First let me say that I agree with Glenn in spirit. The immensity of geological time entitles us to postulate more improbable coincidences than a court of law would allow but, even so, there are limits.
Cumulative selection is the key to all our modern explanations of life.
The more we can get away from miracles, major improbabilities, fantastic coincidences, large chance events, and the more thoroughly we can break large chance events up into a cumulative series of small chance events, the more satisfying to rational minds our explanations will be. But in this chapter we are asking how improbable, how miraculous, a single event we are allowed to postulate. So, there are some levels of sheer luck, not only too great for puny human imaginations, but too great to be allowed in our hard-headed calculations about the origin of life. If the origin of life is such an improbable event that it happened on only one planet in the universe, then our planet has to be that planet. Begin by giving a name to the probability, however low it is, that life will originate on any randomly designated planet of some particular type.