Dark skin

Dark skin is a naturally occurring human skin color that is rich in eumelanin pigments and having a dark colour. People with relatively dark skin are referred to as brown, and those with very dark skin are often referred to as black, although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.

Evolution of dark skin pigmentation began 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 by 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 a superior protection against the deleterious effects of ultraviolet radiation. This helps the body to retain its folate reserves and protects against damage to the 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 attributed as possibly due to lower presence of Vitamin D binding agents (and thus higher bioavailability) in dark-skinned individuals.

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