Earth's oxygen history
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After most of our planet formed about 4 to 4.5 thousand million years ago, Earth was continuously struck by comets and asteroids for hundreds of millions of years. After this incessant rain of impacting objects stopped, life appears to have established itself comparatively quickly, perhaps within 10 to 100 million years. As soon as life began on Earth, it started to disturb the atmosphere's chemical equilibrium.
Oxygen built up continuously in the atmosphere over the next three thousand million years because it was a waste product expelled by some single-celled organisms. Then, around 600 million years ago, multicellular creatures developed and the oxygen content of the atmosphere rose suddenly. This trend was assisted by plants, which established themselves on the continents around 450 million years ago. Concurrently, evolution produced animals that breathed oxygen, instead of excreting it as a waste product. Without life, the oxygen content of the atmosphere would have sunk to undetectable levels.