The recent Cosmos remake has a good episode that touches on this. I think it's http://en.wikipedia.org/wiki/The_Immortals_%28Cosmos:_A_Spac...

One theory set forth is that earlier in Earth's history, while bacteria were first evolving meteors would periodically strike and extinguish life, but cast rocks with dormant bacteria into space that would later re-enter and re-seed the planet after the dust settled. First time I heard the idea but it's intriguing, and segues into the whole idea of panspermia.