In the Light of Evolution Volume IV: The Human Condition (2010) / Chapter Skim
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9 Human Skin Pigmentation as an Adaptation to UV Radiation--Nina G. Jablonski and George Chaplin
9 Human Skin Pigmentation as an Adaptation to UV Radiation--Nina G. Jablonski and George Chaplin
Pages 167-184
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From page 167... ...
† AnD GeorGe ChAPlin* human skin pigmentation is the product of two clines produced by natu ral selection to adjust levels of constitutive pigmentation to levels of Uv radiation (Uvr)
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From page 168... ...
The causes of variation in human skin pigmentation were much dis cussed long before Darwin's time. observers beginning with hippocrates in the fifth century associated human traits and temperament with the environment and recognized that skin color was part of this package (isaac, 2004)
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From page 169... ...
lacking the covering of dense body hair that protects other mammals and exposed to the myriad physical, chemical, and biological challenges of the environment, human skin evolved under intense pressures of natural selection. The hairless condition itself did not evolve because of a partiality for smooth skin, as averred by Darwin, but primarily because of the need to lose body heat from the skin's surface during exertion and under hot environmental conditions (Zihlman and Cohn, 1988; Jablonski and Chaplin, 2000)
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From page 170... ...
. nonmelanoma skin cancers are common in older individuals from modern lightly pigmented populations inhabiting sunny climes, but they are rarely fatal or incapacitating (ricotti et al., 2009)
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From page 171... ...
The effects of skin cancers on reproductive success in humans today are modest, and were probably statistically inconsequential in the centuries before rapid, long-distance travel and migration. This inference is further supported by genetic evidence indicating no significant association of 15 snPs and skin cancer risk (nan et al., 2009)
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From page 172... ...
The near absence in African populations of functionally significant variation in the coding region of the melanocortin 1 receptor (MC1R) , one of the major genes regulating human eumelanin production, indicates the action of purifying selection maintaining dark pigmentation under intense selective pressure (harding et al., 2000; John et al., 2003; Makova and norton, 2005)
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From page 173... ...
. natural selection to promote continued vitamin D production through loss of constitutive pigmentation under conditions of reduced Uvr was strong, and its independent action on hominin populations dispersing to low-Uvr habitats was inferred before genetic evidence demonstrating positive selection for depigmentation became known (Jablonski and Chaplin, 2000)
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From page 174... ...
, specifically whether the pattern and nature of polymorphisms in the vDr is related to UvB levels and/or length of habitation under specific UvB regimes. GEOGRAPHIC VARIATION IN UV RADIATION Mounting genetic evidence demonstrating the role of natural selection in establishing and maintaining darkly and lightly pigmented cutaneous phenotypes near the equator and poles, respectively, prompts a closer look at the nature of the prime selective agent, Uvr.
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From page 175... ...
Human Skin Pigmentation as an Adaptation to UV Radiation / FiGUre 9.1 (A) Annual mean UvB (305 nm)
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From page 176... ...
outside of the tropics, average UvB levels are much lower and exhibit a single peak at the summer solstice (Chaplin and Jablonski, 2009)
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From page 177... ...
. As previously stated, naked skin was the primary interface between the human body and solar radiation throughout most of the history of the genus Homo.
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From page 178... ...
, leading to the evolution of permanently dark constitutive pigmentation, and the ability to increase eumelanin production in response to seasonal increases in UvB. This was accomplished genetically by positive selection leading to elimination of polymorphism at the MC1R locus and continued purifying selection acting on the same locus.
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From page 179... ...
UvA + Photoconversion of excess vitamin D3 to inactive metabolites UvB +++ Production of cyclobutane pyrimidine dimers and damaged nucleotides requiring repair resulting from DnA absorption of photons; activation of folate-dependent DnA repair processes UvB + Direct photolysis of folate (as 5MThF in serum) , reducing the amout of folate available for cell division and regulation of tyrosinase activity in melanogenesis UvB + Competition for folate: increased folate needs for DnA damage repair and as 1-carbon donor in methylation of DnA competing with folate needed for melanogenesis UvB no effect sunburn UvB no effect Damage to DnA and its repair system and alterations of the immune system lead to progressive genetic alterations and the formation of nonmelanoma skin cancers UvB - Cutaneous photosynthesis of vitamin D3 UvB - Greater need for vitamin D in females probably causing increasing sexual dimorphism in pigmentation: exaggerated by sexual selection in some populations aThe estimated strength of natural selection operating to darken and lighten pigmentation is indicated by numbers of " + " and "−" signs, respectively.
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From page 180... ...
outside of tropical latitudes, the slow ramp-up of UvB in the spring to levels capable of inducing photosynthesis of previtamin D3 provides a head start for vitamin D3 production and storage before facultative pigmentation developed by the DTr competes for UvB photons in the skin. Under conditions of slowly increasing UvB, sunburns would have been rare and would not have posed a risk to survival or reproductive success.
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From page 181... ...
CONCLUSIONS The visualizations of UvB and UvA levels and variation presented here permit elaboration of the nature of the selective mechanisms involved in the evolution of variation in skin pigmentation and, notably, the evo lution of tanning phenotypes in relation to seasonably variable levels of Uvr. skin pigmentation is probably one of the best examples of natural selection acting on a human trait.
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From page 182... ...
Thus, human skin is a perfect model to demonstrate the mechanism of evolution by natural selection in each of its required parts. Considerable antagonism toward evolution is based on the common understanding of the word "theory" in its colloquial sense as a hunch.
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From page 183... ...
it is not necessary to model orbital parameters for the ToMs dataset because the data are direct readings and automatically compensate for orbital effects, something that latitude studies do not (relethford, 1997)
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