Earths biosphere was originally anerobic,, the evolution of photosynthesis pretty much wiped out all life that existed at the time (though it was all bacteria at that point).
adjective
Living or occurring in the absence of free oxygen.
"anaerobic bacteria."
Of or relating to anaerobes.
Not requiring air or oxygen for life; -- applied especially to those microbes to which free oxygen is unnecessary; anaërobiotic; -- opposed to aërobic