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850 million years ago and earlier oxygen levels were below 6%, which is the minimum for human survival. But at oxygen levels below 14% we start to be significantly impaired, and we have evolved with current levels of 20%. 14% was attained about 500 million years ago, but there have been significant low oxygen events since, notably at the start of the ...


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Roughly 3,000,000,000 years ago Earth was a water world. This means terrestrial flora and fauna was 0%. Not much fun breathing in the atmosphere with limited photosynthesis from plants. And a nitrogen cycle that’s unrecognizable. To be fair anything before the Cambrian explosion would have been a harsh, most likely inhospitable environment. https://www....


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