Monday, June 3, 2019 - 11:00
Event Host: 
Bob Hazen

Dr. Carol Cleland of the University of Colorado Boulder presented on "Searching for unfamiliar forms of life without a definition of life."

According to the 2015 Astrobiology Strategy, a central goal of astrobiology is to provide a definition of life. Similarly, the 2018 CRC Handbook of Astrobiology contains an extended discussion of definitions of life. Despite an impressively large number of candidates, however, there is currently no consensus on a definition of life. As I argue elsewhere, this is not surprising. The scientific project of defining life is fundamentally misguided, both logically and empirically. Astrobiologists thus seem to be faced with a dilemma: They cannot formulate a satisfactory definition of life but without such a definition they are unlikely to recognize truly novel forms of extraterrestrial life as cases of life. In this talk I proposes a strategy for circumventing this difficulty: Use tentative (vs. defining) criteria to search for biologically promising anomalies (vs. life per se). The function of tentative criteria is not to decide the question of life but instead to identify geochemical phenomena that resist classification as living or nonliving. Such phenomena are anomalous vis-à-vis our current Earth-centric concepts of life and nonlife. The strategy being recommended has the advantage over a definition-based approach in being much less likely to classify a novel form of life as nonliving. Instead, it will be classified as worthy of further investigation for the presence of biological processes. In addition, tentative criteria will do just as well as defining criteria at flagging extraterrestrial life that closely resemble familiar Earth life as living things. The proposed strategy resembles current life-detection mission approaches, which search for different lines of evidence (qua biosignatures) for life.  As will be discussed, however, the proposed strategy differs from them in a way that increases the likelihood of noticing those forms of extraterrestrial life that provide the greatest challenges to our current understanding of life.

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