http://io9.com/5874963/meet-the-molecule-responsible-for-giving-earth-all-of-its-oxygen
About 2.5 billion years ago, our planet had virtually no oxygen, and lifeforms were primitive. Then, oxygen levels suddenly spiked, the entire landscape of the planet changed, and we were on our way to complex life. Now, at last, we know why.
Earth probably wouldn't have gotten much past simple multi-cellular organisms without the Great Oxidation Event, let alone give rise to intelligent life. Aerobic organisms are able to harness far more energy than their anaerobic counterparts, and that means much more complex lifeforms can evolve than would otherwise be possible.
But there's a mystery here. Before the Great Oxidation Event 2.4 billion years ago, all organisms were naturally anaerobic. That doesn't just mean they couldn't use oxygen — the gas was actually toxic to them. And yet the only way to generate oxygen on a planetary scale was for organisms to release it as part of photosynthesis. That means anaerobic organisms effectively committed mass suicide to pave the way for their aerobic successors. Indeed, scientists suspect the Great Oxidation Event kicked off the first and most massive extinction event in our planet's history.
And yet all that is exactly what cyanobacteria apparently did do, starting around 2.8 billion years ago. For the first 400 million years, all the oxygen they produced was captured by a mix of organic matter and dissolved iron. But at 2.4 billion years ago, these materials could no longer absorb oxygen, and so oxygen started accumulating in the atmosphere. The oxygen revolution had begun, and cyanobacteria had just signed their own death warrant.
Rather surprisingly, oxygen is actually the original toxic waste.
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