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The emergence of life is probable on any wet, rocky planet. Serpentinization gives rise to alkaline hydrothermal vents that form: (i) simple organics; (ii) catalysts that direct primordial metabolism (iii) micropores with cell-like properties; and (iv) proton gradients equivalent to the proton-motive force. Thermodynamic constraints dictate that all anaerobic chemolithotrophic cells must depend on chemiosmotic coupling, explaining the near-universal use of proton gradients today. But proton gradients also limit the evolutionary potential of prokaryotes. Only a rare and stochastic event, an endosymbiosis between prokaryotes, permitted the evolution of morphologically complex life on Earth, as only such an endosymbiosis made it possible for chemiosmotic coupling to be controlled by multiple small genome outposts across a wide area of internal membranes. This leap in bioenergetic capacity in turn enabled the expansion in cell volume and genome size characteristic of eukaryotes. The origin of life and evolution of prokaryotes is therefore deterministic and probable (necessity), while the evolution of more complex eukaryotic life is stochastic and improbable (chance). These bioenergetic principles are likely to apply throughout the universe.