Harry E. Keller

Cred: Chemistry professor, Northeastern University

'Somewhere in the universe, there must be, have been, or will be other advanced civilizations. The chances of anyone alive today seeing or even communicating with them are very slim. However, such problems should not stop us from imagining what they might be like.

"First, a real alien is bilaterally symmetrical, endothermic with excellent manipulative abilities, and has a hard container for a brain.

"Eyes? Of course. You have to see to build a civilization. The squid has better eyes than we do, and the eye seems to have evolved separately a few times at least. Our alien will have eyes that may only resemble ours superficially. A lens and iris are almost an absolute requirement. The whites do not have to be white. Neither does the iris have to be colored in anything like the way ours are. How many eyes? One just will not do. Inability to perceive distance will lead to rapid extinction. Unless you can find a good reason for it, extra eyes create evolutionary baggage and will not persist, spiders not withstanding. Eyes being important, they will be recessed and capable of being covered, as necessary. Our aliens are not likely to be bug-eyed, although we cannot totally rule that out.

"Nose? An alien must have the means to breathe. It must be symmetrical. The nose does not have to be in the middle of a face, although that is convenient for some purposes such as sharing the air pipe with a mouth for times of exertion. You can imagine other arrangements.

"Mouth? Must have means for ingestion of food. All large animals have some means to capture and swallow food. Some masticate it; others do not. Teeth are rather common on Earth but other systems of eating are possible. The problem is that plants and animals have been in an evolutionary war for millenia. We inherit the outcome of that war. Teeth were necessary to eat plants that became more fibrous, an evolutionary reaction to being eaten. There are other ways to sheer off plant food, such as what birds do. They 'chew' in their gizzards.

"Legs? Most certainly. Other means of locomotion will be relatively slow and be relegated to armored animals and of those living in holes. Neither of these lifestyles will lead naturally to greater intelligence. How many legs? In our case, we adapted the forelegs for manipulation from animals with four legs. No land animals with endoskeletons have more legs. The implication is that aliens with two legs are more likely than those with four.

"Fur? Hair? Feathers? Scales? Something else truly alien? The skin covering could be anything that makes sense. Real feathers are not so likely. Because feathers are used for flight, brains will be small. Scales are unlikely because they are particularly well-adapted for ectotherms, although a scaly sort of skin might be possible. Fur versus hair is hard to sort out because of my inherent bias. Fur has good reasons for being that are not related to intelligence. If furry, our aliens probably have short fur.

"Head? If you define head as a bony enclosure for the brain as well as the location for eyes, nose, and mouth, then it's likely. The nose could be located above or below the mouth. Above works better for swimming, but that's not a strict evolutionary requirement.

"Neck? Not at all necessary. Useful to swivel the head, but you can imagine other ways to look around. It's an annoying place for big cats to grab and strangle you with their powerful mouths.

"Arms? You must have something with which to manipulate things. Tentacles are unlikely because of their inability to perform fine manipulation. They usually have suckers on them and a very different purpose than toolmaking. The number of joints could be greater, and the direction of bend could be different.

"Fingers? You must have a way to grab tightly and a way to pick up small objects and twist and turn them. Two fingers are probably too few. Five is overkill. Three or four (as cartoon characters have) is about right. Our three joints in our fingers is about right. One would not do. Two, as with our thumbs, is unlikely."