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Xenomorphs are one of the most dangerous, deadly aliens in movie history. They breed in massive numbers, live in an ant-like society, and have blood that doubles as concentrated acid.
Most of those aren't a problem to figure out as we have many creatures on Earth with those. The biggest difficulty with designing this creature lies in its blood. As far as I know no creature, anywhere on earth, ever has had acid blood.
What would the evolutionary history be for a creature with acidic blood?
The internal salt levels and acidity of the liquids in our bodies are largely set by our evolutionary history since pH changes how proteins fold.
So if a creature evolved in an extremely acidic environment then later had to adapt to being able to live outside that environment then its internal biochemistry could still end up locked to being very acidic.
The problem is that the Xenomorph blood in the films is very very very acidic. It eats through many layers of metal and even hardened glass. That's strong acid.
But it's implied that Xenomorphs are not natural creatures but rather biological weapons of some kind.
This opens another option: If you're using bioweapons you don't want your enemies to be able to take your weapons apart and analyze them easily. With engineered creatures even a spatter of blood could be used to recover details of how to achieve capabilities you've engineered in.
So how might you protect the details of your creatures biochemistry? well, you could engineer the creatures cells to include tiny hardened Teflon (one of the few substances which can survive it) vacuoles of, say fluoroantimonic acid (pH of −31.3 one of the few materials potent enough to match the capabilities of the alien's blood in the movies) engineered to burst once the cells are detached from the main body.
Detached or damaged tissue now dissolves into unusable gunk, possibly even taking the floor beneath with it.
That the xenomorphs in the Alien/Predator/Prometheus movie series have been shown to be artificial bioweapons is a significant datum that means that their capabilities have not merely evolved, but have been intelligently designed, and may therefore have traits incapable of evolving naturally.
Another significant datum is that the movie series has shown them to have originated as a 'black oil' - likely to be some sort of nanotech in my opinion. That the xenomorphs anecdotally have "Acid for blood" is not proof that their blood is acidic: another plausible explanation is that their body fluids appear to dissolve solid substances because their body fluids contain nanites that carry metabolites while circulating within their bodies, but on being spilled, change from metabolite transporters to nano-disassemblers. That they don't keep disassembling stuff forever is simple conservation of energy: each nanite has a limited supply of energy, and disassembling stuff takes energy. Once the energy has been consumed, the disassemblers stop functioning.
Another significant datum is that the xenomorphs have been shown to be able to grow in environments rich in organic matter - like humans - and also in the absence of organic matter, such as the interior of a space ship or a fusion powerplant. This is highly suggestive that the xenomorphs are able to harness the electrical or other energy available in such environments in order to extract carbon in the form of carbon dioxide from the atmosphere in order to grow in the apparent absence of 'food'.
The observed behaviors and 'life cycle' of xenomorphs show that they are as much terror weapons as bio-/nano- weapons. While the trait of requiring biological hosts is a limiting factor on the growth of their population, there are more efficient ways that a designed bio-/nano-weapon could achieve the goal of exterminating large animal-like beings that don't involve growing macroscopic xenomorphs.
Based on the internal evidence of the first two Alien movies, there are suggestions that the Alien Xenomorphs are not "evolved" creatures at all, but are somehow the products of an alien biotechnology.
If this is true, then it not only explains a lot about the behaviours of the Xenomorphs, but also suggests why the blood is so acidic: it is an electrolyte for an alien biological "battery".
If the Aliens, especially the drones and warriors, are completely self contained then there is no need to develop different species to operate in different environments; they will continue to function in vacuum, poisonous atmospheres, underwater or indeed anywhere they can gain a foothold into an ecosystem and begin capturing beings to use as hosts for the next generation. (You notice there is little indication that Kane [John Hurt] was actually suffering from the alien chest burster consuming him or the nutrients from his blood during incubation, it just mechanically burrowed out when it was ready to begin its life cycle).
So in a secret lab somewhere among the stars, a culture created a mobile, self directed and self reproducing weapons system to overwhelm alien ecosystems to meet whatever needs the creators had in mind.
What about facehuggers, nobody mentions those things. I think they are a different species of creatures, they carry the egg, plop it in your mouth and let it evolve. Then when the job is done they run back to the nest to get another one. It’s symbiosis, the aliens get their young nurtured and the facehuggers are probably groomed and fed to keep them healthy and alive.
