Prometheus, the universe and life

I went to see the film last week with one of my sons. We came out with a lot of questions and not necessarily in a good way.

 A voyage into the unknown that conjures more questions than it answers … That's what we were told to expect from Ridley Scott's long-awaited sort-of-Alien-prequel Prometheus, ... We thought they were talking about the characters' journey into the depths of space to meet the extraterrestrial originators of life on earth. It turned out, though, to refer to us poor lugs in the audience, stumbling out into the night scratching our heads.

So I was interested to find this article which asked the same questions and then a couple more ...
Prometheus: what was that about? Ten key questions | Film | guardian.co.uk . But then there's one I asked that's not on their list. In fact it's the same basic question I asked of at least one of the Alien films and also regularly ask about other sci-fi and/or horror films. it's a bio-ecological question. In the case of Prometheus it comes into sharpest focus with the alien beastie that comes out of Elizabeth's womb in the emergency c-section/abortion. That beastie is about the size of a human neo-nate in terms of mass (why Elizabeth didn't 'show' more is another good question btw). When we next see said beastie it has become I would say four to eight times the womb-leaving size. ... How? Leaving aside the rate of growth which I'm willing to suspend my disbelief over- where has the critter found the protein and other compounds it would need to have ingested to convert into that kind of body mass. The room it was trapped in was sterile and had no significant amount of biological material to convert into more of said beastie's body. This issue is also part of at least one other of the the Alien films.

A related issue is the sheer amount of alien carcasses in what turns out to be the Engineers' ship: again; what have the beasties fed on to become so big? While some Engineers clearly were colonised, we seem to be talking about an aggregated mass of alien body-stuff many times that of the captured Engineers' body-stuff. There are well-known ratios of predators to prey in established sizes of ecosystems; the Aliens seem to have the secret of making body-stuff out of thin air. In which case, why bother feeding off other living beings?

I prefer my sci-fi to have greater plausibility in the face of such questions.

PS: I've just (3 July) found a review of the film that actually gives it a lot more sense symbolically but also helps some of the plot plausibility by reckoning that the black goo is an adaptable stuff that is shaped by the emotional atmosphere of those around it -in this case the greedy and rapacious homo sapiens around it produce their own nemesis; but had 'they/we' been more gentle or convivial, the goo would have shaped into a life form of friendlier and more helpful disposition. I'm expecting, on this basis, for the next film to be called Nemesis!