Dark skin is a human skin color that is rich in melanin pigments, especially eumelanin. 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.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 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, and 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, and Africa. Studies into these populations indicates dark skin is a retention of the pre-existing high UVR-adapted state of modern humans before the out of Africa migration and not a later evolutionary adaptation. 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.