I only have the first volume, and I really have no idea what's going on in it, so I'll have to wait until the Omnibus' get that far along before I'll really be able to comment. From what I gathered, though, Homeworld put forth the possibility that the Predators evolved naturally here on earth, but that once they'd attained space-travel they migrated out into the galaxy leaving the terrestrial eco-system open for humans to evolve. It suggested that this is why hunting on earth continued to be such an important part of their culture (I suppose, in a way, it would be similar to visiting Mecca at least once in a Muslim's life).