The Abiogenesis Timescale

If two physical timescales are independent, i.e. they depend on different physics, then (statistically) there is no reason to believe that their values should be equal, even to an order of magnitude. The timescale for abiogenesis $\tau_{AB}$, which depends primarily on prebiotic-chemistry, is expected to be independent of the planetary habitability timescale $\tau_{Hab}$, which depends primarily on the sun and therefore on nuclear forces and gravity. Therefore, we expect that either $\tau_{AB} \ll \tau_{Hab}$ or $\tau_{AB} \gg \tau_{Hab}$. The correct inequality is universal and a single example should suffice to resolve this binary choice. Here I argue that, contrary to a well known anthropic selection effect, our existence (which entails life on Earth) can be considered evidence that the correct choice is the former. A Bayesian analysis, taking into account that our existence is old evidence, implies that the probability of the hypothesis $\tau_{AB} \ll \tau_{Hab}$ is > 0.91, assuming equal priors. The Bayes factor, which depends only on the evidence, is > 10 and suggests strong to decisive support for the short abiogenesis timescale hypothesis, according to the Jeffreys interpretation.