just last week scientists discovered 715 new planets, some of which orbit in the habitable zone of their parent stars.
http://earthsky.org/space/kepler-con...cc5c-393668969
I recently read that it is likely that half of all the stars we see in the night sky, potentially all of them, have planets orbiting them. probability would indicate that an astronomical number of these would be in the habitable zone of their parent stars. Therefore, IMHO, it is absolutely irrational to believe that we are the only planet out of that huge number that had a comet crash into it bringing the ingredients necessary for life, if indeed that is what happened. Whatever happened to us, it has to be obscenely unlikely (almost impossible) that it didnt happen to any other planets.