Epochal work by Nina G. Jablonski and George Chaplin of Department of Anthropology, California Academy of Sciences.
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
545 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, Science 201, 625–626;
Jablonski, 1992, Proc. Australas. Soc. Hum. Biol. 5, 455–462, 1999,
Med. Hypotheses 52, 581–582), a metabolite essential for normal
development of the embryonic neural tube (Bower & Stanley, 1989,
The Medical Journal of Australia 150, 613–619; Medical Research
Council Vitamin Research Group, 1991, The Lancet 338, 31–37) and
spermatogenesis (Cosentino et al., 1990, Proc. Natn. Acad. Sci.
U.S.A. 87, 1431–1435; Mathur et al., 1977, Fertility Sterility 28,
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 D3
necessary 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.
THE EVOLUTION OF HUMAN SKIN COLORATION - INTRODUCTION can be found by clicking HERE.