Editing Melanin (section)

== Human adaptations ==
===Physiology===
Melanocytes insert granules of melanin into specialized cellular [[Vesicle (biology)|vesicles]] called [[melanosome]]s. These are then transferred into the [[keratinocyte]] cells of the human [[Epidermis (skin)|epidermis]]. The melanosomes in each recipient cell accumulate atop the [[cell nucleus]], where they protect the nuclear [[DNA]] from mutations caused by the [[ionizing radiation]] of the sun's [[ultraviolet]] rays. In general, people whose ancestors lived for long periods in the regions of the globe near the [[equator]] have larger quantities of eumelanin in their skins. This makes their skins brown or black and protects them against high levels of exposure to the sun, which more frequently result in [[melanoma]]s in lighter-skinned people.<ref>{{Cite web |date=20 June 2012 |title=Human Skin Color Variation |url=http://humanorigins.si.edu/evidence/genetics/human-skin-color-variation |access-date=24 August 2019 |website=The Smithsonian Institution's Human Origins Program |language=en}}</ref>

Not all the effects of pigmentation are advantageous. Pigmentation increases the heat load in hot climates, and dark-skinned people absorb 30% more heat from sunlight than do very light-skinned people, although this factor may be offset by more profuse sweating. In cold climates dark skin entails more heat loss by radiation. Pigmentation also hinders synthesis of [[vitamin D]]. Since pigmentation appears to be not entirely advantageous to life in the tropics, other hypotheses about its biological significance have been advanced; for example a secondary phenomenon induced by adaptation to parasites and tropical diseases.<ref>{{cite book |last=Berth-Jones, J. |title=Rook's Textbook of Dermatology |volume=3 |page=58.9<!--58-59?--> |year=2010 |editor-first=Tony |editor-last=Burns |editor-first2=Stephen |editor-last2=Breathnach |editor-first3=Neil |editor-last3=Cox |editor-first4=Christopher |editor-last4=Griffiths |chapter=Constitutive pigmentation, human pigmentation and the response to sun exposure |edition=8th |publisher=Wiley-Blackwell |isbn=978-1-4051-6169-5}}</ref>

===Evolutionary origins===
[[Early humans]] evolved dark skin color, as an adaptation to a loss of body hair that increased the effects of UV radiation. Before the development of hairlessness, early humans might have had light skin underneath their fur, similar to that found in other [[primate]]s.<ref>{{Cite news |last=Wade |first=Nicholas |date=19 August 2003 |title=Why Humans and Their Fur Parted Ways |url=https://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html |access-date=24 August 2019 |work=The New York Times |language=en-US |issn=0362-4331 |archive-date=18 June 2009 |archive-url=https://web.archive.org/web/20090618134300/http://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html |url-status=live }}</ref> [[Anatomically modern humans]] evolved in Africa between 200,000 and 100,000 years ago,<ref>{{Cite journal |vauthors=Tishkoff SA, Reed FA, Friedlaender FR, etal |date=May 2009 |title=The genetic structure and history of Africans and African Americans |journal=Science |volume=324 |issue=5930 |pages=1035–44 |bibcode=2009Sci...324.1035T |doi=10.1126/science.1172257 |pmc=2947357 |pmid=19407144}}</ref> and then populated the rest of the world through migration between 80,000 and 50,000 years ago, in some areas [[Archaic human admixture with modern humans|interbreeding]] with certain [[archaic human]] species ([[Neanderthals]], [[Denisovans]], and possibly others).<ref>{{Cite news |date=22 September 2016 |title=A Single Migration From Africa Populated the World, Studies Find |url=https://www.nytimes.com/2016/09/22/science/ancient-dna-human-history.html |work=[[The New York Times]] |access-date=2 March 2017 |archive-date=2 May 2019 |archive-url=https://web.archive.org/web/20190502133043/https://www.nytimes.com/2016/09/22/science/ancient-dna-human-history.html |url-status=live }}</ref> The first modern humans had darker skin as the indigenous people of Africa today. Following migration and settlement in Asia and Europe, the selective pressure dark UV-radiation protecting skin decreased where radiation from the sun was less intense. This resulted in the current range of human skin color. Of the two common gene variants known to be associated with pale human skin, ''[[Mc1r]]'' does not appear to have undergone positive selection,<ref>{{Cite journal |last1=Harding |first1=Rosalind M. |last2=Healy |first2=Eugene |last3=Ray |first3=Amanda J. |last4=Ellis |first4=Nichola S. |last5=Flanagan |first5=Niamh |last6=Todd |first6=Carol |last7=Dixon |first7=Craig |last8=Sajantila |first8=Antti |last9=Jackson |first9=Ian J. |last10=Birch-Machin |first10=Mark A. |last11=Rees |first11=Jonathan L. |date=April 2000 |title=Evidence for Variable Selective Pressures at MC1R |journal=The American Journal of Human Genetics |language=en |volume=66 |issue=4 |pages=1351–1361 |doi=10.1086/302863 |pmc=1288200 |pmid=10733465}}</ref> while ''[[SLC24A5]]'' has undergone positive selection.<ref>{{Cite journal |vauthors=Lamason RL, Mohideen MA, Mest JR, etal |date=December 2005 |title=SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans |journal=Science |volume=310 |issue=5755 |pages=1782–6 |bibcode=2005Sci...310.1782L |doi=10.1126/science.1116238 |pmid=16357253 |s2cid=2245002}}</ref>

