For more than its first two billion years, Earth’s atmosphere held almost no free oxygen. Cyanobacteria had been releasing O₂ long before, but only around 2,400 million years ago did it build up enough to rise into the air, rusting the iron dissolved in the sea into banded formations and poisoning the anaerobes then in charge. From there the O₂ line climbs in steps toward today’s 21%, ice reaches near the equator around 700 million years ago, and five mass extinctions flash across the final stretch of 4.54 billion years.
For more than its first two billion years, Earth’s atmosphere held almost no free oxygen. Cyanobacteria had been releasing O₂ long before, but only around 2,400 million years ago did it build up enough to rise into the air, rusting the iron dissolved in the sea into banded formations and poisoning the anaerobes then in charge. From there the O₂ line climbs in steps toward today’s 21%, ice reaches near the equator around 700 million years ago, and five mass extinctions flash across the final stretch of 4.54 billion years.
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