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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?

A list of all of the Anatomically Correct questions

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  • $\begingroup$ a distributed stomach that doubles as blood vessel system (or viceversa even better)? $\endgroup$ – Erik vanDoren Apr 29 '16 at 17:28
  • $\begingroup$ I remember reading somewhere that Xenomorphs are silicon-based beings. Might be useful to explain why their blood does not melt their own bodies. $\endgroup$ – SilverCookies Apr 30 '16 at 15:35
  • $\begingroup$ Nerdist video to the rescue. youtube.com/watch?v=ZHJpmWYncuo $\endgroup$ – John Mar 16 '17 at 19:22
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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.

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    $\begingroup$ One issue I could see in having a fluorine-based biochemistry like you're suggesting is where exactly would the Xenomorph acquire the needed fluorine during its life cycle. Fluorine is only found in trace amounts in any animal's body, including humans; only a few plant species synthesize fluorine-based compounds, and purely for defense purposes at that; and it's highly toxic to all animals even at low concentrations, anyway, so inducing the host to acquire fluorine is highly unlikely to be work unless the gestating Chestburster somehow extracts the element at a very fast rate. $\endgroup$ – MarqFJA87 Sep 11 '17 at 11:25
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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.

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  • $\begingroup$ To continue the pseudo-canon - - it's revealed in the Predator series that the Alien species was in fact designed (or bred , I forget) to be a worthy opponent for hunting. $\endgroup$ – Carl Witthoft Jun 14 at 15:25

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