Every planet has a ship / species

Looks like it is not a new standalone film. It is a prequel in the traditional sense.

So what is the evolutionary link to the arachnid facehugger if so? Big mystery

Thats one thing i suspected, that indeed the Xeno was around prior to the Prometheus mission. Did the Jockeys create the Xeno some how via experiment and genetic manipulation? or did they come across it and took it back to LV223 to perform experiments on it. Something went wrong, this nearly killed off all the Engineers/Jockeys and so they decided to move what remained of the Xeno off LV223 to some other destination but then something escaped again and they had to make a emergency landing on LV426 and warn the Engineers on LV223 that the mission had been compremised. Bit like if the USA created a Virus maybe to use as a Cure for Cancer, but it mutated and mutated the patients into Zombies, they then manage to contain the Virus (wipe out Zombies) and get the remainder supply to a safe location away from civilisation say some remote island, and as the ship transporting said virus, gets hit by a Storm and instead crashes off the coast of say the Fiji Islands it would let out a warning to warn the USA that they could not get the Virus to a safe uncivilised place and that it has been compremised and on the Fiji Islands.

i'm dyeing to know what the hammer head cobra snake thing could be is it a previous incarnation of something we already know or a complete new species!.... so many questions roll on 1/6/2012.

@BigDave The jockeys are biomechanical engineers. They made the Xeno for their own purposes. there is no indication in any of the trailers that the Xeno causes the stacks of bodies lying about in that temple. Given that an organism bursts from the remains of the Engineer skull is a key part of this. A huge part of the plot hinges on what that thing is.