Här är ett möjligt svar, som finns på en io9-artikel: länk
Medan det är mer av en spekulation, är det väl argumenterat och dokumenterat och det kan hjälpa andra som delade min nyfikenhet.
[redigera] Relevant utdrag från io9s artikel ingår nedan
The digital orrery that David activates indicates that Earth is just one planet that the Engineers were working with. (It would have been surprising had it been anything else.) What was taking place here was nothing less than a monumental program of experiments involving DNA accelerants, making it tempting to ascribe to that black goo a primordial/mythic status. Scott has said that 1981 caveman drama Quest for Fire was a big influence for this film, and one wonders how far to take that comparison. If that black goo really is the fire a la Quest (and the Prometheus legend itself), then the Engineers never created the accelerant, but rather stumbled across it. Or else were given it by their creators. This notion of the Engineers falling short of the status of unmoved mover — that they're not at the apex of the cosmic hierarchy—is given further weight by another tantalizing statement by Scott, namely, that he drew heavily on Sumerian myths in creating the universe of Prometheus. When we consider the extent to which those myths feature gods with divine helpers, it leaves one wondering whether the Engineers were merely the servants of something greater. After all, the Engineers seem pretty damn engineered themselves. They all look the same, and their art seems no less religious than do the cave-drawings back on Earth (a la that wall-image of the xenomorph, to say nothing of the gigantic head). We're thus led down a disquieting chain of logic: just as robots can turn the tables on humans... just as the xenomorphs turned the tables on the Engineers, and just as the Engineers feared humans would do the same — perhaps the Engineers themselves were rebelling against a greater master, left off-screen (and possibly long dead.)