Overused Silly Sci-Fi Science.

[Ekaros]

Originally Posted by VydorScope

To be fair I would expect this to be true. Not because all nonterrestrial life eats/breaths/etc the same as we do, but because we would for the most part only interact with the subset that does. Logically we would not travel to a place like Jupiter to have tea with the natives. We might talk with them via some communications system, but in that case the eat/breath/etc stuff does not come in to play.

Still, I find it questionable there isn't some toxins, bacteria(specially digestive), yeast(other fungus) or enzymes which would be bad for species which systems haven't had experience with them. Just think how regular bacteria from different area can cause diarrhea and there is same ancestors for them, something from different ecosystem could be much much worse and some species might not be able to adapt for such...

[dworth]

It might be a case of one extreme or another. Bacteria and viruses on Earth have evolved to infect particular species and use them to breed and spread to others. It might be the case that you could no more catch alien flu than you can catch fowl pest from a chicken. Or: if somehow they were able to infect you, your body would have no defence whatsoever against a new disease and the result would be a repeat of what happened to the Aztecs when they caught measles from the Conquistadors, a disease hitherto unknown in the Americas.

Food from other planets might also prove to be indegistible. Not toxic, perhaps, but your gut flora might not be able to break down unfamiliar cells and compounds. It might be like trying to eat wood. It wouldn't poison you, but you would get very little nutrition from it.

[Steven Lyle Jordan]

Originally Posted by dworth

Or: if somehow they were able to infect you, your body would have no defence whatsoever against a new disease and the result would be a repeat of what happened to the Aztecs when they caught measles from the Conquistadors, a disease hitherto unknown in the Americas.

Food from other planets might also prove to be indegistible. Not toxic, perhaps, but your gut flora might not be able to break down unfamiliar cells and compounds. It might be like trying to eat wood. It wouldn't poison you, but you would get very little nutrition from it.

I find both of these to be a very likely scenario, even if we should find a planet almost exactly like Earth and with vegetation and a humanoid population. (Not that I expect an Earthlike planet to be a likely scenario...)

[ScalyFreak]

Originally Posted by Ekaros

Still, I find it questionable there isn't some toxins, bacteria(specially digestive), yeast(other fungus) or enzymes which would be bad for species which systems haven't had experience with them. Just think how regular bacteria from different area can cause diarrhea and there is same ancestors for them, something from different ecosystem could be much much worse and some species might not be able to adapt for such...

HG Wells had that feature pretty prominently in one of his books, as I recall... It's astonishing that so few other sci-fi writers seem to think about this.

[QuantumIguana]

Each of us as individuals may need day and night, but we have many institutions that go about their business day and night. On a space station, ships would be arriving at all hours of the night. Their crews are going to want some place to spend their money, I would expect the Promenade to be open 24x7, especially on a space station where day and night are totally arbitrary.

I think that we would be safe from alien viruses, we would be too alien for a virus to infect. Bacteria are a different matter, they might pose quite a problem. We might be able to eat alien food, it would likely have the proteins, carbohydrates and fats that we need. But there might be nutrients that are lacking, or their could be toxins. For example, vitamin C might be unknown on some alien world, and we would have to bring our own. On the other hand, vitamin C might be a toxin to some alien species.

[Penforhire]

On the other hand, since we haven't been able to find much life different than ours then perhaps it is natural to assume even more compatibility. Who's to say which assumption is more correct?

Also, even plain human diseases taken from a different time and place tend to ravage us. It was one of the ways the New World was conquered (through the spread of European disease).

[Steven Lyle Jordan]

Originally Posted by Penforhire

On the other hand, since we haven't been able to find much life different than ours then perhaps it is natural to assume even more compatibility. Who's to say which assumption is more correct?

Every creature on this planet has a common biological heritage, obtained from the chemical makeup of this planet. I wouldn't assume any other planet would duplicate Earth's chemical makeup, so I wouldn't assume any organism that developed on that planet would duplicate our biological makeup.

We already know that all terrestrial organisms use a "right-handed" molecule configuration, though why that is, no one knows. Organisms featuring "left-handed" molecules would not be able to interact with us on a molecular level; foodstuffs would pass undigested through us, antibodies would not be able to recognize alien bacteria, etc.

