Detecting Traces of Primitive Life on Earth (in Context of Solar System Exploration)
Dawn Y. Sumner (UC Davis)
The vast majority of ancient organisms leave no discernible evidence of their existence beyond a minute contribution to the average chemistry of their environment. Preservation of distinctive chemical or morphological signatures is unfortunately (for paleobiologists) a very rare occurrence. Thus, detecting the traces of early life on earth requires three approaches: 1) a deep understanding of how the chemistry of environments is influenced by both abiotic and biological processes; 2) a targeted search for distinctive chemical signatures of specific types of life where they can be well preserved; and 3) wise use of morphological signatures to identify and distinguish abiotic and biological structures. We use all three approaches to identify and understand the early evolution of life on earth, and these same three approaches are appropriate to use in searching for evidence of life elsewhere in the solar system.
Identifying traces primitive life is difficult on earth, and the challenges are somewhat different elsewhere. Earth is currently saturated with life, unlike any other body in the solar system, making distinguishing evidence of ancient versus younger ecosystems extremely important. Such a distinction is unnecessary on any other body where the detection of any trace of any life would be a momentous discovery. However, studies of the primitive terran biosphere demonstrate that traces of life are difficult to unambiguously identify even when life was abundant. These studies raise the caution that sparse life elsewhere may be exceedingly difficult to identify, especially in fossil form.