Variation is everything in terms of a species. As competition increases and the struggle for survival and reproductive success heightens, variations will be weeded out and some will continue and others multiply. As humans have evolved, we have maintained a degree of variation that we all recognize. In human terms we refer to this variation as race.
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Branching a major theme of evolution. Everyone agrees that all human beings are members of a single biological species - Homo Sapiens, Sapiens. We also recognize that there are geographical variants in terms of skin color, hair form, facial morphology, body proportions, and a host of less obvious traits. Race, therefore, is one's variation relative to all other humans created by the interplay of geography and inheritance. Race, in this sense, is defined by a set of physical characteristics that cluster together with some degree of predictability within distinct geographic regions. Asians, for example, are predisposed to have "yellow" skin coloration, wide, flat cheekbones, epicanthic fold (skin folds are the edges of eyes), straight black hair, sparse body hair, and "shovel-shaped" incisors (your front teeth). If you placed yourself in any city in China, you probably would find almost everyone conforms to a large degree to this definition. However, place yourself in a city such as Bangkok in Thailand or Tehran in Iran and you still find yourself in "asia". But, the traits would be less distinctively "asian" in both cases. If we take just the epicanthic folds around the eyes, we would possibly place Khoisans (Bushman from southern Africa) as "asians" since they too possess these distinctive traits. Shovel shaped incisors also occur in relatively large numbers in Sweden as well as among North and South American Indians.
So what is our conclusion? The straightforward biological traits of human variation do vary from region to region but they do so independently and not as sets. More importantly, there is no trait or set of traits that are inherently associated with one of these varieties of humans. There is no "race gene".
We are highly visual animals and the color of one's skin is one of the first things we identify. It is a superficial difference between us very much like nose shape, the texture of our hair, or other physical characteristics. Obviously, differences in skin color often play a role in terms of prejudice. Studies in pyschology have shown that the darkest skinned children in a single family are treated less well than other children by teachers, their peers, and even their parents. So what controls our skin color. The answer is melanin.
We know that in mice there are more than 50 different genes that influence how meaning forms and when and where it's deposited. It is likely that a similar number of genes will occur in humans. All the colors of melanin have a common amino acid that in turn forms a compound called dopaquinone. It appears that dopaquinone follows two routes - one leading to black and brown pigments and other to red and yellow pigments.
The principle enzyme in all this is something called tyrosinase. If the gene for this enzyme is defective, the result is a person with albinism, someone who makes no melanin at all. Perhaps, more significant, geneticists have discovered that most of us have enough of this enzyme to make us black and very black in deed. In those people with light skin, something prevents the enzyme from functioning at full strength. There is apparently a switch that creates this limiting factor.
So it turns out that what separates Africans from Europeans is not different numbers of genes or certain genes that provide pigmentation, but rather a collection of tiny genetic differences in the way our genes interact and regulate each other. This provides the chance for mutations to play significant roles in contributing to color variations.
In animals, melanin comes and goes at the dictates of evolutionary pressures. Coloration is one of the easiest changes for organisms. Some of the most darkly pigmented people in the world live on the Solomon Island. They rarely have any skin cancer (carcinoma or melanoma). Europeans living in Hawaii, on the other hand, have the highest documented skin cancer rate in the United States. It is clear that darker pigmentation protects from skin cancer. It is less clear how humans benefit from lesser amounts of pigmentation however. The most popular theory is based on the fact that the exposure of our skin to ultraviolet light is necessary for the formation of the precursor of Vitamin D. Vitamin D is necessary for proper bone formation. Lighter levels of pigmentation at higher latitudes enables the processing while higher levels of pigmentation near the equator prohibits the production of too much. In evolutionary terms this makes sense.
It is worthy to note that there are people that have evolved toward lighter or darker skin color than that of their closest relatives. The Negritos of the islands of Luzon and Mindanao in the Philippines, for example, superficially resemble other dark-skinned peoples living in Africa and Australia. Yet their overall genetic affinities turn out to be far stronger to the lighter-skinned Asian peoples who surround them geographically. This suggests that these Negrito peoples descended from a lighter-skinned ancestry and that they independently evolved toward people of Africa or Australia in terms of pigmentation. This suggests again that it is relatively easy for human populations to evolve colorations dependent upon evolutionary processes.
Did you know that you possess melanin in your brain? You do of course. It is one of thousands of compounds unique to the brain. We have very little idea as to what it actually does. We do know that a lot of it is found in an area of the brain called the substantia nigra (translated as "black substance" due to the dark coloration of this part of the brain.) It is also known that this area of the brain is most likely destroyed in people who have Parkinson's disease. Other than that, nothing else is clear except that this melanin, called neuromelanin isn't obviously related to skin pigment (or anything else that one can tell.)
Interesting as it may be, there are indications that people with lots of skin melanin are less prone to hearing damage than more lightly pigmented individuals. It is also known that melanin of the skin variety is indeed found in certain cells of the inner ear. How these two things relate is unclear. Melanin also has been connected with a benefit to smokers. Tobacco smoke tends to stimulate production of skin melanin, particularly in the cells lining the mouth. One medical study has actually suggested that smokers have less noise-induced hearing loss than nonsmokers. While this relatively insignificant benefit of smoking does not outweigh the negatives, it is an oddity. It may mean only that smokers can hear the wheeze of their lungs as they cloud over with smoke related damage.