I've made up alien races for RPGs for a long time, and here are my own guidelines:
Based on essays by Isaac Asimov ("Life as We Do Not Know It," and "Life Somewhat As We Know It"), I think there are four basic bio-chemistries, depending on two flags -- water-based or ammonia-based, and hydrogen-breathing or oxygen-breathing.
Water-based oxygen-breathers we know about.
Other respiratory gases look unlikely because of problems of cosmic abundance. There just isn't much chlorine around, for instance. But there's loads of hydrogen. The question is, are there planets that have hydrogen but have enough of anything ELSE to build life? Look at the gas giants in our system. They're almost all hydrogen; the challenge is to scrape together enough non-hydrogen gunk to make life, or for that matter rocks. However, we are just beginning to explore the range of possible planet types, so there may well be hydrogen-breathing life out there. It might even be on Jupiter, if that planet is more complex than a ball of wind.
Ammonia is the best alternative to water because, like water, it is a polar moelcule and so will assist organic compounds to dissolving and reacting. So there may well be ammonia-based life. But I bet it isn't as common as water-based life. First, oxygen is more cosmically abundant than nitrogen, so there is jsut more water than ammonia. Second, ammonia has a smaller range of temperatures in which it is liquid. Third, frozen ammonia is denser than liquid ammonia, so ammonia oceans might have a tendency to acumulate ice at the bottom and become mostly frozen.
Of course, it is fun to get wilder and speculate about cryonic creatures based on superconductivity and the properties of superfluid liquid helium, or "energy beings" made of plasma and living in stars or nebulae. But in those cases, dramatic encounters with humans will never be face-to- face but ship-to-ship, or telepathic, or with one party or the other suited up (or equivalent) ... or purely by communications medium, e.g. the Internet.
Many years ago, Poul Anderson wrote a book called "Life on Other Worlds," an extended and intelligent speculation about aliens. In general, Anderson on aliens, either in fiction or non-fiction, is worthwhile.
He pointed out some general considerations for the design of an intelligent being:
They are unlikely to be sessile. Plants and polyps just don't have that much to think about. They are unlikely to be slow-moving like starfish. An that means they are unlikely to be radial in symmetry like a starfish. If they are active and quick-moving, they probably have a front end they can point quickly in the appropriate direction rather than having "front all the way around."
If they have a front, gravity leads to a top and a bottom, and you have the basic bilateral symmetry of most motile Earth animals.
Anderson suggested a mass range between 10 kg and 1000 kg. That is, between a terrier and an elephant. Plenty of room for creativity there.
Anderson suggested they were likeliest to be omnivorous, next likeliest to be carnivorous, but herbivores were not impossible. I think the odds are even closer than that. Gorillas and orangoutans are essentially herbivorous, and are among the smarted critters on Earth.
He thought they would have sexuality, though it certainly need not be our kind.
He suggested they would probably be warm-blooded, and probably produce only a few young, since hordes of young suggest little interaction with the parents and thus little transmission of culture.
So what does this leave us? A fantastic range of possible variety. Erect bimanual bipeds like us obviously fit in, but you can look VERY weird and still be an erect bimanual biped. Other plausible forms are centauroids, elephantines, supine bipeds (like birds and bipedal dinosaurs), and many more.
Add to this the range of sizes, and things like fur, feathers, crests, coloration, muzzles, tusks, tails, and so on, and you have so many variables that you are NOT going to get a trekky "humanoid" unless you invoke someone like Grandfather to produce them deliberately.
Also, all Anderson's recommendations (like Asimov's about biochemstry) are nothing more than educated guesses. Pick one and think up a good reason to violate it, and you get a very interesting alien, though one that is probably exotic by galactic standards.
One more message to go.
Earl Wajenberg
Several people have already recommended looking over Earthly animals and foreign cultures for the bases of alien ones. I second that and strongly recommend reading any of the more popular books by Konrad Lorenz, one of the founders of ethology, the science of animal behavior.
Not only are these very readable and interesting, Lorenz is always making connections between animal behavior and human, firmly convinced that animals are more "aware" and humans have more instincts -- and so we are closer together -- than popular wisdom would have it.
Applying this to alien-building:
Carnivores need more land per head to support themselves. Thus they are likelier to be touchy about matters of privacy and property than us omnivores, and WE are touchier on these subjects than herbivores. (A grossly general trend. But the major point is that, for whatever reason, territoriality is one point on which species are seen to vary a lot.)
Animals with built-in weapons -- like fangs or horns -- generally have built-in restraints about using them. Wolves will snarl and snap, but generally don't REALLY tear out one another's jugulars. Otherwise, there would be no more wolves. Humans, with niddling little teeth and fists, can batter away at each other for a long time before doing serious damage. Unfortunately, our instincts don't change when we pick up a spear or a gun, with results you can see in the headlines. The upshot is that a ferociously -built race, e.g. the Aslan, may bluster MORE but be actually SLOWER to attack than a creampuff species. So beware the bunnies from Beta Lyrae. And Niven's Kzinti are probably way off base.
Other instincts to "tune" are hierarchy, sociability, mating cycle, hunting instinct, and migratory instinct.
But remember, when these are part of an intelligent species, you have a complex chunk of culture, not a mindless impulse. Humans have a mating instinct, right? But most of us don't go about it like alleycats or salmon. So a race with a strong migratory instinct would not just mindlessly wander, but would, say, have an elaborate tourist industry.
To give another source besides Lorenz, try "GURPS Uplift" from Steve Jackson Games (if I may mention a competitor here), based on David Brin's Uplift series, and with original material written for it by Brin.
Earl Wajenberg
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Joseph Heck
(joe@mu.org)
21 August 2000