Genetically, Africa is the most diverse continent in the world. Two Africans sampled from the same village could have Y-chromosome or mtDNA lineages that are more divergent from each other than either is to a non-African. This diversity extends also to physical appearance, where there is a broad range in different regions. The features North Americans and Europeans typically associate with Africans are influenced by the populations they have had contact with, notably those from west-central Africa during the slave-trading era. These people, speaking Bantu languages, have certainly had a large influence on the appearance of African populations, and today their genetic legacy is as widespread as their linguistic influence. But Africa contains a far wider range of appearances, as you can see in Figure 7.
The range of variation in Africa is extraordinary, containing both the tallest (the Maasai) and the shortest (Pygmies) people on Earth. Facial characteristics are similarly diverse. The heavy features of the West Africans contrast with the finer-boned features of the San Bushmen. The San also have an epicanthic fold—the extra layer of skin above the eyes that characterizes people from East Asia—while other African groups lack this feature. The skin coloration also varies enormously, ranging from a deep brown in the West Africans to the lighter skin of the San. Overall, the main uniting feature is a generally darker skin color for Africans relative to people from northern latitudes, such as Europeans.
Much has been made of skin color as a defining racial characteristic. Yet most human variation is found among individuals within populations, and less than 10 percent serves to distinguish between racial groups. In other words, beneath the skin we are all much more similar than our surface features would lead us to believe. The reason that our surface appearance varies so much is probably due to the survival of different genetic variants over time. Skin color has probably been subject to strong selection over the past 50,000 years, and the lighter skin of Europeans is most likely due to the climatic influences they encountered living far from their African homeland.
Africa is the most tropical continent in the world—the vast majority of it is located between the Tropics of Cancer and Capricorn. As any tourist on a tropical holiday can attest, the sun there is incredibly intense, resulting in painful sunburns for those whose skin is not used to it. The dark skin of Africans—and other tropical populations, such as southern Indians and Melanesians—is probably an adaptation to the sun's intensity. Since we are a relatively hairless species, much of our body surface is exposed to the scorching effects of the sun. Melanin, which makes skin dark, is a natural sunscreen, and since we evolved in the tropics, our early ancestors probably had dark skin to protect them from the sun's rays. As painful and debilitating as sunburn is, long-term exposure of light skin to the tropical sun can lead to skin cancer. Lighter skin also allows enough ultraviolet radiation to penetrate to the deeper skin layers and break down folic acid, which can lead to anemia and (in gestating fetuses) neural tube defects. Clearly it made sense for early humans living in the tropical zone to have darker skin. Consistent with this, the ancestral form of the major gene for skin coloration, known as melanocortin recaptor (MC1R), produces darker pigmentation.
As humans began to migrate out of the tropics, though, they encountered regions with much less UV light. When they started moving into these regions, they experienced a relaxation in the selection pressure needed to maintain the ancestral form of the MC1R gene, and once they moved far enough north the selection pressure reversed itself. This is because some UV light must penetrate the skin in order to biosynthesize vitamin D, an essential vitamin—so called because it has to be supplemented in the diet, or (in sunny conditions) synthesized. Without enough vitamin D, the children of dark-skinned people would have fallen prey to rickets, a nasty childhood disease in which the long bones are deformed due to an inadequate supply of calcium.
The need to allow more UV light to penetrate the skin would have favored mutations in MC1R that produced lighter skin. The function of MC1R can be reduced in several ways, and there are several different mutations that contribute in varying degrees to lighter skin. All would have been favored, though, during the cold northward trek of our ancestors.
Why has the marvelous adaptation become such an integral part of racist ideology, causing lighter-skinned
Europeans to feel superior to the darker-skinned peoples of the tropics? It is probably due to the general correlation between latitude and levels of economic development over the past 2,000 years. As Jared Diamond has argued persuasively in his book Guns, Germs, and Steel, the peoples of the tropics have tended to be poorer than Eurasians during this period, and Africans have been among the poorest of the poor. What originated as an ice age adaptation to climatic diversity has been conflated with mental and cultural superiority, when the two phenomena have nothing to do with each other apart from geography. Again, the differences in skin color that distinguish human races are literally not more than skin deep.
If Africans have had dark skin for 50,000 years, it is a certainty that our distant ancestors were dark-skinned. They also were likely to have been thin people of a height similar to African hunter-gatherer populations today (perhaps 5'6" on average), since the stoutness of the Eurasian Neandertals was a biological adaptation to the climate evolved over hundreds of thousands of years. Since humans migrated into the northern parts of Eurasia relatively recently in comparison, they have had to adapt culturally for their African body proportions. Basically, though, our earliest ancestors of 50,000 years ago probably looked very much like modern Africans.
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