Dark skin

Dark skin is a type of human skin color that is rich in melanin pigments. People with dark skin are often referred to as black people, although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.

The evolution of dark skin is believed to have begun around 1.2 million years ago, in light-skinned early hominid species after they moved from the equatorial rainforest to the sunny savannas. In the heat of the savannas, better cooling mechanisms were required, which were achieved through the loss of body hair and development of more efficient perspiration. The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against folate (vitamin B9) depletion, and to a lesser extent, DNA damage. The primary factor contributing to the evolution of dark skin pigmentation was the breakdown of folate in reaction to ultraviolet radiation; the relationship between folate breakdown induced by ultraviolet radiation and reduced fitness as a failure of normal embryogenesis and spermatogenesis led to the selection of dark skin pigmentation. By the time modern Homo sapiens evolved, all humans were dark-skinned.

Humans with dark skin pigmentation have skin naturally rich in melanin, especially eumelanin, and have more melanosomes which provide superior protection against the deleterious effects of ultraviolet radiation. This helps the body to retain its folate reserves and protects against damage to DNA.

Dark-skinned people who live in high latitudes with mild sunlight are at an increased risk—especially in the winter—of vitamin D deficiency. As a consequence of vitamin D deficiency, they are at a higher risk of developing rickets, numerous types of cancers, and possibly cardiovascular disease and low immune system activity. However, some recent studies have questioned if the thresholds indicating vitamin D deficiency in light-skinned individuals are relevant for dark-skinned individuals, as they found that, on average, dark-skinned individuals have higher bone density and lower risk of fractures than lighter-skinned individuals with the same levels of vitamin D. This is possibly attributed to lower presence of vitamin D binding agents (and thus its higher bioavailability) in dark-skinned individuals.

The global distribution of generally dark-skinned populations is strongly correlated with the high ultraviolet radiation levels of the regions inhabited by them. These populations, with the exception of indigenous Tasmanians, almost exclusively live near the equator, in tropical areas with intense sunlight: Africa, Australia, Melanesia, South Asia, Southeast Asia, West Asia, and the Americas. Studies into non-African populations indicates dark skin is not necessarily a retention of the pre-existing high UVR-adapted state of modern humans before the out of Africa migration, but may in fact be a later evolutionary adaptation to tropical rainforest regions. Due to mass migration and increased mobility of people between geographical regions in the recent past, dark-skinned populations today are found all over the world.

Due to natural selection, people who lived in areas of intense sunlight developed dark skin colouration to protect against ultraviolet (UV) light, mainly to protect their body from folate depletion. Evolutionary pigmentation of the skin was caused by ultraviolet radiation of the sun. As hominids gradually lost their fur between 1.2 and 4 million years ago, to allow for better cooling through sweating, their naked and lightly pigmented skin was exposed to sunlight. In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight. Indigenous populations' skin reflectance (the amount of sunlight the skin reflects) and the actual UV radiation in a particular geographic area is highly correlated, which supports this idea. Genetic evidence also supports this notion, demonstrating that around 1.2 million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus Homo. The effect of sunlight on folic acid levels has been crucial in the development of dark skin.

The earliest primate ancestors of modern humans most likely had pale skin, like our closest modern relative—the chimpanzee. About 7 million years ago human and chimpanzee lineages diverged, and between 4.5 and 2 million years ago early humans moved out of rainforests to the savannas of sub-Saharan Africa. They not only had to cope with more intense sunlight but had to develop a better cooling system. It was harder to get food in the hot savannas and as mammalian brains are prone to overheating—5 or 6 °C rise in temperature can lead to heatstroke—there was a need for the development of better heat regulation. The solution was sweating and loss of body hair.

Sweating dissipated heat through evaporation. Early humans, like chimpanzees now, had few sweat glands, and most of them were located in the palms of the hand and the soles of the feet. At times, individuals with more sweat glands were born. These humans could search for food and hunt for longer periods before being forced back to the shades. The more they could forage, the more and healthier offspring they could produce, and the higher the chance they had to pass on their genes for abundant sweat glands. With less hair, sweat could evaporate more easily and cool the bodies of humans faster. A few million years of evolution later, early humans had sparse body hair and more than 2 million sweat glands in their body.