Somehow i have a bad feeling about going back into Cambrian era:
http://en.wikipedia.org/wiki/File:Phanerozoic_Carbon_Dioxide.png
Probably a lot of factors were different back then, like the sun being younger and smaller
More recent data shows that we are in a peek of co2:
http://en.wikipedia.org/wiki/File:Carbon_Dioxide_400kyr.png
