Skin Pigmentation

In My Humble Opinion
1

This is less than cosmic and very interesting.

Journal of Human Evolution Vol. 39, No. 1, July 2000
http://www.idealibrary.com/links/doi/10.1006/jhev.2000.0403

The evolution of human skin coloration pp. 57-106 Nina G. Jablonski, George Chaplin njablonski@c… gchaplin@c…

Skin color is one of the most conspicuous ways in which humans vary and has been widely used to define human races. Here we 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
and its direct and indirect effects on fitness. Using remotely sensed data on UV radiation levels, hypotheses concerning the distribution of the skin colors of indigenous peoples relative to UV levels were tested quantitatively in this study for the first time.

The major results of this study are: (1) skin reflectance is strongly correlated with absolute latitude and UV radiation levels. The highest correlation between skin reflectance and UV levels was observed at 45 nm, near the absorption maximum for oxyhemoglobin, suggesting that the main role of melanin
pigmentation in humans is regulation of the effects of UV radiation on the contents of cutaneous blood vessels located in the dermis. (2) Predicted skin reflectances deviated little from observed values. (3)In all populations for which skin reflectance data were available for males and females, females were found to be lighter skinned than males. (4) The clinal gradation of skin coloration observed among
indigenous peoples is correlated with UV radiation levels and represents a compromise solution to the conflicting physiological requirements of photoprotection and vitamin D synthesis.

The earliest members of the hominid lineage probably had a mostly unpigmented or lightly pigmented integument covered with dark black hair, similar to that of the modern chimpanzee. The evolution of a naked, darkly pigmented integument occurred early in the evolution of the genus Homo. A dark epidermis protected sweat glands from UV-induced injury, thus insuring the integrity of somatic
thermoregulation. Of greater significance to individual reproductive success was that highly melanized skin protected against UV-induced photolysis of folate (Branda & Eaton, 1978, Science201, 625-626; Jablonski, 1992, Proc. Australas. Soc. Hum. Biol.5, 455-462, 1999, Med. hypostheses 52, 581-582), a metabolite essential for normal development of the embryonic neural tube (Bower & Stanley, 1989, The Medical Journal of Australia150, 613-619; Medical Research Council Vitamin Research Group, 1991, The Lancet338, 31-37) and spermatogenesis (Cosentino et al., 1990, Proc.
Natn. Acad. Sci. U.S.A.87, 1431-1435; Mathur et al., 1977, Fertility Sterility28, 1356-1360).

As hominids migrated outside of the tropics, varying degrees of depigmentation evolved in order to permit UVB-induced synthesis of previtamin D3. The lighter color of female skin may be required to permit synthesis of the relatively higher amounts of vitamin D3necessary during pregnancy and
lactation.

Skin coloration in humans is adaptive and labile. Skin pigmentation levels have changed more than once in human evolution. Because of this, skin coloration is of no value in determining phylogenetic relationships among modern human groups.

Key Words: ultraviolet radiation, UVB, melanin, integument, folate, photoprotection, neural tube defects, vitamin D, depigmentation, thermoregulation, race concepts

Full text, subscribers only:
http://www.idealibrary.com/links/doi/10.1006/jhev.2000.0403/pdf

So how slowly did hominids migrate outside of the tropics?

2

Um, not to be picky or anything, but isn’t there a group of Polynesian Islanders with real dark skin living nowhere near the tropics? I think I saw it in Time or Life ages ago.

3

Could be, people have moved all over the planet.

4

I’ll buy the first three results, but number four seems to be an interpretation, not a result. The reason for skin lightening may well be to allow more vitamin D synthesis, but the correlations they give don’t actually prove that, or even add much support over what was already known.

It’s also highly speculative to say when in human history changes ocurred. Did skin start light, then darken, then migration north occured, then skin lightened again in those northern populations, as suggested? Or did skin start light, then the population split/migration north occurred, then the southern branch of humans subsequently darkened? For that matter, chimps may have light skin under their hair, but other of our closely related simians are dark. How do we actually know we started light?

The data is interesting, but the speculations seem to go far beyond what they can support.

5

#3 bothered me. That’s been a known artist’s convention since butter got slapped on bread or earlier. I’ve never considered that it might actually be true and it doesn’t seem to be true of the basis of what I can observe here in the US. I probably just means I need to take a world tour!

If our rate of travel is 8 miles per year (amount of time taken to spread over the Americas and Europe/miles=8 miles per year) I can’t see how we got nordic coloring and why only one place on earth has skin color this pale.

And would measuring skin reflectance instead of some form of absolute skin color remove the differences between brownish and yellowish tones?