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 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.