Kayli Jade Gertsen Skin Evolution Essay Human skin pigmentation is the product of two clines produced by natural selection to adjust levels of constitutive pigmentation to levels of UV radiation (UVR). One cline was generated by high UVR near the equator and led to the evolution of dark, photo protective, melanin-rich pigmentation. The other was produced by the requirement for UVB photons to sustain cutaneous photosynthesis of vitamin D3 in low-UVB environments, and resulted in the evolution of pigmented skin.
As hominids dispersed outside of the tropics, they experienced different intensities and seasonal mixtures of UVA and UVB. Extreme UVA throughout the year and two equinoctial peaks of UVB prevail within the tropics. Under these conditions, the primary selective pressure was to protect foliate by maintaining dark pigmentation. Photolysis of foliate and its main serum form of 5-methylhydrofolate is caused by UVR and by reactive oxygen species generated by UVA.
Competition for foliate between the needs for cell division, DNA repair, and melanogenesis is severe under stressful, high-UVR conditions and is exacerbated by dietary insufficiency. Outside of tropical latitudes, UVB levels are generally low and peak only once during the year. The populations exhibiting maximally pigmented skin are those inhabiting environments with the lowest annual and summer peak levels of UVB. Development of facultative pigmentation (tanning) was important to populations settling between roughly 23° and 46° , where levels of UVB varied strongly according to season.
Depigmented and tannable skin evolved numerous times in hominin evolution via independent genetic pathways under positive selection. Variation in skin color is the most noticeable of human polymorphisms. As visually dominant mammals, we readily notice differences in skin color in each other. As primates who uniquely use language to create categories, we readily give names to these differences. Since the mid-18th century, skin color has been the single most important physical trait used to define human groups, including variously named varieties, races, subspecies, and species.
Charles Darwin observed variation in human skin color while abroad during the voyage of the H. M. S. Beagle (1831–1836), but he soundly rejected the notion that physical differences such as skin color constituted the basis for distinguishing separate human species (1). Darwin’s rejection of the existence of distinct human species was based upon his observation that human groups “graduate into each other, and that it is hardly possible to discover clear distinctive character between them” (1, 226).
His aversion to the separation of humans into discrete species was also motivated by his vehement aversion to slavery, which in his lifetime was defended and promoted on the basis of the superiority and inferiority of allegedly distinct human species (2). It is also well known that early in his career, Darwin collected copious notes on human origins and descent (3), but “without any intention of publishing on the subject, but rather with a determination not to publish, as I thought that I should thus only add to the prejudices against my views” (1, 1).
Darwin thus deflected potential criticism of natural selection in the first decade after publication of The Origin by avoiding almost entirely discussion of humans in an evolutionary context. The causes of variation in human skin pigmentation were much discussed 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 (4).
The association of dark skin pigmentation with intense sunshine and heat was further developed by Aristotle and his followers as part of a comprehensive “climatic theory,” which related human features, dispositions, and cultures to the environment. By the mid-18th century, naturalists such as John Mitchell and, later, Samuel Stanhope Smith recognized a pronounced latitudinal gradient of skin pigmentation among the world’s peoples—from dark near the equator to light toward the poles—and related it mainly to differences in sunshine heat experienced by people at different latitudes (5, 6). This general uniformity in the effect,” Smith wrote, “indicates an influence in climate that, under the same circumstances, will always operate in the same manner” (5, 34). It is thus surprising that Darwin, who was so keen to identify adaptations of organisms to “different conditions of life,” rejected a causal association of skin pigmentation with climate in favor of the notion that variations in skin color had evolved primarily through the agency of sexual selection
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