dark skin dark skin dark skin is a type of human skin color that is rich in melanin pigments. people with very 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. a woman with dark skin 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: australia, melanesia, new guinea, south asia, southeast asia, west asia, africa, and the americas. studies into these 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. evolution 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. savannas in east africa, where most of the hominid evolution of dark skin may have taken place the earliest primate ancestors of modern humans most likely had light 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 east 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. hairless skin, however, is particularly vulnerable to be damaged by ultraviolet light and this proved to be a problem for humans living in areas of intense uv radiation, and the evolutionary result was the development of dark-coloured skin as a protection. scientists have long assumed that humans evolved melanin in order to absorb or scatter harmful sun radiation. some researchers assumed that melanin protects against skin cancer. while high uv radiation can cause skin cancer, the development of cancer usually occurs after child-bearing age. as natural selection favours individuals with traits of reproductive success, skin cancer had little effect on the evolution of dark skin. previous hypotheses suggested that sunburned nipples impeded breastfeeding, but a slight tan is enough to protect mothers against this issue. a 1978 study examined the effect of sunlight on folate—a vitamin b complex—levels. the study found that even short periods of intense sunlight are able to halve folate levels if someone has light skin. low folate levels are correlated with neural tube defects, such as anencephaly and spina bifida. uv rays can strip away folate, which is important to the development of healthy foetuses. in these abnormalities children are born with an incomplete brain or spinal cord. nina jablonski, a professor of anthropology and expert on evolution of human skin coloration, found several cases in which mothers' visits to tanning studios were connected to neural tube defects in early pregnancy. she also found that folate was crucial to sperm development; some male contraception drugs are based on folate inhibition. it has been found that folate may have been the driving force behind the evolution of dark skin. as humans dispersed from equatorial africa to low uvr areas and higher latitudes sometime between 120,000 and 65,000 years ago, dark skin posed a disadvantage. populations with light skin pigmentation evolved in climates of little sunlight. light skin pigmentation protects against vitamin d deficiency. it is known that dark-skinned people who have moved to climates of limited sunlight can develop vitamin d-related conditions such as rickets, and different forms of cancer. a 2022 study revealed that traits such as dark skin show strong signals for convergent evolution and selective pressure (positive selection). other hypotheses the main other hypotheses that have been put forward through history to explain the evolution of dark skin coloration relate to increased mortality due to skin cancers, enhanced fitness as a result of protection against sunburns, and increasing benefits due to antibacterial properties of eumelanin. darkly pigmented, eumelanin-rich skin protects against dna damage caused by the sunlight. this is associated with lower skin cancer rates among dark-skinned populations. the presence of pheomelanin in light skin increases the oxidative stress in melanocytes, and this combined with the limited ability of pheomelanin to absorb uvr contributes to higher skin cancer rates among light-skinned individuals. the damaging effect of uvr on dna structure and the entailing elevated skin cancer risk is widely recognized. however, these cancer types usually affect people at the end or after their reproductive career and could have not been the evolutionary reason behind the development of dark skin pigmentation. of all the major skin cancer types, only malignant melanoma have a major effect in a person's reproductive age. the mortality rates of melanoma have been very low (less than 5 per 100,000) before the mid-20th century. it has been argued that the low melanoma mortality rates during reproductive age cannot be the principal reason behind the development of dark skin pigmentation. studies have found that even serious sunburns could not affect sweat gland function and thermoregulation. there are no data or studies that support that sunburn can cause damage so seriously it can affect reproductive success. another group of hypotheses contended that dark skin pigmentation developed as antibacterial protection against tropical infectious diseases and parasites. although it is true that eumelanin has antibacterial properties, its importance is secondary to 'physical adsorption' (physisorption) to protect against uvr-induced damage. this hypothesis is not consistent with the evidence that most of the hominid evolution took place in savanna environments and not in tropical rainforests. humans living in hot and sunny environments have darker skin than humans who live in wet and cloudy environments. the antimicrobial hypothesis also does not explain why some populations (like the inuit or tibetans) who live far from the tropics and are exposed to high uvr have darker skin pigmentation than their surrounding populations. biochemistry and genetics darkly pigmented skin dark-skinned humans have high amounts of melanin found in their skin. melanin is derivative of the amino acid tyrosine. eumelanin is the dominant form of melanin found in human skin. eumelanin protects tissues and dna from the radiation damage of uv light. melanin is produced in specialized cells called melanocytes, which are found at the lowest level of the epidermis. melanin is produced inside small membrane-bound packages called melanosomes. people with naturally-occurring dark skin have melanosomes which are clumped, large and full of eumelanin. a four-fold difference in naturally-occurring dark skin gives seven- to eight-fold protection against dna damage, but even the darkest skin colour cannot protect against all damage to dna. dark skin offers great protection against uvr because of its eumelanin content, the uvr-absorbing capabilities of large melanosomes, and because eumelanin can be mobilized faster and brought to the surface of the skin from the depths of the epidermis. for the same body region, light- and dark-skinned individuals have similar numbers of melanocytes (there is considerable variation between different body regions), but pigment-containing organelles, called melanosomes, are larger and more numerous in dark-skinned individuals. keratocytes from dark skin cocultured with melanocytes give rise to a melanosome distribution pattern characteristic of dark skin. melanosomes are not in aggregated state in darkly pigmented skin compared to lightly pigmented skin. due to the heavily melanised melanosomes in darkly-pigmented skin, it can absorb more energy from uvr and thus offers better protection against sunburns and by absorption and dispersion uv rays. darkly-pigmented skin protects against direct and indirect dna damage. photodegration occurs when melanin absorbs photons. recent research suggest that the photoprotective effect of dark skin is increased by the fact that melanin can capture free radicals, such as hydrogen peroxide, which are created by the interaction of uvr and layers of the skin. heavily pigmented melanocytes have greater capacity to divide after ultraviolet irradiation, which suggests that they receive less damage to their dna. despite this, medium-wave ultraviolet radiation (uvb) damages the immune system even in darker skinned individuals due to its effect on langerhans cells. the stratum corneum of people with dark or heavily tanned skin is more condensed and contains more cornified cell layers than in lightly-pigmented humans. these qualities of dark skin enhance the barrier protection function of the skin. although darkly-pigmented skin absorbs about 30