Fascinating topic, and one that I’ve been studying for some decades. (As an artist with some scientific training and interest, not as a scientist.)
I’m assuming that by “human variation,” John Keates means normal variation. Not deformity, but something along the lines of “why do some people have blond hair, when most don’t? Why do some have epicanthal folds? Why do some noses have high bridges and others have low ones?” and things of that nature. I apologize if I’ve misunderstood. I confess that I haven’t read all the posts.
It’s my contention that people know a lot more at an unconscious, emotional level than they do at a cognitive level. That’s why we have visceral, emotional reactions to things, but can’t articulate quite why it’s striking us like that.
As artists, it’s our job to manipulate emotional reactions; to get people to feel what we want them to feel when they look at our work. The more we understand about those things that most of us know only unconsciously, the more we can use those things in our pictures, and the better we are at our job.
So it pays to know as much as possible about why things are the way they are. I believe that’s as true for why humans look the way they do as for anything else.
It’s not so important when we’re using live models or doing portraits. We can just look at the person in front of us, and the only things we have to decide are what features to emphasize, and which to downplay (as previously mentioned.)
But when we start to design people, especially non-human people, to fit into their environments (or not, as the case may be,) then it is, I believe, vitally important to know what combination of attributes will do that.
And not just human attributes. We can incorporate non-human variables, too; but only if we understand them.
For instance, it has been discovered that there are clusters of attributes that show up together, for whatever reason. (I’m not a geneticist, so I’m not going to talk about genes and alleles.) But, for some reason, when animals are bred for docility, they also come out with short legs, short round snouts, droopy ears, soft hair, curly tails, and piebald coats. (Research from the 40+ year Belyaev Silver Fox experiment.)
We can use this.
If you want a figure to look docile, friendly, and non-threatening, give him short arms and legs, a short face, bulbous nose, soft hair, and round, floppy ears. On the other hand, if you want him to look wild, and a little dangerous, give him long limbs, a long face, stiff hair, and sharply pointed ears. Freckles for the funny guy or hair that’s darker on the top and lighter on the bottom for the wild fellow can underline the effect.
Human variations are based on several factors; genes, environment, and the way that the person habitually holds their face and body.
The habitual stuff can be a product of the environment, for example, the “seaman’s squint” (holding the lower eyelids partially closed to protect the eyes from reflected glare off the water.) Or it can be learned behavior, for instance the way the British Aristocracy habitually hold their chins and eyebrows higher than average, which gives them a very distinctive “look.” Or it can be the result of their normal emotional state, for instance, someone who seems to be always scowling, or looking worried. All of these leave marks on the face, over the course of years, and are what gives an older face its “character.”
We have learned that, at a genetic level, there’s no such thing as “race.” And yet, most people from central Africa have quite dark skin, and people from China have epicanthal folds. This isn’t because they are closely related; they aren’t. It’s because the environment left its stamp on the people living in it, over the course of many generations.
Skin color is a response to ultraviolet radiation. Where there’s a lot, the skin has a lot of melanin, to protect the deeper tissues from damage. Where there is little, skin has lost its melanin, so that enough can penetrate to allow the formation of precursor vitamin D3. (Research from Jablonski and Chaplin, 2000.)
The epicanthal fold allows an extra layer of fat to surround the eyeball, protecting it in areas where the temperature becomes extreme, like the interior of China. In fact, there is an extra subcutaneous fat layer all over the body in Chinese people, because of the extreme cold of the winters in the interior. This is why their muscle definition is softer, and their surface veins not as prominent, as, say, people from Africa (where cold isn’t a problem.)
If anyone is interested, I can give many more examples; but this post is already quite long. The important thing is that we can use these facts to tell the story in our paintings and animations.
If you have two picutres of humanoid people in a forest, and one is very pale, while the other is deeply pigmented, the people looking at the picture will get the impression that the first is in a Northern forest, while the second is in a hot, tropical setting. This will happen even if the trees aren’t at all like trees on Earth, and the heavily pigmented individual is purple or green. The second will just “feel” hotter than the first.
You can make this even clearer by putting small leaves on the trees in the first picture, and large leaves on the plants in the second, because these variations are as true for plants as for animals.
So, ummmm, can we please stop arguing about what variations are esthetically pleasing, and talk about where the variations come from? 
