (blue green algae) began using sunlight to make food and released oxygen into the water and air.

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Oxygen levels rose slowly. Iron in the ocean reacted with oxygen and formed rust on the seafloor, and the ozone layer began forming in the sky.

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Green plants became common on land and made even more oxygen. The ozone layer got stronger, which protected life from UV rays, helping animals move onto land.

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humans and animals depend on gases in the air.

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Oxygen levels stayed stable, supporting a variety of life. Humans and animals now depend on the balance of gases in the atmosphere, but pollution threatens this balance.