EXTREME LIFE, PRECAMBRIAN PALEONTOLOGY AND THE ASTROBIOLOGICAL EXPLORATION OF THE SOLAR SYSTEM

Advances in molecular biology have radically altered our view of the biosphere by dramatically expanding the environmental limits of life on Earth and the role that microbiological processes have played in shaping geological history. In parallel, our knowledge of the fossil record of extreme environments has also grown, providing broader access to the early (Precambrian) history of terrestrial life. These and related discoveries have had an important impact on how we view the potential for life's origin and persistence elsewhere in the Solar System. Especially important is the discovery that microbial life is common on Earth in the deep subsurface, where interactions between water and rock yield energy for metabolism. The discovery of a subsurface biosphere significantly improves the possibility that life may have originated on other planets, like Mars, where surface conditions are presently unfavorable for life, but where subsurface habitats may have been present for most of the planet's history. In a similar vein, exploration of the outer Solar System has also revealed the likely existence of subsurface habitable zones of liquid water- saline brines (perhaps oceans), beneath icy crusts of Europa, Callisto and Ganymede, along with plausible energy sources from the chemical disequilibria between oxidized and reduced compounds. Opportunities to systematically explore these and other potentially habitable environments in the Solar System have helped catalyze the development of Astrobiology, an emerging interdisciplinary science that seeks to understand the origin, evolution, distribution and future of life in the Cosmos. Studies of the Precambrian fossil record are a central element of Astrobiology and provide a test bed for the development of the basic conceptual and technological tools needed to explore for a fossil record of microbial life on other worlds. In this talk, I will review work on the nature of the microbial fossil record in some extreme terrestrial environments, with strategic implications regarding how to explore for fossil biosignatures elsewhere in our Solar System.