For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today:
80% Nitrogen
20% Oxygen
Small proportions of various other gases, i.e. CO2 , H2O and noble gases
One theory suggests that during the first billion years of the Earths formation, there was intense volcanic activity that released gases that formed the early atmosphere.
Volcanoes also produced nitrogen (N2 ) which gradually built up in the atmosphere & there may have been small proportions of methane (CH4 ) and ammonia (NH3 )
Water vapour condensed to form the oceans , CO2 dissolved in the water and carbonates were precipitated producing sediments, reducing the amount of CO2 in the atmosphere.
Oxygen increased by Algae & plants producing the O2 that is now in the atmosphere by photosynthesis
Over the next billion years plants evolved and the % oxygen gradually increased to a level that enabled animals to evolv
Carbon Dioxide decreased by Algae and plants, decreasing the % CO2 in the atmosphere by photosynthesis.6CO2+6H2O−>C6H12O6+6O
