If xenomorphs evolved on another world, then how nightmarishly violent would their native environment have to be in order to sustain them? And what if they ever made it to Earth? Whole ecosystems would surely perish as this ultimate invasive species chased humanity into extinction.
Much like the vampires of legend, the xenomorph doesn't seem a very sustainable creature. On any given world, it's only a matter of time until they extinguish all viable food and host organisms -- and that just might be the point.
While reports are shrouded in mystery, a 2089 exploratory mission to the distant moon LV-223 provides some possible answers. There, the crew of scientists, mercenaries and dreamers encountered an ancient and highly advanced extraterrestrial civilization -- the same civilization, in fact, responsible for the derelict ship the Nostromo crew discovered on LV-426. These cosmic Engineers wielded staggering biomechanical and genetic technology, including something that can only be described as weaponized evolution.
The crew of the Prometheus found a vast storehouse of ampules, each containing an exceedingly contagious viral mutagen. Within hours, this inky black slime infected human crew members, as well as a native species of worm. The slime quickly spawned several incredibly hostile life-forms, all of which displayed a pattern of parasitic impregnation like that of the xenomorph species.
While details of the doomed Prometheus mission are still emerging, the biological weapon eventually impregnated and absorbed genetic characteristics of both humans and Engineers to produce a slender, phallus-headed biped with a pharyngeal jaw.
Was this being, code-named the deacon, a precursor to the xenomorph? Or does the Prometheus disaster simply refute any species-oriented understanding of the xenomorph at all? Perhaps it is less a true creature than it is death made manifest -- a cancerous, genetic force that exists only to extinguish all life, save its own, and then perish amid the bones of decimated worlds.
For now, these mysteries remain unanswered.