HOLY S***! IT'S AN ALIEN!!!
Why do alien organisms always have to be equated to cnidarians? Cnidarians occupy aquatic niches and are fairly simple, sedate animals- other organisms with other lifestyles (like that of an intelligent species) would presumably have adaptations befitting them. Evolution is driven by by natural selection, not strangeness, after all.
That said, human bodies evolved the way they did for a reason. It's not unreasonable to assume that aliens who are also intelligent tool-users will have followed a similar evolutionary course and wind up with two arms and two legs.
That is true, but many of those reasons will probably not exist for other organisms- physics and chemistry are universal, obviously, but the evolutionary heritage and bauplan possessed by humans is fairly unique. If a dinosaur were (unimpeded by extinction) to evolve into an intelligent being, it would most likely look quite distinct from us, despite being a two-armed biped with which we also share such features as vertebrate eyes, jaws, nostrils, digits, etc (perhaps they would look something like
this artwork, which is profoundly closer to actual dinosaur physiology than Dale Russell's 'reptoman').
Presumably an alien creature with ancestors reminiscent of a slug or crustacean rather than our lancelet-like ancestors, would be even more unusual when compared to us.
We look like them, which is sort of kinda like logical, right?
Being our seeding engineers and all.
Thinking about how it would fit into what the fossil record says of the history of humanity (and life in general) is a headache-inducing activity. Not necessarily a fault of Prometheus, but of the "Ancient Aliens" genre in general.
