Quantifying the origins of life on a planetary scale

Abstract

A simple, heuristic formula with parallels to the Drake Equation is introduced to help focus discussion on open questions for the origins of life in a planetary context. This approach indicates a number of areas where quantitative progress can be made on parameter estimation for determining origins of life probabilities, based on constraints from Bayesian approaches. We discuss a variety of "microscale" factors and their role in determining "macroscale" abiogenesis probabilities on suitable planets. We also propose that impact ejecta exchange between planets with parallel chemistries and chemical evolution could in principle amplify the development of molecular complexity and abiogenesis probabilities. This amplification could be very significant, and both bias our conclusions about abiogenesis probabilities based on the Earth and provide a major source of variance in the probability of life arising in planetary systems. We use our heuristic formula to suggest a number of observational routes for improving constraints on origins of life probabilities. T he question of whether or not life exists beyond the confines of the Earth environment is intimately related to the question of life's origins on Earth or anywhere else. For example, if just one instance of life with an independent origin is detected-whether in the solar system or on a suitable exoplanet-estimates of the Bayesian probability for life across the universe will be significantly improved. Specifically, the rate of abiogenesis per young planet could, in principle, be constrained to be at least one event per Gyr (1). Equally, if a detailed mechanism for the terrestrial origins of life (OoL) were identified and tested, a complementary estimate of the rate of planetary abiogenesis should be possible. Furthermore, the architecture of a given solar system might also lead to widely different probabilities (e.g., the proximity of Mars and Earth and their potential exchange of material might result in a fundamentally larger search space in which OoL events could occur). Discovering complex organic molecules on Mars could help elucidate this possibility (2).At present, OoL science involves a wide range of approaches, theories, and opinions. A considerable body of work now exists on hypothetical early terrestrial life and its precursor chemistry including the RNA-world hypothesis, prior polymerization chemistry, autocatalytic processes, and metabolic origins, as well as the presumed transition to fully cellular organisms. Significant work has also been undertaken on inorganic "template" chemistry, the planetary evolution of suitable material precursors, and important substrates for the emergence of a biology-including the study of electrochemical gradients in hydrothermal vents systems and processes such as serpentinization as energy sources. In addition, the genomic exploration of the modern Earth's microbial life is providing ever-deeper insight into the nature and evolution of the biosphere's nested metabolic processes (3), which may provide clues to the ori...

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