Being the splendidly reactive stuff that it is, molecular oxygen doesn't survive very long in any atmosphere: generally there's always something that needs oxid

Being the splendidly reactive stuff that it is, molecular oxygen doesn't survive very long in any atmosphere: generally there's always something that needs oxidising. So it's thought to be a good indicator of life, only arising on Earth after photosynthesis began: https://en.m.wikipedia.org/wiki/Great_Oxidation_Event https://en.m.wikipedia.org/wiki/Great_Oxidation_Event