Exobiology: not for the meek.
There are a lot of people that believe that the Earth was 'seeded' with life from outer space.
But before anyone takes this to heart any theories should undergo rigorous review.
(The latest 'bacteria-in-metorite find' splashed all over the news did not, as it's turning out.)

More:
http://en.wikipedia.org/wiki/Carbonaceous_Chondrite
http://en.wikipedia.org/wiki/Coprolite
http://scienceblogs.com/pharyngula/2011/03/did_scientists_discover_bacter.php
http://en.wikipedia.org/wiki/Fred_Hoyle#Rejection_of_chemical_evolution
http://en.wikipedia.org/wiki/Panspermia