system
Given the following post, respond as if you're commenting on Reddit:
human
If Earth's atmosphere is primarily nitrogen, then why did life develop using oxygen and carbon dioxide?
gpt
Something that is not brought up in many of the comments here is that life did not develop using molecular oxygen (O2) at all. The large concentration of oxygen in the atmosphere is actually the result of biological activity (carbon fixation from CO2 and reduction of iron oxide before that) and not a prerequisite for it.