Giger's 1978 concept painting 'Alien Hieroglyphics' always intrigued and bothered me, as it shows a race of beings purposefully impregnating each other to create aliens. (I know that this concept was never shown in the film and is not considered part of the canon at all, but it is one of Giger's more impressive concept works IMO). This post is shamelessly speculative and full of blundering assumptions, but let's say that Ridley has
been informed by this artwork when figuring out his origin story for Prometheus (of course there may not even be an origin story in Prometheus).
What I never understood about this idea was, if the beings depicted were an advanced form of the alien then their one-for-one breeding program wouldn't of worked out too well. When we think of how they might of evolved we end up with a 'chicken and the egg' paradox, so you can assume that they didn't evolve on their own. Unless of course they are not the adult aliens at all but actually slaves to the aliens' masters, an oppressed sub-race engineered to be hosts for these 'weapons' and maybe even anatomically structured to be able to survive the 'birth'. (It would be handy in this case if you had a reversed rib cage.)
I speculate that what we are seeing dead on the floor is one of these 'slaves'. To loosely quote Ridley: "what they find is civilization, but with very uncivilized behavior". I also speculate that they are, somehow, related to human beings. These are the cats that challenge their 'gods' and are punished for it.
And if the round thing on the ground is a head, it looks far more like one of these guys than it does an SJ IMO.
P.S. I am just guessing, don't hurt me!