Significance of Evolution, Reproductive Fitness and Cultural Adaptation in Skin Coloration (2014)

Skin color is a physical trait among Homo sapiens that is prejudicial at first social encounter between individuals. It is a trait that superficially presumes one’s biological and cultural data.  However, the biological and cultural phenomenon of one’s skin color is a significant factor of evolution, cultural adaptation and reproductive fitness.  In this paper, I take consideration of various anthropologists’ evaluations of the relationship between skin reflectance and absolute latitude, melanin function, reproductive success, sexual dimorphism, sexual selection and cultural adaptations. I will also focus on the primary roles of natural selection.

Natural selection and sexual selection creates a protection and regulation of UV radiation exposure on and within the surface of the skin. This biological and cultural phenomenon is also relative to one’s environment.  A person’s environment is the variable that pre-determinates melanin function and reproductive success. Of course, a specific environment is also reflective of the pressure that it exerts and/or its impact of cultural practices. The first thing to consider about skin coloration is the varied changes and reactions encompassed within the forces of one’s climate and geography.  Jablonski and Chaplin (2000) postulate that spatial properties engage with biological dynamism thereby developing characteristics that may sustain reproductive fitness.  In this aspect, the sun’s UV radiation distribution varies according to the proximity to the equator and in deference to relative hemisphere.  Whereas, the habitable land located in the southern hemisphere has the largest percentage of the land mass on Earth and is closest in latitude to the equator, it receives the highest distribution of electromagnetism from the sun.  This wider span of distribution in the southern hemisphere has a more intense electromagnetic penetration into the skin and other biological characteristics than the scattering in the northern hemisphere. Thus, it expresses a higher concentration of pigmentation among the inhabitants of the southern area. In contrast, the surface of the northern hemisphere is pierced with a lighter concentration of electromagnetism, thus producing light to moderate pigmentation.  Electromagnetic waves interact in various ways at the outer layer of the epidermis from reflection to slight alteration. However, deeper below the surface of the skin articulates the chemical reaction between the immersions of UV waves and binding at the molecular level, expressing a phenotype output that is rendering to an environment’s input.  This packaged product – melanin – has the power to lessen and scatter radiation through an optical filter, as well to filter harmful chemicals that are toxic or carcinogenic.  Denser or richer melanized bodies living in a higher UV-induced area have a higher advantage in this photoprotection from extreme UV radiation.  Before earlier hominids lost their hair, the skin was of a lighter pigment.  However, in the advent of hair loss, the body’s sub-layer tissue adapted to direct sun exposure by increasing the sweat glands as a mode for thermoregulation and by developing melanin to protect the tissue.  Chaplin (2004) says that climate-sourced variables such as rain, forest cover, altitude levels, temporal seasonality or temperature also impact skin reflectance. Speaking in particular to sweat glands and thermoregulation, he studies the effects of some seasonal warm rain:

“…summer rain, particularly in warmer regions, promotes sweating, which causes blood to rise into the dermis to supply the sweat glands. Under these conditions, the blood and its nutrients will be more susceptible to UVR damage.” (Chaplin, 2004: 293-300)

After the loss of hair, melanization became an evolutionary agent for photoprotection against nutrient photolysis, disintegration or depletion (Jablonski and Chaplin, 2000).  

Reproductive fitness is reactive to melanin health and skin color evolution.  Folate is a critical resource in maintaining individual reproductive success. It helps to maintain male sperm fertility, promote the nervous system of embryos and other organic functions.  In areas with high UV energy piercing, folate is vulnerable to breakdown.  However, folate deficiency is very rare in such areas with high UV exposure because of the protection from highly melanized skin and folate-enriched dietary intake (Jablonksi and Chaplin, 2000).  Conversely, UV particles synthesize and positively contribute to Vitamin D3 in the human body by converting the body’s 7-dehydrocholesterol in to previtamin D3, then transforming to Vitamin D3 at body temperature.  However, deep, concentrated melanin doesn’t absorb the nutrient as effortlessly and requires an extended length of exposure of sunlight for sufficient conversion and absorption. Insufficiency in levels of exposure could make one vulnerable to Vitamin D3 deficiency.  The significance of the nutrient in the body is that it “… is essential for normal growth, calcium absorption and skeletal development. Deficiency of the vitamin can cause death, immobilization, or pelvic deformities which prevent normal childbirth.”(Jablonksi and Chaplin, 2000: 63)  Vitamin D3 is in most dire need during pregnancy and lactation for the transfer of calcium and construction of a baby’s skeletal system.  Whereas deep melanin has a greater advantage in protecting and maintaining folate levels, it is non-adaptive and has a slighter drawback to attaining Vitamin D3.  Jablonski champions Loomis’ refutation of Robins’ argument that suggested for depigmentation for the deeply melanized bodies that have relocated to more northern latitudes or where there’s a lesser UV-radiated environment.  Loomis, on the other hand, suggests that depigmentation is not necessary in northern latitudes.  He references the Vitamin D3 sufficiency of earlier Homo species that migrated to the northern hemisphere even while coated with fur or thicker skin. As indicated in Jablonski and Chaplin’s (2000) review, Robins’ challenge to Loomis argues that the deficiency of vitamin D3 in contemporary times would likely be due to “industrialization, urbanization and overpopulation”.   Paraphrasing Robins, Jablonski and Chaplin (2000) also noted that most known cases of Vitamin D3 deficiency in low UV energy regions “are urban dwellers, deprived of natural sunlight and the opportunity to synthesize previtamin D3.” Nevertheless, disparate levels can be adjusted with sufficient sun exposure during appropriate seasons. Vitamin D can be stored in the organs during low exposure seasons.  

