In my earlier article about varieties of alien life we may encounter in the future I strove to illustrate that it is difficult to impossible to predict what form it may take especially when we see the vast diversity of organisms we find right here on earth. But it does serve us to note that there does appear to be advantageous to certain standards of adaptation. For instance at least on this planet it seems to be preferable by the laws of our adaptation to have bilateral symmetry (ie. two arms, two legs etc). This serves us well for walking, dexterity and manipulation. But it behooves me to add that science fiction writers (Star Trek in particular) have developed beings with massive and complicated intelligences with many limbs which are individually directed to perform separate tasks. Bearing in mind that just about anything is within the boundaries of possibility as far as adaptation goes, we have to be imaginative but try to maintain a modicum of restraint.
It also seems to be quite advantageous to have stereoscopic vision and hearing as we have. The old fantasy novelists loved to write of three-eyed monsters or cyclopses and the fact is that such things carry disadvantages. Two eyes spaces equidistantly as do ours offer us depth perception and stereoscopic view. Three eyes would not allow for this and would create distorted and imbalanced perceptions. A single eye in the middle of the head (or anywhere else for that matter) would not give depth perception and carry a much shallower field of vision. So it is safe to say that binocular vision is most likely what an advanced alien race would possess. We can also consider what variations would occur to an alien’s vision if his planet was closer or farther from its star. If daylight was brighter there it might have an intrinsic filter to deal with this or if its light was dimmer than ours their eyes may be larger to allow in more light.
The effect of gravity is also something to be considered. If a planet’s gravity was lighter than ours then it is likely that this would give form to a taller stature than ours with a more slender structure. On the antithesis if a planet’s gravity were heavier than ours then it is safe to say that its residents would have developed shorter and more heavily structures.
Along these lines if a planet’s atmosphere were thinner than ours then its inhabitants would almost necessarily require some adaptation for that such as larger lungs to process more oxygen. It could also have developed some sort of organ that would store oxygen and distribute it efficiently throughout its body. On the other side if a world’s atmosphere were thicker than ours then the opposite would likely be the case.
Although I doubt that we will encounter three-headed fifty foot tall races with one thousand arms, the fact remains that if we just look at the variety of life right here on earth it is clear that all sorts of combinations are possible…Jersey Devil not withstanding that is.