Epochal work by Nina G. Jablonski and George Chaplin of Department of Anthropology, California Academy of Sciences.
Three zones representing different potentials
for UV-induced vitamin D3 synthesis
in light-skinned humans were identified
(Figure 1). The annual average UVMED
and average skin reflectance measurement
for each zone are presented in Table 1.
Zone 1 was delimited as the area in which
the average daily UVMED was sufficient to
catalyze previtamin D3 synthesis throughout
the year.
RESULTS and CHARTS are available at the PDF document HERE.
CONCLUSIONS
The results presented here demonstrate that
skin coloration in humans is highly adaptive
and has evolved to accommodate the
physiological needs of humans as they have
dispersed to regions of widely varying annual
UVMED. The dual selective pressures of
photoprotection and vitamin D3 synthesis
have created two clines of skin pigmentation.
The first cline, from the equator to the poles,
is defined by the significantly greater need
for photoprotection at the equator in particular
and within the tropics in general.
Deeply melanized skin protects against
folate photolysis and helps to prevent UV-induced injury to sweat glands (and subsequent
disruption of thermoregulation). The
second cline, from approximately 30N to the
North Pole, is defined by the greater need in
high latitudes to accommodate as much previtamin
D3 synthesis as possible in areas of
low annual UVMED. Humans inhabiting regions
at the intersection of these clines demonstrate
a potential for developing varying
degrees of facultative pigmentation (tanning)
(Quevedo et al., 1975). Moderately melanized
skin would appear to be at risk of vitamin
D3 deficiency and rickets under conditions
where UV radiation is restricted as a result of
latitude, cultural practices or both.
The results of this study suggest that skin
pigmentation is relatively labile, and that
adaptations to local UVMED conditions
can occur over relatively short periods of
geological time. Thus, it is likely that some
human lineages through time may have gone
through alternating periods of depigmentation
and pigmentation (or vice versa) as
they moved from one UVMED regime to
another. As the pace of human migrations
has quickened in recent centuries, more and
more populations are finding themselves living
under UV irradiation regimes to which
they are inherently poorly adapted (e.g.,
the English who settled in Australia in the
nineteenth and twentieth centuries, and the
Indians and Pakistanis who have moved to
northern England in recent decades), with
major public health consequences (Kaidbey
et al., 1979; Henderson et al., 1987). Cultural
practices such as sun-bathing and purdah
have in some cases exacerbated these
conditions and mitigated others. Because of
its high degree of responsiveness to environmental
conditions, skin pigmentation is of
no value in assessing the phylogenetic
relationships between human groups.
SKINIPEDIA - your Skin Encyclopedia