AN OCEAN OF AIR
Above the Earth's surface lies the atmosphere, an ocean of air that sustains life, drives our weather, and aids the breakdown of rocks into their constituent minerals.
The composition of the atmosphere is constantly evolving. If you went back three billion years in time, you would be able to breathe. At that time, the atmosphere contained almost no oxygen- it was mostly carbon dioxide and nitrogen. Today, Earth's air is mostly oxygen and nitrogen.
Oxigen began to accumulate in the atmosphere only when primitive life forms (first bacteria and then plants) developed photosynthesis, excreting oxygen as a waste product of the process. Not only does oxygen let us breathe, it also reacts with sunlight to form ozone, which protects us from harmful ultraviolet rays.
Carbon dioxide acts as a "greenhouse gas" , allowing sunlight to reach the surface, but preventing heat from escaping. In this way, it helps warm our atmosphere, just as an abundance of it turned Venus into an inferno. Today, the burning of fossil fuels is releasing carbon dioxide into the air, subtly changing the constitution of the atmosphere and contributing to a global increase in temperatures.