how did the atmosphere evolve to the chemical compositionthat is it today?

Question

natural sources and chemicals

Answer 1

I just answered that question, cept it was for water.

History of Earth
http://www.mansfield.ohio-state.edu/~sabedon/biol1010.htm

Formation of the Earth's Atmosphere and Oceans
http://zebu.uoregon.edu/internet/l2.html

Answer 2

Most of the credit goes to carbon dioxide eating plants which take the carbon out of the air and give off the oxygen (the dioxide part of carbon dioxide).