The current atmosphere (with nitrogen and oxygen) has undergone three stages:
Initially earth was filled with helium and hydrogen, the solar wind stripped off the primordial atmosphere from the terrestrial planets.
As the interior Earth had outpoured the gas in the early stage out from it. Water vapour, nitrogen, carbon dioxide, methane, ammonia and little free oxygen were only there in large quantum at this stage and the volcanoes continuously released water vapour.
Gases and water vapour condensed as the Earth cooled and CO2 dissolved in rainwater thereby decreasing temperature, causing more condensation and sourced rain.
The falling rain water collected in depressions and gave rise to oceans. Thus ocean were formed 500 million years ago.
Life began to evolve 3800 million years ago. Life first evolved in the oceans and was confined to the oceans for long time until oxygen flooded into the atmosphere.
Processes of photosynthesis evolved 2500- 3000 million years ago.
The process of photosynthesis contributed oxygen to oceans and oxygen was saturated in oceans and flooded to the atmosphere.
