The Great Oxidation Event
The air fills with oxygen — and almost everything dies
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For hundreds of millions of years the oxygen was mopped up by dissolved iron in the ocean — the banded iron formations we mine today are the residue. Once the iron was exhausted, atmospheric oxygen rose sharply. The result was probably Earth's largest mass extinction measured as a fraction of life lost. The oxygen also destroyed the greenhouse gas methane and helped trigger the long Huronian glaciation. Aerobic respiration, made possible afterwards, yields roughly sixteen times more energy per sugar molecule than fermentation.
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Bacteria had been breathing out oxygen for hundreds of millions of years, and eventually the air filled with it. For most creatures of the time oxygen was pure poison, and vast numbers died out. But the survivors gained a powerful new way of getting energy.
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Part of Terra Saga — the story of Earth on a timeline. terrasaga.org