how is the air we breath today different from 3.6 billion years ago?
http://en.wikipedia.org/wiki/Earth's_atmosphere#Evolution_of_the_Earth.27s_atmosphere
http://eesc.columbia.edu/courses/ees/climate/lectures/earth.html
http://en.wikipedia.org/wiki/Earth's_atmosphere#Evolution_of_the_Earth.27s_atmosphere
http://eesc.columbia.edu/courses/ees/climate/lectures/earth.html