What can rapid terrestrial biogenesis tell us about life in the universe ?

It is sometimes asserted that the rapidity of biogenesis on Earth suggests that life is common in the Universe.We critically examine the assumptions inherent in this argument. Using a lottery model for biogenesis in the Universe, we convert the observational constraints on the rapidity of biogenesis on Earth into constraints on the probability of biogenesis on other terrestrial planets. For example, if terrestrial biogenesis took less than 200 Myr (and we assume that it could have taken 1 billion years) then we find the probability of biogenesis on terrestrial planets older than ∼ 1 Gyr, is > 36% at the 95% confidence level. However, there are assumptions and selection effects that complicate this result: although we correct the analysis for the fact that biogenesis is a prerequisite for our existence, our result depends on the plausible assumption that rapid biogenesis is not such a prerequisite. Our existence on Earth can tell us little about how common life is in the Universe or about the probability of biogenesis on a terrestrial planet because, even if this probability were infinitesimally small and there were only one lifeharboring planet in the Universe we would, of necessity, find ourselves on that planet. However, the rapidity with which life appeared on Earth gives us more information. We find ourselves in the group of planets on which biogenesis has necessarily occurred – we have of necessity won the biogenetic lottery some time in the past. And we also find that biogenesis has occurred rapidly – we won soon after the tickets went on sale. From radioactivity to lotteries, more probable things happen more rapidly. If life were rare it would be unlikely that biogenesis would have occurred as rapidly as it seems to have occurred on Earth. During and immediately following the Earth’s formation there was a period without life (∆tfrustrated), followed by a period during which life evolved (∆tbiogenesis), followed by a period during which life has been present (∆tlife). The sum of these intervals adds up to the age of the Earth (Fig. 1): ∆tfrustrated +∆tbiogenesis +∆tlife = ∆tEarth. (1) Inserting observational estimates for these durations (Lineweaver & Davis 2002, hereafter LD) yields 0.5± 0.4 + ∆tbiogenesis + 4.0 +0.4 −0.2 = 4.566 ± 0.002 Gyr (2) or ∆tbiogenesis = 0.1 +0.5 −0.1 Gyr. (3)