The evolution of skin color on modern Homo sapiens’ bodies also engage with the relevance and influence of sexual dimorphism and sexual selection. Human females relatively possess a lighter skin reflectance than to human males that live in same populations.  Madrigal and Kelly (2007) considers differing of opinions on the cause of this skin color sexual dimorphism.  They reference Garn et al’s view of how “…skin color can be attributed to influences of hormones on melanocyte activity, with estrogens and androgens…” (Madrigal and Kelly, 2007:471).  Also, suggested reasoning is given to males’ denser blood at the integument layer of the skin as theorized by Smith and Mitchell and Edwards and Duntley (Madrigal and Kelly, 2007).  According to Jablonski’s and Chaplin’s (1998) reasoning, the female advantage in this particular distinction of sexual dimorphism is that the lighter skin reflectance grants her a higher allowance of UV permeation, an effectiveness of previtamin D3 synthesis and an increase in of 1.25 dihydroxyvitamin D for a greater benefit of calcium absorption for pregnancy and lactation.  The sexual dimorphism, however, also translates to a particular magnetism in mating between male and female.  In general, Stanford, Allen and Anton (2013) discuss the struggle between two males – of various species - to attract a female based on the appeal and variation of their own traits in which would likely implicate their fitness.  The female has the power of selection based on her own particular attraction to these males that would enhance her own reproductive success.  In contrasts, Jablonski and Chaplin (2000) explain sexual selection in deference to the human male point of view in specifics to lighter skin reflectance of females.  Aside from the physiological and reproductive benefits and appeals, there appears to be an attraction of females with notable lighter skin reflectance by men within the same population.  However, Jablonski and Chaplin (2000) contend that mating successes of this particular context wouldn’t necessarily evolve pigmentation traits unless the female is significantly lighter; albeit, the anthropologists don’t seem to expound on the pleiotropic influence of skin coloration. 

It appears that in some cultures, cultural evolution or adaptation occurred after the biological phenomenon of sexual dimorphism and sexual selection of lighter skin. A suggestive is made in reference to women in some populations that attempt to avoid the sun to which contextualizes her attractiveness to the opposite sex. Madrigal and Kelly (2007) indicates that such regions with strongest sense of sexual selection are areas with lighter UV radiation.  Jablonski and Chaplin (2000) remark that the light skin reflectance “began as a trait tied to increased fitness and was subsequently reinforced and enhanced in many human populations by sexual selection.”  Thus, this point considers how natural and sexual selection can eventually shape or intersect culture, thereby developing cultural adaptations.  

Gene culture co-evolution is a theoretical concept that van de Berghe and Frost (1986) considered to play a factor when cultural behavioral traits and biological traits both mesh and express a particular relationship together.  However, there are two particular positions under the concept: minimalist and maximalist.  The minimalist believes that “cultural evolution has increasingly ‘taken over’ genetic evolution…”  The maximalist feels that the “causality of genes and culture are reciprocal” (van de Berghe and Frost, 1986: 87).  Natural selection still performs a major role in cultural evolution for the sake of reproduction.  For example, either approach can be reasoned in analyzing the sex and class differentiation in skin pigmentation among Japanese adolescents.  The females had a lighter skin reflectance than the male, as was discussed earlier is likely originated as a natural form sexual dimorphism and then followed into a cultural performance. Although, Frost (1988) negates the idea that human skin color sexual dimorphism is shaped by gender role constructions and occupation because the trait is a universal phenomenon, whereas gender roles and occupations are not, an inversed causality should be reviewed.  For example, the upper class had a lighter skin reflectance than the lower class and poor.  Lighter-skinned women in Japan have typically been known to mate with men of a higher status.  It appears that a cultural preference for fair-complexion women among the higher class have developed and solidified.  van de Berghe and Frost (1986) then hypothesize on how racism could develop within this gene culture co-evolution.  I offer that classism and sexism should also be evaluated as a cultural projection onto a woman with the presumptions that her physical characteristics are already preset with particular socio-cultural parameters.

In this discussion, I reviewed the considerations of various anthropologists’ explanations of the relationships between skin reflectance and absolute latitude, melanin function, reproductive success, sexual dimorphism, sexual selection and cultural adaptations.  Although there are some nuances and relative associations, the referenced theories about skin coloration do appear to be centered on the role that evolution and natural selection performs universally.   

References

 Chaplin, George
2004 Geographic Distribution of Environmental Factors Influencing Human Skin Coloration. American Journal of Physical Anthropology 125(3):292-302. 

Frost, Peter
1988 Human Skin Color: A Possible Relationship between its Sexual Dimorphism and its Social Perception. Perspectives in Biology and Medicine. 32(1): 38-58.

Jablonski, Nina G., and George Chaplin
2000 The Evolution of Human Skin Coloration. Journal of Human Evolution 39(1):57-106. 

Madrigal, Lorena, and William Kelly
2007 Human Skin‐Color Sexual Dimorphism: A Test of the Sexual Selection Hypothesis. American Journal of Physical Anthropology 132(3):470-482. 

Stanford, Craig, John S. Allen, and Susan C. Anton
2012 Exploring Biological Anthropology. Pearson Higher Ed. 

Van den Berghe, Pierre L, and Peter Frost
1986 Skin Color Preference, Sexual Dimorphism and Sexual Selection: A Case of Gene Culture co‐evolution? Ethnic and Racial Studies 9(1):87-113.