And there are so many ways in which even the same chemical organisms can be different from ours; imagine, for example, an alien race that had a protein-based chemical composition, but in which those proteins were straight, not tightly twisted like ours? It would require a very different chemical system to process those proteins, and that could determine the difference between a being similar to us, a plant, or a sentient rock.

One of SF's favorite tropes is of finding things that are "like us." It makes for easier storytelling. I think it much more likely that anything we find out there will be overall nothing like us.

[QuantumIguana]

Viruses are very specialized. A human is never going to become infected with the tobacco mosaic virus, that's too big a jump to make. Yet a tobacco plant and a human are at least distantly related, an alien species will be totally unrelated. Viruses can make a from one animal species to another, but it isn't easy. Living in close proximity to animals can give the virus enough opportunities to hit on the correct mutation to infect humans. Think of a virus like a key used to unlock the DNA of the cell. If the virus doesn't have the right key, it can't infect the cell. They key required to infect a pig is a bit different than the key required to infect a human, but a fortuitous (from the viruses perspective, that is) mutation can leave it with the right key to infect a human cell. The key required to infect a cell from a tobacco plant is so different than the key required to infect a human, that it is just not going to be able to mutate enough.

A bacteria just needs someplace warm, wet and with food.

[Penforhire]

You mean like avian flu, which DID jump species?

[QuantumIguana]

I did say that viruses could jump between species. That's the downside of raising animals, it puts us in close proximity to animals, and gives the virus opportunities to mutate into a strain which can infect humans. Viruses pass between humans with ease, but pass only between species with difficulty. Viruses can mutate from an avian virus into one that can infect humans only because humans and birds are related, if rather distantly.

An alien would be utterly unrelated to us. Think of a virus as a computer program. A program written for one version of Windows will run properly on the intended version (or at least it should). It might not run so well on some other version. Maybe it will run on Windows 98, maybe it won't. But it won't run on a Mac at all.

[Penforhire]

You're still assuming aliens would be, well... alien. I'm not saying it is a terrible assumption but our experience suggests the opposite isn't a terrible assumption either.

Carbon-based energy cycles (making and breaking those bonds) line up nicely with the amount of environmental energy available from our class of star and our rough orbit (as opposed to, say, silicon). Molecule handedness may be equally necessary.

All I'm pointing to is this particular SF trope may just as easily turn out to be more natural law than SF fallacy.

[VydorScope]

Originally Posted by Penforhire

You're still assuming aliens would be, well... alien.

Star Trek theorizes that all alliens in their universe share a common ancestor. Some modern scientist today believe life on earth was seeded from space, via a comet or something. So I could easily see how some would say that the Star Trek idea is possible/reasonable. In that case, then the vast majority of popular sci fi writers are vindicated.

[altworld]

Originally Posted by VydorScope

Star Trek theorizes that all alliens in their universe share a common ancestor. Some modern scientist today believe life on earth was seeded from space, via a comet or something. So I could easily see how some would say that the Star Trek idea is possible/reasonable. In that case, then the vast majority of popular sci fi writers are vindicated.

We are all Cylons!

[Ekaros]

Originally Posted by Penforhire

You mean like avian flu, which DID jump species?

Yeah, but only because lot of genetic material is same. Viruses need host with suitable material to transfer, they can't replicate by themselves they need host. Without common cell and proteins they can't do it. So alien virus which doesn't have link to same genetic material has no means to replicate inside human.

Whole Star Trek theory is bit questionable, how all the species reached same level in relatively same time, or how to different race can have childs... But on otherhand it's not anyway hard-scifi...

[WT Sharpe]

Originally Posted by ScalyFreak

HG Wells had that feature pretty prominently in one of his books, as I recall... It's astonishing that so few other sci-fi writers seem to think about this.

Of course, the fly in Well's ointment was that no microbes of any type had ever evolved on Mars; a rather fantastic proposal. That the Martians could have been destroyed by Earth bacteria which was foreign to their systems is perfectly reasonable, but for a species to have evolved on a planet where no bacterial life of any kind had evolved is a stretch.