Not counting temporary events, like mass extinctions.
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$\begingroup$ The Ozone Layer didn't develop until around 600 million years ago, so Even if Oxygen availability was present about a billion years ago during the Great Oxidation event. What good is a breathable atmosphere if you die of skin cancer $\endgroup$– LazyReaderOct 4, 2021 at 2:03
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2 Answers
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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 Jurassic 200 million years ago when levels were 60% of current concentrations.
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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.science.org/content/article/earth-s-ancient-atmosphere-was-half-thick-it-today-rev2
