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niceatheist
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The early atmosphere was a reducing atmosphere, dominated by carbon dioxide and gases like hydrogen sulfide and methane. But the earliest life was anaerobic, so a lack of free oxygen wasn’t a problem. In fact the largest extinction event in the planet’s history occurred about 2.5 billion years ago when the cyanobacteria first appeared, and began photosynthezing and excreting O2. A great many anaerobic organisms couldn’t tolerate the growing O2 levels and died off. But we also know that sulfur metabolizing genes are among the oldest genes of life, indicating life’s earliest times were in an atmosphere quite different from the one that was in place by about two billion years ago.