===Effects===
As with peoples having migrated northward, those with light skin migrating toward the equator acclimatize to the much stronger solar radiation. Nature selects for less melanin when ultraviolet radiation is weak. Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of [[sun tanning]]. Dark-skinned people, who produce more skin-protecting eumelanin, have a greater protection against [[sunburn]] and the development of melanoma, a potentially deadly form of skin cancer, as well as other health problems related to exposure to strong [[solar radiation]], including the [[photodegradation]] of certain [[vitamin]]s such as [[riboflavin]]s, [[carotenoid]]s, [[tocopherol]], and [[folate]].<ref>{{Cite journal |last1=Jablonski |first1=Nina G. |last2=Chaplin |first2=George |date=11 May 2010 |title=Human skin pigmentation as an adaptation to UV radiation |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=Supplement 2 |pages=8962–8968 |bibcode=2010PNAS..107.8962J |doi=10.1073/pnas.0914628107 |pmc=3024016 |pmid=20445093 |doi-access=free}}</ref>

Melanin in the eyes, in the [[Iris (anatomy)|iris]] and [[choroid]], helps protect from [[ultraviolet]] and [[high-energy visible light|high-frequency visible light]]; people with [[Eye color| blue, green, and grey eyes]] are more at risk of sun-related eye problems. <!--
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--> Furthermore, the ocular lens yellows with age, providing added protection. However, the lens also becomes more rigid with age, losing most of its [[accommodation (eye)|accommodation]]—the ability to change shape to focus from far to near—a detriment due probably to [[protein]] crosslinking caused by UV exposure.

Recent research suggests that melanin may serve a protective role other than photoprotection.<ref>{{Cite journal |vauthors=Liu Y, Hong L, Kempf VR, Wakamatsu K, Ito S, Simon JD |date=June 2004 |title=Ion-exchange and adsorption of Fe(III) by Sepia melanin |journal=Pigment Cell Research |volume=17 |issue=3 |pages=262–9 |doi=10.1111/j.1600-0749.2004.00140.x |pmid=15140071}}</ref> Melanin is able to effectively [[chelation|chelate]] metal ions through its carboxylate and phenolic hydroxyl groups, often much more efficiently than the powerful chelating ligand ethylenediaminetetraacetate (EDTA). Thus, it may serve to sequester potentially toxic metal ions, protecting the rest of the cell. This hypothesis is supported by the fact that the loss of neuromelanin, observed in Parkinson's disease, is accompanied by an increase in iron levels in the brain.