  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.
