Biomarkers (molecular fossils) as geochemical indicators of life Original Research Article

Biomarkers (also termed chemical fossils, molecular markers, indicators or organic tracers) are specific organic compounds which are useful in assessing the genetic sources of lipidic or bituminous organic matter. Biomarkers in geological samples on Earth are products derived from biochemical (natural product) precursors by mainly reductive but also oxidative alteration processes (e.g., cholestanes from cholesterol). They have chemical structures which can be related back to their precursor compounds in contemporary or extinct biota. Generally, lipids, pigments and biomembranes are preserved best over longer geological times. Thus the detailed characterization of biomarker structures and mixture compositions permits the assessment of the major contributing source species of extinct and/or extant life. Biomarkers cannot be synthesized by abiological processes and each precursor generally is altered by diagenetic and catagenetic processes to a sequential series of derivative products. The utility of biomarkers as indicators of biogenic, paleoenvironmental and geochemical processes on Earth has been widely accepted. Here I redefine and discuss the application of this concept to extraterrestrial exploration for evidence of life or prebiotic organic synthesis. Nonbiomarkers are also considered in this context.