This explains it, pretty much exactly what I thought..fuond it online last night in my nerdom
After David opened the chamber door, he atmosphericly compromised the room’s air which had been sealed for around 2000 years (we know this because they carbon dated the headless body by about 2000 years). Fresh air caused the murals on the wall to degrade and the temperature difference of the fresh air caused the canisters to sweat, like when you put ice in a glass of water. This also had an effect on the biological liquids in the canisters, causing them to start expanding (liquids can expand when the temperature radically canges).
When David first walks into the chamber, we see (although none of the crew notice) that he disturbs some form of larvae on the ground (they come up from under the ground after he steps on it). These are most likely left over from the space jockey head left in the room (these may also be what causes the head to explode later during the examination, but that’s a whole other topic). The black liquid in the canisters is a bio weapon, from what I understood in the murals we saw in the walls, the main one showed the alien xenomorph at it’s centre, suggesting that the biological black liquid is designed to grow into the alien xenomorph – each canister when activated will become one alien drone, most likely a soldier that can NOT reproduce (otherwise there would be no way to control them once the space jockeys have unleashed them on their target, the xenomorph would NOT be capable of implanting an embryo in another being). The black liquid though, because it does create a living organism from nothing, would have to contain some kind of advanced steroid that affects biological matter in order to create a life form.
When the black liquid pools on the ground, the larvae we saw on the ground began to bathe in it and feed on it. As a result, it bioformed them as it’s capable of doing, the end result being an agressive more advanced worm creature, the creatures encountered by Fifield and Millburn. When the creature sprays acid on Fifield’s helmet it begins to melt it. Fifield then falls into the black liquid, but when he stands the helmet has been melted enough that it collapses inwards. The mixture of melted helmet and the black liquid burns into his face, causing him to pass out, falling face first into the black liquid, where it begins to biologically change him as it did the larvae worms.
Meanwhile, Holloway, who has ingested a single drop of the black liquid, has been undergoing internal changes. As he ingested it, and only in a small amount, his change has taken more time, working more subtly, changing his internal organs first including his reproductive system. Because of this, and unawares to either of them, he impregnates Shaw with an embryo that is human based, but has been part changed by the black liquid. Holloway begins to suffer radical changes with the black liquid changing him into a xenomorph drone. He’s not as far along as Fifield when we last see him, and his behavour and mind has not been changed yet when he sacrifices himself.
The real surprise is that because the creature that Shaw gives birth to is a hybrid of human and xenomorph genetics, it contains all of the genetic traits of the xenomorphs, intelligence, aggression, but it now also contains in some form, a reproductive system, meaning that it is able to implant an embryo in a being (as we see at the movie’s end), which it will take traits from and evolve into the xenomorph we know.
If one could look at the xenomorph as an final good (in the economics sense of the word), I think perhaps:
the black goo is the raw material, which produces:
an infected human as the first intermediate good, which produces:
a facehugger as the secondary intermediary good, which produces:
a human carrying a xenomorph embryo, the teritary intermediate good, which produces:
the xenomorph, the finished good, which can convert humans into eggs.
once the first two steps are accomplished, the last three independently cycle provided there is an availability of inputs.