"....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. Some single-celled microbial life lacking a nucleus that segregates their internal DNA or RNA ("prokaryotes") from the surrounding cytoplasm may have flourished in darkness within cracks in Earth's seafloor crust and around deep, boiling hot ocean springs (known as volcanic vents or "black smokers") without a need for light or free oxygen in the oceans or atmosphere. Adapted to their very hot but watery enviroment, these microbes metabolized hydrogen-rich compounds or dead or live organic materials to derive the energy that sustains anaerobic life, including sulfate-reducing bacteria that produce Hydrogen Sulfide (H2S), fermentative bacteria that produce carbon dioxide and alcohol (-OH), and methanogenic bacteria -- the methanogens found in sewage and mudflats today -- that produce methane (CH4) gas as a waste product."
http://www.solstation.com/life.htm