It has been suggested elsewhere on SF+F that the Xenomorph species in the Alien franchise is bio-engineered, possibly with specific focus on targeting the human species. Apparently this may be a plot point in the upcoming firm, Prometheus. What evidence is there in material relating to Prometheus or the wider Aliens canon that Xenomorphs did not evolve naturally and were designed as a bio-weapon (as opposed to Weyland-Utani wanting to turn them into a bio-weapon)?
As far as I can tell, you will find nothing written in canon, about the Xenomorphs being an engineered species. There has been plenty of opportunity to discuss it in the multiple Dark Horse Comics series where they show entire societies of the creatures.
I can only surmise it because of the evidence presented by the species. Yes, this is entirely speculative but the references are from the canon materials.
- almost all forms of the creature has a volatile hydro-sulfuric acid for blood, highly uncommon with no known analogs among tens of millions of potential lifeforms on Earth as a reference.
- the creature is able to eat nearly every form of life it comes in contact with; and if my theory is correct, it must also be able to eat a wide array of inorganic things to convert whatever it uses for a biological process to create acidic blood.
- the creature has an incredibly durable form of life capable of existing in an egg state for at least one hundred years or more. (Alien)
- the creature has a skin composed of a natural-occurring, fast growing, highly elastic polymer. This polymer is resistant to hydro-sulfuric acid and able to be created at an incredible rate from materials that don't normally contain it (like us). We don't know how they convert us into them, but their biology has to be astounding! They are also able to continue to secrete even more of this resin over time to protect and fortify their hive spaces.
- the queen is capable of suspending her life force under what humans would consider hostile conditions (subzero temperatures) for over a thousand years and still remaining viable (Aliens vs Predator: Requiem, 2007)
- The species is able to attack a wide array of life forms from different biomes, biospheres and completely divergent DNA profiles, including humans, dogs, Predators, as well as the creatures from the prison world seen in Predators (2010) and spawn from them. Seeing that the Predators would carry them from world to world, lends to the idea of their extremely versatile bio-systems being compatible with any number of other worlds.
- The ability to eat creatures with extremely different genetic profiles should provide either no nutritional value at all, or be potentially toxic to the creature consuming them. These creatures appear to be able to adapt their digestive process completely to survive.
- The Xenomorph is able to engage in horizontal gene transfer, copying genetic traits from the host creature to itself with little impairment of the resulting offspring.
- Horizontal gene transfer almost never occurs in nature except in the case of bacteria. For it to occur in such an advanced lifeform, indicates genetic engineering for the feature to even be available in the organism. We have no examples of such capacity in our own higher lifeforms.
- Such a wide range of genetic capability and adaptability could only occur in a weaponized lifeform. Literally designed to attack nearly any kind of prey, under any biological condition, even in environments inimical to human life (the birthing chamber on LV426) for example.
With all of this in mind, it would be far easier to explain such an incredible life form, able to adapt to nearly any environment, horizontally blend its DNA to create completely new versions of itself, becoming even more well adapted to a new environment, eat nearly everything on every world it finds itself on, is eusocial and still independently intelligent, if that lifeform had been engineered to do so. The only thing on Earth with that much versatility is a virus!
Finding such a powerfully adaptive creature that evolved independently would ultimately mean they would eventually conquer every planet they landed on as the ultimate in invasive species. With such versatile and energetic biology, they would have eaten themselves out of house and home once they reach the confines of the land masses on their home planet, unless they had some form of predation, competition, or other limiting factor on their homeworld.