The atmosphere on early Earth was strikingly different from that of today. Primitive Earth's atmosphere consisted of methane, ammonia, hydrogen, water vapor, and only a negligible amount of free oxygen.
This is a very important point because with a lot of free oxygen available, life probably could not have arisen from inorganic compounds. This is because oxygen tends to oxidize substances, which means that electrons are removed. The early atmosphere was actually highly reductive (capable of gaining electrons and forming more complex molecules).