BIOMARKERS AS TRACERS FOR LIFE ON EARLY EARTH AND MARS

Biomarkers in geological samples are products derived from biochemical (natural product) precursors by reductive and oxidative processes (e. g., cholestanes from cholesterol). Generally, lipids, pigments and bio membranes are preserved best over longer geological times and labile c ompounds such as amino acids, sugars, etc. are useful biomarkers for r ecent times. Thus, the detailed characterization of biomarker composit ions permits the assessment of the major contributing species of extin ct and/or extant life. In the case of the early Earth, work has progre ssed to elucidate molecular structure and carbon isotopic signals pres erved in ancient sedimentary rocks. In addition, the combination of ba cterial biochemistry with the organic geochemistry of contemporary and ancient hydrothermal ecosystems permits the modeling of the nature, b ehavior and preservation potential of primitive microbial communities. This approach uses combined molecular and isotopic analyses to charac terize lipids produced by cultured bacteria (representative of ancient strains) and to test a variety of culture conditions which affect the ir biosynthesis. On considering Mars, the biomarkers from lipids and b iopolymers would be expected to be preserved best if life flourished t here during its early history (3.5-4 x 10(9) yr ago). Both oxidized an d reduced products would be expected. This is based on the inferred oc currence of hydrothermal activity during that time with the concomitan t preservation of biochemically-derived organic matter. Both known bio markers (i.e., as elucidated for early terrestrial samples and for pri mitive terrestrial microbiota) and novel, potentially unknown compound s should be characterized.