When water, CO2 methane and ammonia are combined and electric spark is added a primordial soup of amino acids soon forms. Chemical Apparatus For Synthesizing Biochemical Molecules - This chemical apparatus is designed to synthesize complex biochemical molecules by energizing a mixture of simple chemicals. In the diagrammed procedure, the gases are placed in the upper bulb to simulate the primordial Earth atmosphere and then energized by spark-discharge electrodes. After about a week of recycling the gases through the lightning like electrical discharge, amino acids and other complex molecules can be found in the trap at the bottom, which simulates the primordial oceans into which heavy molecules would have fallen from the atmosphere. Oxygen atmosphere gets added later.
Life creates the present atmosphere.
Without life the oxygen rich atmosphere we have would go away via chemical reactions. A fossilized organism found in sediments radioactively dated as 2 billion years old. This primitive system possesses concentric spheres or walls connected by smaller spheroids. The roundish fossils here measure about a thousandth of a centimeter. So Perhaps Life is not so Uncommon Mars rock.