Life arose on Earth sometime in the first few hundred million years after the young planet had cooled to the point that it could support water-based organisms on its surface. The early emergence of life on Earth has been taken as evidence that the probability of abiogenesis is high, if starting from young-Earth-like conditions. We revisit this argument quantitatively in a Bayesian statistical framework. By constructing a simple model of the probability of abiogenesis, we calculate a Bayesian estimate of its posterior probability, given the data that life emerged fairly early in Earth's history and that, billions of years later, sentient creatures noted this fact and considered its implications. We find that, given only this very limited empirical information, the choice of Bayesian prior for the abiogenesis probability parameter has a dominant influence on the computed posterior probability. Thus, although life began on this planet fairly soon after the Earth became habitable, this fact is consistent with an arbitrarily low intrinsic probability of abiogenesis for plausible uninformative priors, and therefore with life being arbitrarily rare in the Universe.
Their work undermines what was formerly thought to be one of the more reliable assumptions in the Drake equation, namely, that the probability of life emerging on a planet in a habitable zone is relatively high. According to Spiegel and Turner, that assumption is biased by the fact that we are here: it has taken about 3.5 (American) billion years for intelligent life to emerge on earth, and that could only have happened if life emerged quite quickly. But the point of their Bayesian analysis is to remind us that the probability of life emerging at all is independent of the fact that it emerged quickly in this instance. In other words, the fact that life emerged quickly on earth tells us nothing about the probability of life emerging in general.