By 3.0 billion CO 2, H 2 O, N 2 dominant. O 2 begins to accumulate. Continued release of gases from the lithosphere. Water vapor clouds common in the lower atmosphere. Chemosynthetic bacteria appear on the Earth at 3.6 billion.
Life begins to modify the atmosphere.
500 million years ago concentration of atmospheric oxygen levels off. Humans begin modifying the concentrations of some gases in the atmosphere beginning around the year 1700. As the Earth continued to cool, the water vapor found in the atmosphere condensed to form the oceans and other fresh water bodies on the continents. The emergence of living organisms was extremely important in the creation of atmospheric oxygen and ozone . Without ozone, life could not exist on land because of harmful ultraviolet radiation .