A GEOBIOLOGICAL PERSPECTIVE ON BIOSPHERIC EVOLUTION

Earth scientists scarcely need to be reminded that life is very old and that it has been predominantly microbial through most of its history. We also understand that the long Precambrian interval of microbial hegemony was, as well, the time during which environments inhabitable by plants and animals took shape. A principal question that needs to be addressed in partnership with microbiologists is how metabolic and physical processes interacted to determine environmental evolution on the early Earth. Microbial phylogeny provides a genealogical template for interpreting microbial evolution, and geobiological observations of Precambrian sedimentary rocks constrain the timing of events inferred from phylogeny. When the resulting timetable of evolutionary innovation is compared with the history of environmental state change reconstructed from geobiological and other geochemical data, it becomes apparent that evolution alone cannot account for Earth's long-term environmental trajectory. Evolution defines the dimensions of biological participation in the Earth system, but it is the incompletely understood interactions of biology with tectonics, oceanic circulation, and climate that govern environmental history. Viewed from this perspective, geobiology emerges as the key link between biology and the physical Earth sciences in accelerating research on Earth system history.