Everyone of us knows that there was an Intense Volcanic activity for the first billion year of Earth. Due to the circumstances, the amount of gases in our atmosphere was very unsupportable for existence of life(Very similar like the atmosphere of Mars & Venus of today).
Amount of Carbon dioxide was most abundant whereas oxygen was very little. Then, the changes which affect the proportion of gases were:
1) For Increasing Oxygen. - The only and most important source of increasing oxygen was the "Photosynthesis by Plants".
2) For decreasing Carbon dioxide. - (i) Absorbed by plants in order to do the process of photosynthesis.
(ii) Most of the CO2 dissolved in Oceans and many locked in <span>sedimentary rocks, such as limestone, and in fossil fuels.
In that way, Our Atmosphere evolved,and now we're able to survive (If we care about our environment)
