Slothy makes some good points at the beginning of his 'essay' (on Drake's Equation). I found the following thoughts interesting: "It is entirely possible that our backwater of a planet is literally the only one that has ever borne life. The point is that if there were only one planet that had ever borne life, then it would have to be our planet, for the very good reason that 'we' are here discussing the question! If the origin of life is such an improbable event that it only happened on one planet in the universe, then our planet has to be that planet. So, we cannot use the fact that Earth has life to conclude that life must be probable enough to have arisen on another planet. Such an argument would be circular." [Richard Dawkins]. He goes on to say that we have to have some independent arguments about how easy or difficult it is for life to originate on a planet before we can even begin to answer the question of how many other planets in the universe have life.