NASA recently reported that scientists have identified nucleobases that make up our genetic code in meteorites and that it appears they were created in space. It has been theorized that life may have actually migrated to Earth rather than spontaneously appearing here, and this kind of discovery supports that possibility. Popular Science has a nice story on the discovery, as does NASA itself.
Interestingly, the assumption about this material in the meteorites is not that they are passengers who just happened to be along for the ride, but that comets and asteroids may have the chemistry to actually manufacture the stuff of life. Either way, such research lends credence to the likelihood that life exists elsewhere, and the possibility that life here began somewhere else.