the theories i've seen quite alot and one that i happen to agree with is this -
a futile war continued for the space jockys, their numbers were rapidly decreasing as the predator species were gradually wiping them out (jockys were considered prize prey for there sheer size)as the jockys battled against extinction they had to find a way of holding off the predators. so the quest began as the space jockys traveled to the darkest reaches of the glaxies in search of something to tip the scales in their favour, they definately found it. the xenomorphs. a breed of aliens born to kill that spread like a virus, wiping out all in their path. the jockys managed to capture a few of these aliens in early stages of their life cycle (still contained within eggs) and transport them back to aid them in the war. after deployment on the predator infested planets, the xenomorphs began to breed and kill off the predators, almost doubling their species, the predators had never seen anything like this before and struggled to contain them, thus the decision to create high tech weapons to deal with these things. on a routine drop off to a predator colony, a space jockys ship was hit by an astroid and the fail safe systems containing the alien eggs failed thus the eggs were free to open at anytime. disaster struck as memebers of the jocky crew became 'infected' by the alien larve, the ship, in danger, crash landed on a planet called acheron (LV4-26) the last surviving jocky (the pilot) set a final distress signal warning all never to come in contact with the ship, he too was victim to the aliens in the end.........
there it is, in a nutshel.
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