Oxygen is a highly reactive element, and the only way that free oxygen can be a significant part of a planet's atmosphere is if there is some ongoing mechanism to pump it into the atmosphere faster than it can combine with other substances. Free oxygen was not part of the earth's atmosphere until life developed photosynthesis, and this remains the sole source of the earth's atmospheric oxygen.

The earth's secondary atmosphere was capable of supporting life even in the absence of oxygen, and life can develop under a range of atmospheric compositions. The critical aspect about the earth's atmosphere is that it formed at liquid-water temperatures. Venus is closer to the sun than the earth, and its original water was driven into space by solar radiation. Mars is further from the sun, and its water is locked up in the form of ice. On Earth, the presence of large quantities of water further alters the atmosphere by reacting with carbon dioxide to form solid carbonates. This removes carbon dioxide from the atmosphere, which is why the atmospheres of Venus and Mars are mostly carbon dioxide while the earth's is just under 0.04% CO2.
Source(s):
http://www.mahalo.com/answers/science-and-mathematics/if-the-earth-is-4-5-b...

http://zebu.uoregon.edu/internet/l2.html
http://zebu.uoregon.edu/internet/images/atmoss.gif
http://biology.clc.uc.edu/Courses/bio303/bigbang.htm

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