Rock record, survival

Going back before 2.8 billion years ago and back to the end of a good sedimentary rock record 3.5 billion years ago, there were only trace amounts of O2 in the water and there may have been significant amounts of methane in the air. (Trace O2 does not persist in air with methane because of photolysis [34].) Macroscopic evidence includes mobile iron in paleosols and survival of easily oxidizable detrital minerals like pyrite in sediments. 

Efforts to evaluate ancient records prompts an exploration of the factors that influence the sources of organic matter and their alteration during and after deposition (9). Toward this objective, studies of contemporary depositional settings can provide evidence of the principal influences on the production and preservation of organic matter in the rock record. Most importantly, a detailed perspective of contem rary processes permits consideration of the varied controls that determine the survival of both total organic matter and specific organic compounds like isopentenoids. 

Any search for evolution trends in natural products is constrained by the paucity and incompleteness of the ancient rock record, coupled with the fact that segments contain a biased molecular record. All of the process affecting the preservation of organic matter serve to favor the survival of certain components more than others. Specifically, it is in the oxygen deficient environment of sediments that the best prospects for molecular preservation occur, and components derived from aquatic organisms are more likely to survive the process of setding to reach the sediments than those from terrestrial sources... 

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