Space Jockeys are extinct and need our DNA.

I think I have worked out at least a part of what Prometheus may be about. The SJ's did not visit earth since the time of ancient civilization, because there was some kind of extinction event on the planet the Prometheus ship visits. They Bio engineered humans, in their own liking, to store most of their DNA, to react with what ever are in those vials. I dont think the planet is LV426, I think that the Derelict Dallas finds in Alien, is just another Space Jockey ship, just thousands of years old... like the civilisations are where the stone tablets are found, revealing the star maps. The star maps were left by the SJ race, so their creation (man) could find them to restart their civilisation at a later date. Say, once the planets air became breathable again, after the Extinction event. That would tie in with the first words spoken on the trailer "A king has his reign, and then he dies, it's inevitable" Once man has served his purpose, there is no need for humans to exist, they have done their job. Hence, the SJ try to go to Earth to wipe humans out. That is my take on it anyway..