The color of human skin is determined by the amount of two pigments, eumelanin and pheomelanin. These pigments are the basic ones underlying all kinds of coloration in animals — even blue colors like those in the irises of blue eyes result from light reflecting above a layer of dark brown-black eumelanin. The darkest human skin and hair tones contain an abundance of eumelanin, while brown and reddish hair and freckles of the skin contain a large proportion of pheomelanin.
Genes can influence skin and hair pigmentation in many ways. The overall color of the skin results from both the number of pigment-making cells (called melanocytes) and their level of activity. Most skin is capable of tanning, which means that exposure to UV radiation induces greater melanin production. Today, more than 20 genes are known to influence skin pigmentation in humans. Genetic changes can alter the development and migration of melanocytes, the regulation and expression of genes that generate melanin, or the chemical steps in the synthesis of the pigments themselves. As a result of such genetic changes, two people who live in the same environment may have very different shades or patterns of skin coloration.
Some of the genes that influence skin pigmentation also cause variation in hair color or eye color. For example, variation in the gene OCA2 explains most of the variation in eye color in Europeans. People with blue eyes are mostly homozygotes for an allele of this gene; these people also tend to have slightly lighter skin due to this allele. Likewise, the variation in the gene MC1R explains some of the variation in skin color in Europe, but also explains a large proportion of variation in hair color. Red and blond hair each result from some of the distinctive alleles of MC1R.
Dark skin evolved in ancient humans
Relatively light-skinned populations include the native inhabitants of Europe, West Asia, East Asia, the Arctic, and the Americas. The lightest skin tones are found in Europe, while the darkest are in tropical Africa, southern India, Indonesia and Melanesia, and Australia. The level of skin pigmentation shows a close correspondence with latitude — people living near the equator tend to have dark skin, while light-skinned people live nearer the poles.
Selection on skin color depends on the level of UV radiation.
Cline of skin color in global human populations
Skin pigmentation correlates with latitude because it serves as a defense against UV radiation. Like all solar radiation, UV is more intense at lower latitudes, where the sun is more often directly overhead. High-energy UV light damages and destroys the molecules that skin is made of. In sufficient amounts, this UV radiation can cause severe burns, that are painful and leave the skin unable to maintain its normal protective and cooling functions. UV radiation also can cause long-term damage to the DNA of skin cells, resulting in dangerous skin cancers.
Dark-skinned people have a lower incidence of skin cancers in most countries compared to people with less pigmentation. The highest skin cancer rates in the world are suffered by people of European origin who currently live in equatorial places; Australia is presently the highest. Still, skin cancer may be a relatively weak cause of natural selection, because deaths from skin cancer tend to occur later than the mid-30s, relatively late in most peoples' reproductive lifespan.
Dark skin reduces the incidence of skin cancer and sunburn.
Possibly more important was the incidence of heat stroke in severely sunburned people. Today, relatively few people in Western societies succumb to heat exhaustion and heat stroke today, but this was potentially a great danger in the past and remains so in some places today. This danger of sunburn especially influences children, whose smaller masses allow less room for error in water loss and overheating.
Some evidence suggests that dark skin pigmentation first appeared in humans within the last 500,000 years. African apes are polymorphic in skin coloration. Chimpanzees in particular are variable — some chimpanzees have quite light skin, and others have very dark skin; skin color tends to darken with age in these primates. But humans who live in equatorial Africa today show very little variation in skin color. Dark skin has been strongly selected in that population. One gene that contributes to skin pigmentation phenotypes, MC1R, shows evidence for positive selection in Africans sometime between 200,000 and 1 million years ago [1]. This date is interesting — humans first appeared nearly 2 million years ago, and our divergence from chimpanzees was far earlier, at over 6 million years ago. So the evolution of dark skin pigmentation was continuing at a relatively recent date. One suggestion is that people lost their body fur sometime during the last million years. With fur, there was no survival benefit to dark skin, but exposed skin creates the susceptibilities that select for darker pigmentation.
Light skin pigmentation evolved recently
Light skin pigmentation is a more difficult problem than dark pigmentation. The advantages of dark skin are clear, and genetic evidence shows that dark skin has been around for a long time. But light skin evolved relatively recently.
The variation among light-skinned populations helps to illuminate the problem. Europeans and Asians today are broadly similar in their range of pigmentation. Northern Europeans average a bit lighter in skin color than north Asians, but the ranges of variation in pigmentation greatly overlap. Still, there are regional differences. For example, both hair and eye coloration are more polymorphic in Europeans than in living Asians. These phenotypes suggest that different alleles may affect pigmentation in these populations.
Recently, geneticists have identified more than a dozen different genes influencing skin coloration in Europeans and Asians. The variation in pigmentation associated with these genes is mostly explained by new alleles under recent positive selection. For example, northern Europeans carry a new allele from a gene called SLC24A5 at a frequency near 100 percent. This allele has spread as far west as Spain, and as far east as Pakistan; it is also common in North Africa. Yet, the new mutation originated very recently, approximately 6000 years ago. Likewise, a gene called DCT has a new allele common in China, which appears to have originated less than 10,000 years ago. Both Europeans and Asians have 10 or more alleles influencing their light skin pigmentation, but these alleles are only rarely shared between these populations. Variation in eye color in Europeans is mostly explained by a recnet mutation in the gene OCA2. This same gene has another allele under recent selection in China, which does not strongly influence eye color. European hair color variation is mostly explained by variation in MC1R; this gene has many new alleles in Europe, but does not greatly influence hair color in East Asia. In every case, the new mutations occurred recently and have not yet had time to spread and proliferate from one end of Eurasia to the other.
The recent evolution of light skin can only be explained by a strong pattern of selection favoring it. Scientists have focused on ways that dark skin may create disadvantages for people in places with lower natural UV radiation. One way that UV radiation is necessary is in the metabolism of vitamin D. Humans synthesize vitamin D in the skin, where exposure to UV radiation allows the transformation of precursor molecules into the necessary vitamin. Vitamin D is necessary for normal bone development, and people who suffer from a deficiency of vitamin D get a disorder known as rickets, characterized by deformation of the bones. Such abnormalities in bone growth can be potent causes of selection, either by decreasing mating attractiveness or by impeding normal activities. Such problems can extend to reproduction itself, as a pelvis deformed by rickets can make it impossible for a woman to give birth normally.
There is some evidence that dark skin is less capable of maintaining vitamin D metabolism. Most notably, people with darker skin living at higher latitudes in historic times, such as in London, apparently have suffered a higher incidence of rickets. However, today people acquire vitamin D primarily through dietary supplements, including dairy foods enriched with the vitamin, so that dietary differences between peoples of different skin tones in Western nations may partially account for differences in rickets incidence. Nevertheless, vitamin D metabolism remains the most prominent hypothesis to account for the distribution of light skin in the northern parts of the world.
Even so, some differences in skin color are probably explained by other factors. For example, northern Europeans are markedly lighter in skin color than people who live at the same latitude in East Asia. Many Europeans also have less melanin in their hair, which ranges in tone from blond to brown and red, while most high-latitude Asians have black hair.
It is possible that some of these differences may be the result of sexual selection, as different populations create different long-term patterns in sexual attractiveness and mating. Scientists have also applied sexual selection to explain differences in hair form among populations, from short and kinky to long and straight, and differences in hair color among equatorial populations. In all such cases, there is no ready environmental explanation for the differences. Even so, human cultures are very flexible and change rapidly, especially when compared to biological evolution, so that a stable sexual preference for such a characteristic as skin color or hair color, expressed over many hundreds of generations, would appear to conflict with the rapid cultural changes that affect mating preferences.
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