Human skin color

Differences in skin color among individuals is caused by variation in pigmentation, which is the result of genetics (inherited from one's biological parents), exposure to the sun, disorders, or some combination thereof.

The red color underlying the skin becomes more visible, especially in the face, when, as consequence of physical exercise or sexual arousal, or the stimulation of the nervous system (anger, embarrassment), arterioles dilate.

[12][failed verification] Women from darker-skinned populations may have evolved to lighter skin than men so their bodies could absorb more vitamin D during pregnancy, which improves calcium absorption.

Loss of body hair in Homo links to the thermoregulation through perspiration heat dissipation required for activity in hot open environments[19] and endurance running.

[20] Humans as primates have a particular need for this thermoregulation since unlike other mammals they lack a carotid rete that allows precooling of blood to the brain, an organ extremely sensitive to changes in body temperature.

In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight.

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.

[6][10][27][28] In 2017, a study showed that both dark and light pigmentation alleles arose before the origin of modern humans,[29] with the older version of the variants in many cases being associated with lighter skin.

[40] Paleolithic Cro-Magnon groups, as well as Early Holocene Western and central European hunter-gatherers (Western Hunter Gatherers) have been suggested to have been dark skinned based on DNA analysis,[41][42][43] with a number of the most prominent light-skin tone gene variants found in modern Europeans being introduced by Anatolian Neolithic Farmers that migrated into Europe beginning around 9,000 years ago,[44] with selection pressure for lighter skin intensifying from the Neolithic period onwards.

[47] According to Crawford et al. (2017), most of the genetic variants associated with light and dark pigmentation in African populations appear to have originated more than 300,000 years ago.

[49] Huang et al. 2021 found the existence of "selective pressure on light pigmentation in the ancestral population of Europeans and East Asians", prior to their divergence from each other.

[51] The authors argue that lack of significant differences between modern light-skinned and dark-skinned populations in vitamin D deficiency, early death from UV-induced cancers and birth defects — as well as instances of light and dark populations living side-by-side in areas with similar UV — suggest the standard model is insufficient to explain the strong selection drive for pigmented skin.

[51] Jablonski rejects this theory on the grounds that the human tanning response is driven by UV-B exposure, not xeric stress, and that the positive selection for vitamin D production is "well-established".

[54][91] A number of studies have found genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Chinese and East Asian populations.

[107] In hominids, the parts of the body not covered with hair, like the face and the back of the hands, start out pale in infants and turn darker as the skin is exposed to more sun.

[109] The skin of face and hands has about twice the amount of pigment cells as unexposed areas of the body, as chronic exposure to the sun continues to stimulate melanocytes.

The blotchy appearance of skin color in the face and hands of older people is due to the uneven distribution of pigment cells and to changes in the interaction between melanocytes and keratinocytes.

[12] In populations where women have lighter skin than men, it has been hypothesized that the requirement for high amounts of calcium during pregnancy and lactation may be related to the dimorphism.

Natural selection may have led to females with lighter skin than males in some indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infants and to maintain their own health.

Skin may either appear lighter, or darker than normal, or lack pigmentation at all; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling.

On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face.

[125] In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups.

[9] In recent times, humans have become increasingly mobile as a consequence of improved technology, domestication, environmental change, strong curiosity, and risk-taking.

[citation needed] Colonization and enslavement as carried out by European countries became involved with colorism and racism, associated with the belief that people with dark skin were uncivilized, inferior, and should be subordinate to lighter-skinned invaders.

[133] The preference for fair skin remained prominent until the end of the Gilded Age, but racial stereotypes about worth and beauty persisted in the last half of the 20th century and continue in the present day.

African-American journalist Jill Nelson wrote that, "To be both prettiest and black was impossible,"[134] and elaborated: We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look.

[138] Many languages popularly use specific words to describe distinct skin tones - from (for example) "jincha", Puerto Rican slang for "glass of milk" to "morena", literally "brown".

[152][153] In 2020, Der Spiegel reported that in Ghana, "When You Are Light-Skinned, You Earn More", and that "[s]ome pregnant women take tablets in the hopes that it will lead their child to be born with fair skin.

[156] In 2010, four out of ten women surveyed in Hong Kong, Malaysia, the Philippines and South Korea used a skin-whitening cream, and more than 60 companies globally compete for Asia's estimated $18 billion market.

In Japan, the geisha have a reputation for their white-painted faces, and the appeal of the bihaku (美白), or "beautiful white", ideal leads many Japanese women to avoid any form of tanning.