Reason for seeding life?

So, at the beginning of Prometheus we see one of the Engineers seeding life on earth, but we obviously don't know the reason why. We also have the mural of the Alien on the wall of the room with the vases in, which does look religious in nature. Well, could it possibly be that the Engineers seed life on other planets for the Aliens? We know that they need lifeforms in which to gestate, so do the Engineers create life so that they can then transport the Aliens later on when there are animals large enough for the Aliens to use to gestate within? Also, there are different types of bee that while they look different to each other, think bumblebees and honeybees, are still both bees? And whilst I wouldn't worry about rescuing bumblebees from pavements and moving them to safety in my hand, I wouldn't do that with other types of bee - so we know about the differing levels of aggression within the species. Couldn't the same be true of the Aliens as well? I certainly came away with the impression that the one we saw at the end of Prometheus was a different subspecies to the one we saw in Alien - but that could just be me.