They're the most recognisable part of the Alien franchise, the monster in the air vents, the physical representation of the most primal of fears. Designed by the late legendary surrealist H. R. Giger, the alien xenomorph that first reared it's ugly head in Ridley Scott's sci-fi horror classic Alien back in 1979 has terrified generations since, appearing in some form or another in every film of the Prometheus and Alien franchises and video game spin-offs.
"I wanted to make something that would be somehow human but that would also be robot-like, a kind of human that is protected from all kinds of external forces, be they weapons, radiation or whatever else. Instead of having their bones on the inside, they have them, partly at least, on the outside like an exo-skeleton. I mixed these together, this world of bones , mixed it with technical things, that's how these 'biomechanoids' came about from the interplay of the biological and the mechanical." - Giger [Alien Evolution]
From their first appearance to the nightmarish Ripley/Alien Queen hybrid that appeared at the end of Alien: Resurrection, the xenomorphs have existed in a multitude of different forms, and the next two instalments in the franchise promise to switch things up with their design yet again.
The xenomorph is due to return to our screens in Neill Blomkamp's upcoming Alien 5 project, and the concept art is more than convincing.
We know that Alien 5 is intended to take place after the events of Aliens (without disregarding the sequels though - go figure) and though the fate of Newt (Carrie Henn) is not revealed, we also see a badly burnt and scarred but still very much alive Corp. Hicks (Michael Biehn) alongside an older Lt. Ripley (Sigourney Weaver) in the concept art.
The xenomorph we see here is clearly more evolved than the original - it has more arms, a wider skull, what looks to be deadly spikes protruding from it's back and some kind of armoured plating along the top of its head. You wouldn't like to run into one of those in a well lit stadium, never mind a dark alley.
In the final scene of Prometheus we see a smaller, Queen-like xenomorph bursting from the chest of the Engineer, as is their tradition of birth. However the stages of xenomorph evolution differ drastically between Prometheus and Alien, with the former process going from fluid, to worm-like creatures, to a rapidly growing squid-like being then skipping the chest burster stage and going straight to the adolescent looking xenomorph.
Alien and it's sequels have a more solid version of the process: from egg, to facehugger, to chestburster then finally to the xenomorph itself.
So Why The Differences?
Given that there's nearly 30 years worth of canon time between Prometheus and Alien, and that the xenomorphs are shown to be highly adaptable and rapidly developing, it's not a stretch to say that the Prometheus xenomorph is an earlier stage in terms of evolution, or a different offshoot of the species entirely (supported by the convoluted name of the chemical element used by the engineers: A0-3959x.91 – 15, there's probably different versions of the chemical weapons which produce different results).
This would also explain the differences between the Alien/Aliens xenomorphs and the one we see in the Alien 5 concept art.
Though Ridley Scott initially said that he was planning something big for the xenomorphs in act 3 of the sequel, he's now confirmed that Prometheus 2 won't in fact feature them, so it looks like that side of the franchise's iconic monsters will be left to Blomkamp from now on.
"The beast is done. Cooked. I got lucky meeting Giger all those years ago. It’s very hard to repeat that. I just happen to be the one who forced it through because [the studio] said it’s obscene. They didn’t want to do it and I said, ‘I want to do it, it’s fantastic.’ But after four [Alien films], I think it wears out a little bit. There’s only so much snarling you can do. I think you’ve got to come back with something more interesting. And I think we’ve found the next step. I thought the Engineers were quite a good start."
This all kinda makes sense though; Scott has previously said that he didn't intend for Prometheus to be a direct sequel to Alien, rather just one that exists in the same universe. The sequel is set to diverge even further as it follows Elizabeth Shaw (Noomi Rapace) and David (Michael Fassbender) in their search for the Engineers home planet.
Scott's comment that the Engineers were a "good start" suggest that he's got something else antagonistic in mind for Prometheus 2. Something even more deadly than Gods and Aliens? Can't wait.