QUOTE(LessHorrid vanU @ Fri 8th January 2010, 5:18pm)
Also, if you believe in the mechanics of evolution then other world intelligent life is probably going to look something like us generally - binocular vision (and thus upright, and likely bipedal), with focusing eyes and colour vision, grasping appendages at end of non-locomotion limbs, warm blooded oxygen breathers, communications based on sound production and reception at least in part, gender differentiation (may be more than two, but that would mean that the entire higher animal structure would be the same), food ingestion and digestion - and waste disposal.
There would be variations, of course, in height, width, "skin" texture (panting or heat gilled aliens would not need sweat pores, for example, in heat regulation), but even things like skin colour - if vitimin D production is required - will not be too dissimilar.
The markedly thing that will make them "alien" is the culture that shapes them - they same thing that makes 'alien' other human cultures today.
Bleh, the reason we're bipedal is an accident of tetrapod design that "froze" us into having only 4 limbs to work with. Not good. Six (at min) are clearly better. That gives you 4 to run with (never try to outrun a canid-- they're just better, that's all), and two for manipulation (the centaur design, basically). Horsey should approve. The same thing happens with bats and birds-- they ended up without enough limbs to work with after losing two for flying. But a bat with four legs like a tree-squirrel would clearly be a formidable critter.
In a 3-D world you do need at least 2 limbs for manipulation, and indeed squids and octupi, with many limbs to choose from and no bilateral symmetry, still have 2 semi-dedicated graspers. Still, it might be useful to have 3, just to have 2 to hold something and a third to mess with it. I've wanted a third arm since I don't know when.
Eyes? Again we're stuck with 2 only due to a bilateral symmetry design, and there are critters like jumping spiders which have a bunch of accessory ones. You need two for depth perception, but a rear-view eye would help a lot at airports and other places where some boob stands right in the middle of a walkway looking at something, not realizing there's a giant pile-up of traffic behind him.
Vitamin D??!? Another accident. It takes UV to make it everywhere on Earth, but again there's no chemical reason for that. Some other molecule would be doing the same job.
It does seem likely that "animals" are going to need some powersource better than photosynthesis (which is very sparce in energy production for terrestrial planets-- not enough to move very fast), and oxidation of plant fuels collected slowly by "plants" looks like it might be a winner in a lot of places. So I would guess there are a lot of tube-animals out there with "mouths" and "anuses," that need to "eat" solids. On the other hand, there are animals like tube worms on Earth that use O2, but "eat" only small molecules like fungi do, so even solid food intake for an animal (at least a water animal) is not a given (I don't know how intelligent tubeworms insult each other, but not by suggesting to each other that they've mixed up ass and mouth; since they have neither).
Our multisegment design also gives us a directionally polarized central nervous system (not like a hydra or sea urchin) which again gives us a brain at one end, where it ends up being rather exposed. That could happen better-- we "wear" our brains kind of out in in the wind, and it would be better to have them centrally located and better protected, like our hearts are. You can always run nerves farther from your sense organs back to the middle.
Eventually we'll get to the point that we'll have full control over biology, and will be able to try out all these designs ourselves (genetics and body plan becomes a matter of art and culture). So humanity will be able to try on all these designs for itself. The thing we perhaps need to realize about aliens with star travel tech is that they won't look like anything in particular-- they'll look like anything they want to look like. Squids and whales for exploring the ocean, 6 legged birds and centaurs for working on air and land, and so on. The frozen accidents from old evolution that has build our present bodies from the bricolage of past junk, will no longer have to be tolerated. We can start over, and will.
I'm not even sure it will be worth meeting aliens for their different cultures. At the thinking speeds of our future, any separation of minds will result in our own cultural evolution which will speciate cultures far faster than we can find new ones by looking through thousands of light years of space. Probably as strange aliens as we'll ever meet, is when we go someplace for some number of clock cycles, and then come back and meet our isolated selves again. By that time totally transformed, and as unrecognizabe as you like.
MR