I was listening to Science Friday this PM and heard the oldest fossil as yet found on Earth was discovered on the Gunflint trail in MN. Over 2 Billion years old, it was a sulfate reducing bacteria; the same general type that produces the "rotten egg" smell in rotten eggs, or makes sewer gas, or flatulence.

I know Posthole goes up that way from time to time, but I don't think he left it; when this particular bacterium was alive the atmosphere was mostly CO2, CH$ and H2S. Oxygen would tend to thin out these critters, but plants were just begining to use photosynthesis.