  A simple stochastic model for evolution, based upon the need to pass a
sequence of n critical steps (Carter 1983, Watson 2008) is applied to both
terrestrial and extraterrestrial origins of life. In the former case, the time
at which humans have emerged during the habitable period of the Earth suggests
a value of n = 4. Progressively adding earlier evolutionary transitions
(Maynard Smith and Szathmary, 1995) gives an optimum fit when n = 5, implying
either that their initial transitions are not critical or that habitability
began around 6 Ga ago. The origin of life on Mars or elsewhere within the Solar
System is excluded by the latter case and the simple anthropic argument is that
extraterrestrial life is scarce in the Universe because it does not have time
to evolve. Alternatively, the timescale can be extended if the migration of
basic progenotic material to Earth is possible. If extra transitions are
included in the model to allow for Earth migration, then the start of
habitability needs to be even earlier than 6 Ga ago. Our present understanding
of Galactic habitability and dynamics does not exclude this possibility. We
conclude that Galactic punctuated equilibrium (Cirkovic et al. 2009), proposed
as a way round the anthropic problem, is not the only way of making life more
common in the Galaxy.
