I'm still curious about the relief of the xenomorph on the wall myself. If they're just supposed to be another bio-weapon or black goo mutation, why are they the only ones that share the same Giger-esque ribbing and bio-mechanical look as the Engineers?
In the "Art of" book, Ridley talks about the mutations taking on the form of their hosts, but it was shown pretty clearly in the movie that the bio-mechanical detail was only part of a bio-suit, and the Engineers look much more human underneath. So I don't think the xenomorphs could have gotten it that way.
Maybe they were some kind of special creation the Engineers patterned after themselves or something. Or maybe they even come from the same home planet...
I was wondering that. Though, parts of the biosuit on the pilot himself seemed integrated with his skin. Also, why do the hives the xenos make resemble the Engineers' structures?
Maybe the xenos were a mere engineering tool...to create structures. "We pale giants are too lazy to build, let's make some worker ants. But what if they breed incontrollably? We'll engineer them so they require hosts to breed."
So the xenos are their David's, their artificial lifeforms.
Or not. The ALIENS don't seem designed for digging tunnels. Acid for blood is messy, too. They'd need a kill switch to make sure the aliens could be eradicated when their job was done.
And we don't know the end stage of Alien metamorphasis. We think we do. At some point the hive changes into something else, maybe the Newborn alien in Resurrection wasn't a hybrid, maybe it was the next step in the xeno "life cycle". hell, maybe it was the kill switch. It kills all the xenos.