Evolution of the atmosphere

The Earth's early atmosphere was mainly made up of carbon dioxide and water, released by volcanic activity.

As the Earth cooled, the water vapour condensed to form the oceans. 

Carbon dioxide was removed form the atmosphere by dissolving in the oceans and forming sedimentary rocks, and fossil fuels.

The carbon particulates in the water sank to the bottom, where over millions of years, sediment built up and compressed the carbon particulates underneath. Under extreme pressure and heat, fossil fuels and sedimentary rocks were formed.

As organisms began to develop, carbon dioxide was also removed form the atmosphere by photosynthesis, through which oxygen was also added to the atmosphere.