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Nothing new under the sun?: Darwinian surprises of skin colours

FredParisFrance 61 / 7  
Jul 15, 2008   #1

Could you please read my essay and give me some feedback?

The prompt is:

Select a topic that interests you from this week's reading material and write an anatomy and physiology "lecture" (300 to 500 words) to your classmates; describing the topic to them as if your were the instructor teaching the class.

Thank you in advance


From the point of view of anatomy and physiology, skin colour depends on an individual's integumentary system composition. Indeed, an individual's skin colour results from the combination of three pigments. First, the yellow-orange pigment carotene induces an orange colour. Second, haemoglobin, the pigment present in red blood cells, is at the root of a reddish coloration of the skin. Finally, melanin, which is produced by melanocytes and then transferred to keratinocytes, is the "darking" pigment of skin. Contrary to all expectations, all individuals belonging to the Homo sapiens sapiens species have the same amount of melanocytes. Actually, the so notable differences in human skin colours, from pale rose to ebony, emerge from the amount of pigment that those melanocytes secrete. Furthermore, ultraviolet radiation stimulates the production of vitamin D that is consecutively transformed into calcium and phosphorus, from the digestive system into the cardiovascular system.

Apart from aesthetic consideration, the propensity to generate melanin, and by extension skin colour, deeply influences health in ways that are rarely publicized in mass media. Indeed, sub-standard levels of exposition to ultraviolet rays hamper vitamin D synthesis in the long run, which may lead to a detrimental lack of vitamin D if this insufficiency is not compensated by vitamin D supplements (Parra, 2007). Consequently, one could ask why the most visible aspect of the human phenotype, i.e. skin colour, ranges from light to dark.

On the one hand, skin colour differences might have partly arisen from the geographical distribution of ultraviolet distribution. For instance, Jablonski has assumed that the earliest individuals belonging genus Homo were hairy with light skins and that some of them have evolved in contemporaneous Homo sapiens sapiens through the concomitant loss of their initial hairs and darkening of their skin colour, i.e. human beings (2007). Norton, Kittles, Parra, McKeigue, Mao, Cheng, Canfield, Bradley, McEvoy, and Shriver have even suggested that the current geographical distribution of skin colours has originated from the intensity of ultraviolet radiation on different parts of the globe (2007). They have subsequently stated that this phenomenon might derive from an adaptation by means of natural selection (Norton, Kittles, Parra, McKeigue, Mao, Cheng, Canfield, Bradley, McEvoy, & Shriver, 2007). Other scientists have been up to assert that this adaptation has occurred to preserve homeostasis against diseases such as skin cancers for hair has lost its initial protective role against solar radiation (Jablonski, Chaplin, N.G.J., & G.C., 2003). However, Jablonski, Chaplin, N.G.J., & G.C have also hinted at the significance of ultraviolet exposition as regards its effects on strategic nutrients and their corollaries on individuals' reproductive success (2003).

On the other hand, and more surprisingly, skin colour might have stemmed from sexual selection, as well. Actually, Aoki has laid emphasis on the fact that a potential evolution intended for circumventing the apparition of rickets caused by an insufficiency of vitamin D could be most dubious (2002). Indeed, since a light skin colour is not enough for preventing individuals' offspring from developing rickets, Aoki has claimed that the prevailing penchant in most societies for light-skinned mating partners could also explain the presence of light skin in areas of low solar radiation (2002). Madrigal and Kelly have formulated a synthesis by stating that natural and sexual selection intertwine (2007). According to Madrigal and Kelly, natural selective pressure (huge solar radiation) has had the upper hand in sub-tropical and equatorial areas whereas it has been superseded in areas close to the poles by sexual selective pressure (preference for a lighter-than-average skin colour in sexual partners) (2007).

Finally, natural and sexual selection may only have performed the prelude to human evolution. As a matter of fact, although the study of skin colours through the lens of anatomy and physiology has gained from its amalgamation with the field of physical anthropology, it might be interesting now to push further the cooperation with other theoretical frameworks such the biopsychology or the evolutionary psychology to reveal other aspects of the human organism and its homeostasis. For instance, it could be interesting to investigate the aetiology and epidemiology of conversion disorders. Findings emanating from this theoretical research could validate theories related to the influence of an alleged human fear-circuitry, sponsored by evolutionary psychologists, and be applied to real world security issues related with terrorism and epidemic sociogenic disorders, as suggested by Bracha, Yoshioka, Masukawa, and Stockman in an article published in 2005.

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