the link to Alien

If you remember as the team where in the mountain just outside the door where the dead engineer was found david finds some clear sticky resin on the door opening. the holographic image shows engineers running for their lives to get away from something, the team aslo find tons of bodies pile up of enginneers with punctures in there chests and sides, this is the 1st xenomorph . The engineers have been creating xenos for years thats why there is a mural on the temple wall showing xenos and behind the human head is a massive clue that the xenomorph exists when hollaway finds what he refers to as a tomb. The original xeno looks like the creature at the end of the movies, but when it is soaked in that black fluid it mutates into the xeno we al know from alien 1. That black fluid makes things grow and become super in stregth, thats why the little worms you see around the ampules end up becoming like pythons when they get covered in black fluid. That is also why fifield becomes super strong when he gets covered in the black fluid, if you drink the fluid you died and disintegrate (like the engineer in the beginning of the film) but if you are soaked in the fluid you mutate and become super strong. At the end of the movie when that blue xeno bursts from the engineer it is in its original form but it must somehow make its way into the temple and get soaked in that black fluid and mutate into the black xeno we all know from alien.