i have a solid theory but i cant really tie it into the predators its more of the creation of the aliens.
THE CREATION:
The Xenomorphs were created by an ancient race (The Jockey Race) tens of thousands of years ago . They were fighting among themselves in a brutal civil war. Their technology was organic in nature and was so powerful, the death toll ran into the millions. For decades, the war never saw twilight. It seemed to go on forever...until one side got an idea.
It was an experiment to create a biomechanical weapon. It would adapt to its environment, be completely independent....and finally...it would be viscous. This creature would be a living machine with the most grotesque combination of science and nature. They created a species bent only of the art of killing. The experiment was placed on a distant world ("Alien-Proteus"). They designed the species with a hive structure to keep the infestation from running out of control. The "ant" methodology was promoted. The object: To keep the infestation under control by keeping the creatures close to their "queen."
The experiment was a success. The creatures we know as Xenomorphs were created. The standard infiltrator mission was as follows: An egg is deposited in a populace, it impregnates, and an alien bursts out of the host and soon find more host which it can convert into eggs which will also hunt down hosts to impregnate. The creature will kill anything threatening and convert the rest. Soon, the populace is destroyed and the aliens are left alone on the world or spacecraft where the infestation was released. It is assumed the Jockeys that created the Xenomorphs had a procedure to eliminate the new infection.
The Jockeys were ready to unleash their creation on their enemy. The Proteus planet was still under control. The experiment seemed safe. The mother species deposited many life forms to keep their hives intact. These life forms seemed to reproduce in large numbers. When needed, a transport would simply set down and pick up a queen, a drone, an army, or just a bunch of eggs as the mission profile specified. The weapon proved useful and the war ended quickly.
But Pandora's box was open.
The Xenomorphs were designed to adapt to current situations but their basic morphology was not to change...it did. Before the Jockeys could stop their "children", they had evolved. One small little change, something seemingly insignificant connected to the "ant" structure but not included in the alien design...Royal Jelly—the strange substance used to generate a queen. When the Jockeys discovered infested worlds where there shouldn't be, they got worried.
Given the capability to create their own queens, the infestation could not be bottled. It soon broke out of control. The Jockeys found their experiment turning against them. They tried to find a weapon to defeat their own creation but they were already demoralized from a past war. They sent one last ship to pick up "evolved" egg samples from the Proteus planet to study...it never came back.
The Jockey race vanished. After thousands of years, without transportation, the Xenomorphs quickly overran the planets they were on, then promptly died out. They might be long lived, but after ten thousand years, even their lifespan ran out. The eggs fossilized, the bodies disintegrated and the last legacy of the alien species vanished. The ship from the proteus world crashed landed on a world far away from their homeworld.
Everything went quiet.
The universe held its breath.
HUMAN CONTACT
LV-426 was the world were the lone Jockey ship crashed landed, across the galaxy from their home world. The Jockey onboard was killed when an alien egg he carried broke free and impregnated him. Without a pilot, his ship set down on the hostile world. The stasis fields in the cargo bay were damaged and all but one bay of eggs were unsealed. They all soon died from exposure. One bay remained sealed and waited--the last living legacy of a past experiment that failed.
It was several millennia later when the Nostromo set down on the planet to investigate the signal the Jockey ship had sent to keep people away. 57 years later, a colony was placed on that world which was overrun by the species. The infestation was wiped out when the colony's atmosphere processor went critical. The Jockey ship survived, covered in a lethal dose of radiation.
It is unsure how the species got off the world but all bets point towards a human cause. It seemed the race finally died out when Ellen Ripley dove into a furnace on Fiorina 161. The GM is totally open on how the Alien starts its path. Refer to the story for details. A salvage ship could pick up another alien wreck with alien specimens on board. Someone might recover the Sulaco. Someone might even smuggle some life specimens from the radioactive cover of Acheron. Either way...they are out.
THE LIFE AND THE MACHINE
The Aliens are alive. They have a genetic structure. They are born and can die. They have a blood stream; they have a social structure. They have teeth and can scream. The Aliens have a cellular structure partly carbon and partly silicon based. They have a very identifiable brain with sensory organs. They have also shown reasoning. The drones are surprisingly intelligent. They gestate in a living host and come out in pain. The aliens are alive.
They are also a machine. They don't require food, as we know it. They can eat but process it completely different, converting the materials directly into increasing its own size, or storing it for when it needs to create a hive. The aliens, for a better part of the word, are walking batteries, as they run on the acid blood in their veins. They also seem to have a programmed path. One cannot go rogue under the direction of its queen. They seem always under a fixed set of instructions given to them. They don't have a digestive system as we know it nor do they even have eyes. They don't grow as normal creatures do and they also don't have a nervous system, as we know it. In many ways, the Aliens are machines
