>>931392Humans need a partial pressure of 0.16 bar (or 16% O2 in the atmosphere at sea level) to function as we should. For comparison, the partial pressure of O2 on top of Mt Everest is around 0.075 bar (comparable to 7.5% O2 at sea level pressure), so we can survive well below that, or at least some of us can...
Oxgen reached 15% in Earth's atmosphere about 0.4 Bya, so we would be okay at that time, 0.6 Bya it was only 10%, so we would feel similar as being on top of Mt Everest at that time.
In terms of other gases I am not sure, concerning CO2 I'd say that would not have been a problem because most of the O2 in the air was produced by photosynthetic organisms using CO2, so when the O2 levels were high enough to breathe, the CO2 levels were already down to a safe level.
The Cambrian period starts around 0.5 to 0.6 Bya, so thats the time we are getting close in terms of oxygen, so my answer is yes. :D (It would be hard but enough to survive)
