The early Earth's atmosphere was mostly Co2 with virtuall no oxygen. A lot of the early carbon dioxide was absorbed into the oceans and locked up in sedimentary rocks, and fossil fuels. Green plants also steadily removed it and released oxygen. Nitrogen gas began to be produced by organisms such as denitrfying bacteria.

This build up of oxygen killed off early organisms that couldn't cope, but allowed the evolution of more complicated organisms. The oxygen also created the ozone layer ( O3). Present day composition -

78% Nitrogen.
1% Argon.
21% Oxygen.
0.04% Carbon dioxide.

Also - Varying amounts of water vapour, and other noble gases in small amouts.