The mixture of gases that is thought to have made up Earth's early atmosphere is hydrogen, water vapor, ammonia, methane, carbon dioxide. This is previous to life so oxygen was not present because it was produced when green plants existed to produce oxygen through photosynthesis. Ozone was absen and so the UV radiation entered the atmosphere and it is thought that (along with electrical discharges) could vave contributed to the formation of the first organic molecules from the given mixture of inorganic gases.
