Date: Wednesday, August 07, 2002 11:17 AM
From: George Winkel
Subject: Letter to Editor — a comprehensive theory of skin color.
Nina G. Jablonski and George Chaplin’s, “The evolution of human skin coloration” article for the February 2000 Journal of Human Evolution appears to be available for free downloading at this link (it use to cost $35). The 50-page PDF-document is 3Mbytes long.
Briefly, Jablonski and Chaplin, wife and husband anthropologists at the California Academy of Sciences, Golden Gate Park, San Francisco (their e-mails: njablonski@calacademy.org & gchaplin@calacademy.org), may have formulated the first comprehensive theory of skin color. As they say they“present new evidence indicating that variations in skin color are adaptive, and are related to the regulation of ultraviolet (UV) radiation penetration in the integument [sub-skin] and its direct and indirect effects on fitness.”Not surprisingly, their findings relate skin melanin with ambient sunlight at latitude. This has been common knowledge since Hippocrates’s time, at least. What seems new here is first, the evolutionary hypothesis for transition from an ancestral hairy primate to modern humans’ naked skin. Human full-body sweat-gland distribution turned out to be more flexible, adaptive thermo-regulation than body hair over pale, unpigmented ape skin had been. The newly exposed white skin in Africa had brought problems, however. Ultraviolet UV rays burned the new sweat glands. Pulling down the melanin shades blocked the UV rays penetrating the deep skin layers (the integument), thereby protecting the sweat glands. But this required delicate trade-offs. Blocking the integument too much dramatically cuts down needed Vitamin D uptake from sunlight. At the same time, too much UV radiation burned out not only sweat glands. It also cut folate levels in the blood serum of pregnant “white” mothers, resulting in stillbirths from failed development in fetal neural tubes. The same folate drop in “white” men under bright, tropical sunlight lowered or even eliminated their sperm cell counts. It is easy to see how six million years of this would tend to differentiate people’s melanin complexion by degrees of latitude, both to north and south of the equator. Actually, Jablonski estimated the full “color” evolution needed only 10,000 to 20,000 years.
White skin’s greater vulnerability to cancer from prolonged UV radiation was not thought a materially selective influence, because most people are past child-bearing age by the time a skin melanoma shows up.
Continual human demographic movement probably accounts for any observed unevenness in the distribution of human complexions around the world. Interestingly, however, Jablonski states that for lack of sunlight humans could not inhabit regions above 50 degrees latitude (e.g., England, Sweden, Greenland), until they had learned to fish. The Greenlanders’ vitaman D-rich fish diet enables them to survive with melanin; also their arrival within less than 10,000 years is too recent for the predicted white complexions they will acquire, according to Jablonski’s theory.
Jablonski theorized that due to the relative lability of human melanin pigmentation, it is likely that some lineages have cycled through several solar “race changes” as they moved from one UV regime to another. Therefore, she concludes:“Because of its high degree of responsiveness to environmental conditions, skin pigmentation is of no value in assessing the phylogenetic relationships between human groups.”Two other reviews of Jablonski’s article: Gina Kirchweger, “The Biology of Skin Color: Black and White.” http://www.discover.com/feb_01/featbiology.html
Blake Edgar, “Why Skin Comes in Colors.” http://www.calacademy.org/calwild/winter2000/html/horizons.html
Additionally, Jablonski mentioned that most people show their melanin fine-tuning mechanism which works in hours (not 10,000+ years) – namely our familiar experience “sun tanning.” For me her theory of primeval homonids exchanging their fur coats for lively, thermo-regulating human skins rings like a bell. It gives meaning to our common experience tanning brown under summer sunlight, and (in my case) turning white in winter. Other mammals wear colorful fur coats, which may or may not seasonally change. Our glory is our photo-sensitive skin, which burnishes in a wide spectrum of colors which we were strangely blind to, and have barely started appreciating for skins’ own beauty.
George Winkel