[taylorius]

Ok, we don't know how likely life is to evolve in the universe. It could range from impossibly hard (though it cant actually be that, because we exist) to easy enough that it has occurred many times. Of all those possibilities, what difficulty range would result in it occurring once (on earth), but never again - despite the billions and billions of planets that exist. A very, very fine tuned difficulty - or to put it another way, the earth would need to be super,super unique, to be the only place capable of life in the entire universe. That strikes me as less likely that it occurring multiple times.