...Biped works. But so might many other solutions. And nature always settles for the barely adequate, and doesn't plan ahead...
Timo Saloniemi
Nature seems to settle for what ever mutation can survive. Did a spider really need eight legs when so many six leg critters do so well? Probably not, but the spider happened to come about and was not disadvantaged by its eight legs to the point that it died off.
Nature selects for mutations that increase reproductive success. This can one that increases survival probability, but more often means one that increases the probability of getting laid. The spider's extra legs are a useful mutation that increase it's probability for survival, mostly due to it's size. They'd be totally useless if it were as large as a dog. Very small creatures benefit from extra legs. Larger ones don't.
Those mutations which don't provide any reproductive advantage will eventually be bred out, simply because the specimens without the mutation greatly outnumber the one with it. They certainly won't lead to specialization, not without the long-term isolation of a small population that has the muration
There are three requirements for any technological civilization. 1) Tool Use 2) Tool Crafting 3) complex socialization.
Tool use implies grasping hands of some sort. Tool Crafting implies both hands and a certain amount of intelligence. Complex socialization implies the ability to form strong emotional attachments to others, an intrinsic emotional need for social contact, and an intrinsic aversion to killing their own kind.
Such creatures will most likely be bipedal or semi-bipedal, because it is easiest for hands to adapt from forelimbs. This also means that they will most likely not be aquatic, though they might be amphibious. They probably won't be giants, nor will they be very small. They will probably breath oxygen and they will probably have lungs, simply because other methods of oxygen extraction from air don't work well at a reasonable size. They'll have some sort of circulatory system, though possibly not like ours. They will either use iron or copper as an oxygen carrier in their blood, with iron being far more likely because it is more efficient.
Their skin pigmentation will probably depend on the amount of UV light they are exposed to. They may or may not have fur or scales, or armor, but will not have any on their palms or fingers due to the need for precise tactile sensation. If they live in cold climates, they may be very fat, with thick insulating layers just under their skin.
They will probably have an endoskeleton of some sort, probably made from calcium but not necessilarilry.
They'll probably be omnivorous with predatory tendencies, simply because hunter-gatherers are most likely to develop grasping hands and tool use in the first place.They will have some form of complex communication, with spoken language being the most probable due to not needing line of sight, but with a visual language also being possible. If they have spoken language then they will have mouths and vocal cords adapted for speaking, which means that they won't have muzzles or beaks and are likely to have lips like ours. They will have a written language, simply because this is necessary to transfer and preserve knowledge within a large complex civilization. They will live and work in groups of some size. They'll be curious. They'll have weapons. They may or may not engage in warfare, though this is more likely than not.
They will not be able to eat our food, nor we theirs, with any degree of safety. Protien incompatability is highly probable. The most likely result of eating anying from an alien ecosystem is death, possibly a horrific one. Our bodies wouldn't be able to use their protiens and they wouldn't be able to use ours. The best case scenario is simply not getting any calories from the food, the most likely is a chemical chain reaction that causes severe damage on a celular level.
They probably won't fly.
