Global InformationDark skin information
Dark skin is a type of human skin color that is rich in melanin pigments.[1][2][3] People with dark skin are often referred to as "black people",[4] although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.[5][6][7][8]
The evolution of dark skin is believed to have begun around 1.2 million years ago,[9][10] 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.[3][11][12][13][14][15][16][better source needed]
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.[3][17]
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.[3][18] 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.[19][20]
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, New Guinea, 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.[21][22][23] 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.[3][24][25]
- ^ skinned Archived 4 March 2016 at the Wayback Machine Princeton University "naturally having skin of a dark color"
- ^ "Dark-skinned". thefreedictionary.com. Archived from the original on 22 January 2017. Retrieved 24 January 2017.
a person or race having skin of a dark colour
- ^ a b c d e Muehlenbein, Michael (2010). Human Evolutionary Biology. Cambridge University Press. pp. 192–213.
- ^ Oxford Dictionaries. April 2010. Oxford University Press. "belonging to or denoting any human group having dark-coloured skin" "black" (accessed 6 August 2012).
- ^ Dictionary.com: black Archived 19 February 2016 at the Wayback Machine 3.a "a member of any of various dark-skinned peoples" 21.a"specifically the dark-skinned peoples of Africa, Oceania, or Australia."
- ^ "Global Census". American Anthropological Association. Archived from the original on 14 September 2018. Retrieved 10 December 2012.
- ^ Oxford Dictionaries. April 2010. Oxford University Press. "especially of African or Australian Aboriginal ancestry" "black" (accessed 6 August 2012).
- ^ James, Mackers (8 November 1828). "Proclamation". Classified Advertising. Trove. Archived from the original on 26 May 2020. Retrieved 10 December 2012.
- ^ Brana, La (27 January 2017). "New-discovery-in-racial-migration". www.cbc.ca. Canadian Broadcasting Company. Retrieved 13 January 2024.
- ^ Nina, Jablonski (2004). "The evolution of human skin and skin color". Annual Review of Anthropology. 33: 585–623. doi:10.1146/annurev.anthro.33.070203.143955. S2CID 53481281.
genetic evidence [demonstrate] that strong levels of natural selection acted about 1.2 mya to produce darkly pigmented skin in early members of the genus Homo
- ^ Bower, C.; Stanley (1992). "The role of nutritional factors in the aetiology of neural tube defects". Journal of Paediatrics and Child Health. 28 (1): 12–16. doi:10.1111/j.1440-1754.1992.tb02610.x. PMID 1554510. S2CID 45104826.
- ^ Minns, R.A. (1996). "Folic acid and neural tube defects". Spinal Cord. 34 (8): 460–465. doi:10.1038/sc.1996.79. PMID 8856852.
- ^ Copp; et al. (1998). "Embryonic mechanisms underlying the prevenetion of neural tube defects by vitamins". Mental Retardation and Developmental Disabilities Research Reviews. 4 (4): 264–268. doi:10.1002/(sici)1098-2779(1998)4:4<264::aid-mrdd5>3.0.co;2-g.
- ^ Molloy; Mills, J. L.; Kirke, P. N.; Weir, D. G.; Scott, J. M.; et al. (1999). "Folate status and neural tube defects". BioFactors. 10 (2–3): 291–294. doi:10.1002/biof.5520100230. PMID 10609896. S2CID 20128738.
- ^ Lucock, M. (2000). "Folic acid: nutritional biochemistry, molecular biology, and role in disease processes". Molecular Genetics and Metabolism. 71 (1–2): 121–138. doi:10.1006/mgme.2000.3027. PMID 11001804.
- ^ William; Rasmussen, S. A.; Flores, A; Kirby, R. S.; Edmonds, L. D.; et al. (2005). "Decline in the prevalence of spina bifida and anencephaly by race/ethnicity:1995–2002". Pediatrics. 116 (3): 580–586. doi:10.1542/peds.2005-0592. PMID 16140696. S2CID 12765407.
- ^ Nielsen; et al. (2006). "The importance of the depth distribution of melanin in skin for DNA protection and other photobiological processes". Journal of Photochemistry and Photobiology B: Biology. 82 (3): 194–198. doi:10.1016/j.jphotobiol.2005.11.008. PMID 16388960.
- ^ Jane, Higdon. "Vitamin D". Micronutrient Information Center. Linus Pauling Institute. Archived from the original on 8 April 2015. Retrieved 10 December 2012.
- ^ Holick, Michael F. (21 November 2013). "Bioavailability of Vitamin D and Its Metabolites in Black and White Adults". The New England Journal of Medicine. 369 (21): 2047–2048. doi:10.1056/NEJMe1312291. PMID 24256384.
- ^ DeVita Raeburn, Elizabeth (20 November 2013). "Bone Density Higher in Blacks, Vitamin D Lower". MedPage Today. Archived from the original on 8 October 2019. Retrieved 19 June 2014.
- ^ Cite error: The named reference
Jabl00was invoked but never defined (see the help page). - ^ Harding, R; Healy, E; Ray, A; Ellis, N; Flanagan, N; Todd, C; Dixon, C; Sajantila, A; et al. (2000). "Evidence for Variable Selective Pressures at MC1R". The American Journal of Human Genetics. 66 (4): 1351–61. doi:10.1086/302863. PMC 1288200. PMID 10733465.
- ^ Zhang, Xiaoming; Liu, Qi; Zhang, Hui; Zhao, Shilei; Huang, Jiahui; Sovannary, Tuot; Bunnath, Long; Aun, Hong Seang; Samnom, Ham; Su, Bing; Chen, Hua (27 April 2021). "The distinct morphological phenotypes of Southeast Asian aborigines are shaped by novel mechanisms for adaptation to tropical rainforests". National Science Review. 9 (3): nwab072. doi:10.1093/nsr/nwab072. ISSN 2095-5138. PMC 8970429. PMID 35371514.
- ^ O'Neil, Dennis. "Skin Color Adaptation". Human Biological Adaptability: Skin Color as an Adaptation. Palomar. Archived from the original on 18 December 2012. Retrieved 10 December 2012.
- ^ O'Neil, Dennis. "Overview". Modern Human Variation. Palomer. Archived from the original on 5 November 2012. Retrieved 10 December 2012.