It's generally thought that the evolution of complex life was a rare, once-in-4.5-billion-years event. But new research suggests that conditions were right for complex cells to evolve and die off at least once - or perhaps several times - before our lineage even got started. The reason? New evidence that there was enough oxygen in Earth's atmosphere between 2.4 and 2 billion years ago before it dropped off again suddenly. This suggests that the ingredients for complex life were present before the first fossil evidence of complex life. Jeff and Anthony discuss the uses for oxygen, and propose some reasons why it may have fluctuated.

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