https://wikispaces.psu.edu/display/110Master/Prokaryotes+III+-+Evolution+and+Early+Metabolism
"It may be hard to envision life without oxygen, but the first prokaryotes (the first creatures on Earth) evolved in an environment with essentially no molecular oxygen (oxygen gas, or O2). Consequently, the first prokaryotes, which originated between 3.5 and 4 billion years ago, were
anaerobic. Anaerobic is a general term that refers to any organism, environment, or cellular process that lacks or does not require oxygen, and can even be poisoned by oxygen.
The original anaerobic organisms thrived in an environment devoid of oxygen. Many scientists think that the early prokaryotes were chemoautotrophs, obtaining energy from inorganic chemicals, possibly using the then-abundant hydrogen sulfide (H2S)."