I don't think it's necessary for life to be spread through the rest of the galaxy by earth... with all the billions of stars in our galaxy, and the billions of galaxies in the universe, one would have to be pretty arrogant to think Earth is the only place life exists.
*EDIT* And maybe the universe isn't actually a universe. Just because we don't know what's out there, doesn't mean there isn't anything... our "universe" could be one of many just like our solar system is one of many in the galaxy, and the galaxy is one of many in the "universe."