Final Dawn

Sometimes I wonder what would happen if a series of meteorites carrying microbial life made impact with the earth - if they didn't burn up upon entry (thereby destroying organic matter which might reside within the rock) and landed in, say the ocean... >____>
Let us pretend for a moment that the conditions would just happen to be perfect for said organisms, they flourished and over time would begin displacing pre-existing earth-organisms. In a few million years, the earth could be unrecognizable, and pretty neat setting for a sci-fi RPG... or something. :3