Speculation: The Xenomorphs are a genetically-engineered species designed specifically as weapons. I support this with several simple but powerful ideas:
- Their blood is acidic. It would be a very unusual mutation to have blood whose acid content is as powerful as the blood of the Xenomorph. We watch it eat through three decks of metal, easily, in fact effortlessly. Blood with such an acidic level would require a specialized diet to be able to produce such a quantity of acid-laden chemicals (assuming it is an affect of its environment) to create such highly volatile blood.
- They are highly armored, very fast predators, capable of running across walls, floors and ceilings with the same ease as crossing a floor. If they have the opportunity to recreate a viable environment using their secretions, their terrain can even cloak them from visual identification, making them even harder to isolate and target.
- They do not show up under infrared light. This means their body profile is endothermic (cold-blooded). They are very fast for cold-blooded creatures which at least on Earth, is not the norm. Cold-blooded predators on Earth are generally capable at catching, poisoning, grasping or simply outlasting their prey, but there are few track stars in their ranks. This makes them very difficult for creatures who use the visual spectrum, especially the low end, to detect them. Technology for visualizations toward the high end of the spectrum is more sophisticated and less likely to be easily had. Not impossible, just less likely. The most effective way of tracking them, through their chemical signatures, like the Predators do, is one of the most advanced methods possible and likely the hardest to make portable.
- The most damning evidence as a sign of their engineered heritage is their uncanny ability to gestated from a variety of host organisms and the speed of their growth. They do not need to take more than a day or two to reach full adulthood. They are born knowing how to hunt, track and prepare their prey for implantation. We know for certain that they can develop in humans and the Predator species. We have also seen them gestate in dogs, creating functional if not attractive xenomorphs. Since taking human and Predator DNA into account, that is three completely different species who are genetically compatible enough to produce viable offspring. In genetic terms, this would be highly unlikely, on the order of nearly impossible without an advanced technology making it possible.
- Since we have also seen the Xenomorph take the characteristics of the creatures it hosts within (taking on the Predator characteristics for example in AVP and AVP: Requium) this implies an intelligence behind the design of the creature to build an organism able to gestate in a variety of organisms of different genetic structures, from different planets and to produce viable offspring of sufficient strength, speed, coordination, and intelligence to work cooperatively, telepathically (assuming a queen is available), or using a very fast other means of interacting that seems like telepathy and be able to quickly take over an area as effectively as these creatures do.
Their aggression, innate intelligence, weaponry, claws, armor and strength is proportional to the creatures they replicate ensuring their superiority in hand-to-hand combat. Their weaponized blood ensures even if an enemy engages them in physical combat and wins, they risk taking damage or even dying. This creature is the very spitting image of a bioweapon waiting to be used.
Considering the Company seemed to want them as weapons, as part of the premise of the series, it seems a logical assumption. Who could have created such a weapon is unknown but it is certainly well beyond the average capabilities of human technologies of the time.
The counter argument says that nature is adaptable and could conceivably create such a creature with a mutable, species ambivalent, genetic structure (though, no creature on Earth comes to mind with a similar ability except viruses and bacteria). Such a creature would become the apex predator of its world, wiping out all life including itself once all the food sources were destroyed. In which case, it is still the most formidable, macroscopic, biological weapon ever seen.