honestly, i think Ridley left a piece of the puzzle sitting on the table in the middle of Davids lab. In the novelization of the film, its established that David RECREATED what the Engineers had already made.... meaning, the facehuggers David made were actually a recreation from the hugger he found in stasis already. The giant egg on the middle of the table in the film eludes to this idea as it matches the description of the hugger in the book (the egg on his table cut into several pieces). This idea is further alluded to in the short "Advent" when he talks about the "wolf" and how the Engineers cast 'the wolf' from their society aaand how David sought out to recreate that said "wolf".
Also, evidence from Prometheus even further backs this idea with the Deacon being created thousands of years before man (and David). The creature on the wall of the mural IS the Deacon (and NOT a Xeno like some people claim), though an earlier design in the concept work during production of the film (this has been confirmed to by the concept artist Carlos Huarte when I discussed it with him via FB messaging btw). Sooo with that said, the engineers created the Deacon and most likely Xenomorph species LONG before David recreated the facehuggers based upon his learning of them via studying the Engineers "wolf".
