I read about a theory of
Erwin Schroedinger (I believe it was actually in the appendix of the
Riven strategy guide) about how the
universe is, to us,
infinite.
Infinity is such a large number, that is the universe is indeed infinite, then every single possible
scenario of outcomes has been played out. Every wild fantasy of a
science fiction writer, every
daydream or idea that anyone has ever had - it has occured somewhere, sometime, in the
vastness of infinity.
So perhaps there are 1020,000 worlds identical to earth where the spark of life never hit the flint. Still, though, all it would have taken was that one freak chance where the right proteins were in the right place at the time and lighting happened to strike and fuse them and the cell happened to suddenly find itself with a new purpose - propagation. Perhaps we are indeed living on that one world out of 1020,000
In reality though, the chances were probably not as dramatically infinitesimal as you make them out to be. Even as a half-alive organism, the half-bacterium still had the urge to propagate at its core, and it no doubt wandered around bumping into amino acids and proteins until it found the right ones, and eventually it had enough to commence life as we define it.
However, even if the chances are still as minute as 1 in 1020,000, occam's razor says that the easier answer - that it really was just "lucky old earth" - is probably correct. If you think about just how many times each hour life could theoretically arise, and just how many billions of hours the process had as its' disposal, it isn't that far out of the realm of possibility that it just happened to get lucky and hit on the right combination. After all, there's always a chance, however microscopic it is.