Group Report: Long-term Geosphere-Biosphere Coevolution and Astrobiology

This discussion group attempted a qualitatively new synthesis of long-term geosphere—biosphere coevolution, with the aim of understanding and presenting to the other groups the broadest possible context in which to consider Earth system analysis for sustainability. This included the prospects for detecting life and intelligence elsewhere in the Universe, as debated by astrobiology. The chemoton model of life comprising three autocatalytic subsystems (boundary, metabolic, genetic) was adopted. The topology of evolution was characterized as a network in the prokaryote realm and as a tree (or bush) in the eukaryote realm. It was agreed that prokaryotic life is common in the Universe but that eukaryotic life is rare and intelligent life is extremely rare. The appearance of intelligent life on a planet might theoretically involve four or five difficult evolutionary transitions along the way. These probably include the origins of the genetic code, of oxygenic photosynthesis, of eukaryotes, and of language. Optimistic and pessimistic scenarios for the long-term coevolution of the geosphere—biosphere were contrasted. A key finding was that dating of the major transitions in evolution and, to a lesser extent, dating of the major transitions in the state of the environment are subject to large error bars that need to be reduced in order to address the causal relationships of coevolution. A major output was a visualization of a time line of coevolution that includes these uncertainties. New suggestions of coevolutionary connections were also made. The feasibility of unequivocal life detection on extrasolar planets was questioned, but it was recognized that astrobiology is already encouraging a useful broadening of Earth system analysis. The failure of the search for extraterrestrial intelligence (SETI) and the apparent difficulty of the transition to natural language support the view that intelligence (or at least natural language) is extremely rare in the Universe. Habitation was defined as a first-order influence of life on the geochemical cycling of a planet, and it may be important for the maintenance of habitability. Theoretical considerations as well as Earth history suggests that there are limits (albeit rather broad ones) on how globally destructive life can become. A proposal was made to extend an existing model of global coevolution to address this and other Gaia questions. An “autocatalytic Gaia” hypothesis was put forward to suggest that autocatalytic recycling is an almost inevitable planetary phenomenon, once there is life. This is a natural extension of the autocatalytic theory of life (the chemoton model). Some broad lessons of sustainability can be learned from Gaia and the unfolding coevolution of life and its environment on Earth, in particular, the importance of avoiding long time lags for maintaining system stability.