Geological evidence indicates that about 3.6 billion years ago the earth's interior heated up and with the expulsion of volatile components, a primitive atmosphere was formed. The primitive atmosphere contained large quantities of ammonia and methane along with water vapor, carbon dioxide, and nitrogen. There was no free oxygen in the earth's primitive atmosphere. Approximately 2.0 billion years ago free oxygen appeared as an atmospheric element. The major source was the freeing of oxygen through plant photosynthesis.
Photosynthesis is a plant process that combines sunlight.
The four most abundant gases are nitrogen, oxygen, argon and carbon dioxide. The atmosphere contains a variable amount of water vapor. Water vapor, just as the name implies, is water in a gaseous state. The source of atmospheric moisture is through the processes of evaporation from water surfaces and transpiration by plant life. Suspended throughout the lower atmosphere are minute non-gaseous material collectively called dust.