What is atmospheric air made of? Was he always like this?

The atmosphere is made up of different gases, and its density changes with altitude. Three main gases are now observed in the atmospheric air. This is, first of all, nitrogen, which accounts for three quarters of the total volume of air. Oxygen is second and carbon dioxide is third. However, the atmosphere did not always have this composition. Several hundred million years ago, there was almost no oxygen in the air, but there was much more carbon dioxide and there were bacteria and other animals that synthesized it.