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...