You may not have to if they load the spaceship up with fertilized eggs and sperm (or something like that) - then launch it at a likely target with the result being that upon landing - the sperm and eggs combine and kickstart the growth of life (primordial soup). Check out the outrageously hilareous Kurt Vonnegut short story: "The GreatBig Space F***" (found in the Again, Dangerous Visions anthology). Who knows, maybe that's what happened to us? :-) Update: corrected title of story: Big instead of Great.