@ Sluty Badger wasn't an earlier draft of the script of Prometheus which was either Alien Engineers or Paradise suggested that the Engineers lacked the ability to procreate, they found a bizarre angelic ethereal being the Deacon ish creature which they worshiped displayed with the mural in the cave on LV-223, using its blood as a means of creating new life "the whole death and rebirth rising from the ashes aliens cycle". That Deacon's blood was supposedly what the first engineer drank on earth creating the primordial soup, it's said that through the ages they filtered the Deacons blood and didn't have a pure strain and it devolved into the deadly pathogen creating aggressive monstrosities related to the Xenomorph, return. It appears alot of these details changed to not give a definitive explanation and the whole Xeneomorph/ Space Jockey Engineer connection isn't made clear.