I believe in the theory of panspermia. That a piece of bacteria inside a clump of ice from another galaxy billions of billions of billions of billions light years away landed on this fertile planet, melted, evolved to replicating-cells when it came in contact with our atmosphere, and since then has been evolving for over 4 billion years -- into humans, animals, other bacteria, etc.
I saw a documentary on Natial Geographic channel a couple months ago, and they made an experiment. A brick filled with bacteria COULD actually survive the vacuum in outer space, and it COULD survive the impact upon landing on our planet. The likelyness of the ice clump landing on a fertile planet is probably somewhere along the lines of... *dramatic gasp*
0.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.1%
Which is why our planet is the only one that seems to contain life. Where the ice clump with the bacteria came from in the first place, is actually bulletproof evidence that there - at some point in time - have been other forms of life somewhere else. I'm imagining bacteria living on a planet with an environment, getting hit by a meteor, blowing it up, sending the bacteria-infested brick directly towards us. And seeing as it's several hundred minus degrees in most of space, it would freeze, conserving the life.
But of course, it's just a logical theory that has no validated evidence supporting its claims.