Redox proxies, such as the ratio of chromium isotopes in banded iron formations or the ratio of isotopes in sulfide particles trapped in diamonds, tell geologists indirectly whether the Earth' s atmosphere and oceans were reducing (inclined to give away electrons to other atoms) or oxidizing (inclined to glom onto them).

It makes all the difference: the bacterium that causes botulism and the methanogens that make swamp gas are anaerobes and thrive in reducing conditions. Badgers and butterflies, on the other hand, are aerobes, and require oxygen to keep going.