ASK A SCIENTIST
Question: Why do people only have brown, black, blonde, red, white or gray hair instead of bright colors like purple, green, blue or pink?
Answer: It is always much easier to answer a question about why something exists than to settle on why something does not exist. But let's see what we can do. In the case of hair color, since we are mammals (and all mammals have hair or fur), we might ask whether other mammals have any of these unusual hair colors. There are a number of animals that have blue skin and fur. For example, the mandrill monkey (baboon) from Africa has a red and blue snout and its hind quarters have pale blue fur. Also, the wooly opossum from Australia has a pale blue-gray fur. Skin and hair color results either from pigments or from light reflected and scattered in unusual ways. In the case of blue fur, it results from blue light being enhanced and other colors in light being scattered. We do not see this color in people because our hair is not uniform in its pattern of distribution to allow that kind of light scattering. Green fur is found in the vervet monkey from Africa and in the sloth from South America. The vervet fur is probably a combination of blue light being scattered and some yellow pigment. The sloth, on the other hand, has a green tinge because algae grow on its fur during the wet season. This probably indicates how difficult it is to produce green color naturally in fur. Pigments in the skin can show through hair or fur to make it appear colored. Birds get some of their red colors from eating foods with carotene, a red/yellow pigment. This even happens in people: when my son was a baby his skin took on a distinctly yellow appearance because he only liked yellow foods such as sweet potatoes, carrots, and squash, all with lots of carotene. Purple fur does not seem to appear in mammals, but pink or pale red fur does.
Now, as mammals, people do have a remarkable array of hair colors compared with other species. Our hair colors result from two pigments: a brown melanin (eumelanin) and a red melanin (pheomelanin). The tendency toward having none, one, or both of the pigments in hair is inherited from our parents. Human hair color ranges from black (lots of eumelanin) to ash blonde (no melanin) and from bright coppery red (lots of pheomelanin and a little eumelanin) to pale pink blonde (just a little pheomelanin). These hair colors mix, so someone may have dark brown hair with red highlights or light brown hair with rich red highlights. Brown hair is found all over the world; blonde hair is found mostly in Western Europe but also in Australian Natives; and red hair, although common in Scotland, Ireland, and parts of Eastern Europe, is also found in Africa and Australia. Recent genetic studies have shown that even some Neanderthal people from thousands of years ago had red hair! Why we do not have other colors is probably because it is difficult for our bodies to produce the chemicals needed for unusual hair colors. We do see people with green hair from time to time, but this comes from light blondes being exposed to too much copper in the water. I suppose that if it were possible for people to have produced these exotic hair colors naturally, that is, for them to have evolved, it would have happened. People seem to be attracted to others with strange hair colors, as we can see from all of the different hair dyes on the drug store and beauty parlor shelves.
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