The evolution of the atmosphere started with the volcanoes, as they erupted they released green house gases which is Co2-carbon dioxide N2- nitrogen CH4- methane and H2O-water.
As the earth cooled down, the water which was a water vapour turned into a liquid and started turning into seas.
As the water has turned into seas H2O gets absorbed into the water, the ocean which absorbs carbon dioxide from the atmosphere. This is called carbon sink.
As the blue/green algae builds up, more oxygen is released and is out in the atmosphere and the amount of the carbon dioxide has decreased so less carbon dioxide is out in the atmosphere.
When the volcano erupts it releases ashes blocking out the U.V. rays from the sun. Due to this happening 02 is the formed into O3. This then forms the ozone layer.
When oxygen turns into ozone it allows animals and plants to then be able to go on land, without the ozone they wouldn't be able to go on land as they would just get fried. Today's atmosphere makes up of 78% nitrogen, 21% oxygen and the rest is carbon dioxide and water vapour.