"Despite comparatively intense bombardment by large impactors, chemical and radio-isotopic trace evidence of what appears to be biologically processed carbon in Earth's oldest surviving rocks – from western Greenland's Isua greenstone belt that are as old as 3.85 billion years – suggest that self-replicating, carbon-based microbial life became well developed during Earth's first billion years of existence"

( http://www.solstation.com/life.htm )

biologically processed carbon -