[dogscoff]

Do you think the aliens warnings against eating anything not native to earth would be absolute? Seems to me that organic matter is organic matter, is it not?

Someone stop me if my science is off here, but here's how I understand it:

Most of the things we eat are pretty complex- proteins, fats, sugars and vitamins- and they are all produced by plants or animals. For them to exist on another planet, therefore, that planet must have evolved its own plants and animals that produce those same molecules: Parallel evolution might well come up with something similar or identical to Earthly forms, but just as none of us seriously expect to find humanoid aliens with easily recognisable body parts and features, I think we're unlikely to find anything with identical or near-identical bio-chemistry.

I guess we'll never be sure until we get to another planet and find out, but I think it's likely that stuff evolved on another world would be inedible to us.

On the other hand, the bio-chemistry is almost certain to have *some* similarites: Oxygen is almost certainly going to be part of the equation, because no other element is at once stable and volatile enough to contain and then release the energies needed for life. Hydrogen is another likely contender because (A) there's bloody loads of it and (B) it comes packaged up with the oxygen in the form of water. Carbon is likely to be there as well, because not many other atoms can form themselves into such complex molecules. That gives you all the necessary ingredients for carbohydrates (edible sugars). Granted, there are probably a zillion ways those three elements could combine, but I guess it's reasonable that our survivors might find something that could be boiled down into edible carbohydrates.

Finally, there are some simple nutrients we ingest that are available in non-organic forms and therefore would be easy to find on another planet: iron, zinc, magnesium (calcium?) could probably be sourced locally- daresay you could dissolve them into drinking water, if your local friendly mountain stream hadn't already done the job for you.

Anyway, for plot purposes it's quite an interesting question. Anyone with some actual knowledge rather than my pop-science want to chip in?