Just looking at the numbers, it seems impossible that earth could be the only home for life in the entire universe. But of course the probability starts to become more complex when we can't assume that all outcomes are equally likely. I don't know enough about either astrophysics or biology to even begin working that out. Does anyone?

Also, do we know yet even what questions we should be asking? Should it be "How is it possible, in all the vast universe, that earth could be the only place where life developed?"

Or maybe the question is, "How is it that, against fantastic odds, life managed to develop on earth?"

The only honest answer I can offer is a rather unhelpful "I dunno."