Recognizing the geological signature of early life on Earth and Mars

The search for past or present life beyond Earth requires a solid understanding of life's origin and evolution on the only planet on which life is known to exist-the Earth. Hence, the search for evidence of life and environments on the early Earth, where life originated, evolved and later developed complexity are critical components in developing mission plans for Astro/Exobiology space missions. Instruments deployed on Mars or on returned samples to look for traces of past life should be tested on fine-grained terrestrial sedimentary rocks where biosignatures are known to be preserved. Our research is relevant to the search for life on other planets in two ways. Firstly, our ultimate goal is to understand the mechanisms and environmental context of biospheric evolution on the early Earth by identifying the fossils (be they morphological, ultrastructural, or chemical) of early prokaryotes and eukaryotes, determining their biological affinities, and examining their patterns of evolution through intervals of environmental change. This approach, in which sedimentary geology, optical and electronic microscopy, and microchemistry are used in combination, is applicable to Martian sediments. Indeed, terrestrial discoveries provide a comparative data base for evaluating extraterrestrial materials.