For example, iron is oxidized to form iron oxide or rust. The presence of oxygen in a hypothetical primordial atmosphere poses a difficult problem for notions of self-assembling molecules. If oxygen is present, there would be no amino acids, sugars, purines, etc. Because it is impossible for life to evolve with oxygen, evolutionists theorize an early atmosphere without oxygen. In this case the present is NOT the key to the past.
Newer schemes exclude ammonia and methane.
Without this layer, organic molecules would be broken down and life would soon be eliminated. But if you have oxygen, it prevents life from starting. In must be noted at this point that the existence of a reducing atmosphere is theoretical and does not rely on physical evidence. To the contrary, there are geological evidences for the existence of an oxidizing atmosphere as far back as can be determined.