Rethinking the origin of life at seafloor hydrothermal vents

Alkaline vents have dominated a class of hypotheses for the origins of life since their discovery a quarter of century ago, but advances in understanding the nature of these systems at the time of life's origins demand renewed scrutiny. Alkaline vent hypotheses generally reason that strong proton gradients across sulfide mineral membranes at hydrothermal conditions would have driven chemiosmosis and energy harvesting by protocells in seafloor chimneys. While serpentinizing systems would have existed on the ancient seafloor, their different geology would have yielded shallower subseafloor hydrothermal circulation and shorter-lived and less focused venting. Further, because the pH of Lost City hydrothermal fluids, upon which the proton gradient concept is based, only becomes hyperalkaline after cooling, neither modern nor ancient serpentinizing systems are characterized by strong pH gradients at hydrothermal temperatures. Because the ancient oceans were sulfate- and sulfide-poor, and serpentinizing systems, including Lost City, contain little sulfur themselves, sulfide would have been virtually absent from ancient Lost City-type vents. Finally, investigations at Lost City and in the laboratory indicate the generation of complex hydrocarbons, required for protocell metabolism and membranes, would have been frustrated by reaction kinetics at relevant temperatures and timescales. Together, these considerations challenge currently formulated alkaline vent hypotheses for the origin of life. Because these hypotheses have driven speculation for life on ancient Mars, icy moons, and exoplanets, the evaluation presented here demands a reappraisal of the potential for life beyond Earth.