Evolution And History

Studying Human Biological Diversity 293

from their original homelands to other regions, or the

practice of selective mating, or both.

Because skin cancer generally does not develop until

later in life, it is less likely to have interfered with the re-

productive success of lightly pigmented individuals in the

tropics and so is probably not the agent of selection. On

the other hand, severe sunburn, which is especially dan-

gerous to infants, causes the body to overheat and inter-

feres with its ability to sweat and rid itself of excess heat.

Furthermore, it makes one susceptible to other kinds of

infection. In addition to all this, decomposition of folate,

an essential vitamin sensitive to heavy doses of ultraviolet

radiation, can cause anemia, spontaneous abortion, and

infertility.^18

In northern latitudes, light skin has an adaptive ad-

vantage related to the skin’s important biological function

as the manufacturer of vitamin D through a chemical re-

action dependent upon sunlight. Vitamin D is vital for

maintaining the balance of calcium in the body. In north-

ern climates with little sunshine, light skin allows enough

sunlight to penetrate the skin so as to stimulate the

cells than those with light skin, but everyone (except those
with a condition called albinism) has a measure of mela-
nin. Exposure to sunlight increases melanin production,
causing skin color to deepen.
Melanin is known to protect skin against damaging
ultraviolet solar radiation;^17 consequently, dark-skinned
people are less susceptible to skin cancer and sunburn than
are those with less melanin. They also seem to be less sus-
ceptible to destruction of certain vitamins under intense
exposure to sunlight. Because the highest concentrations
of dark-skinned people tend to be found in the tropical
regions of the world, it appears that natural selection has
favored heavily pigmented skin as a protection against ex-
posure where ultraviolet radiation is most constant.
The inheritance of skin color involves several genes
(rather than variants of a single gene), each with several
alleles, thus creating a continuous range of phenotypic ex-
pression for this trait. In addition, the geographic distribu-
tion or cline of skin color, with few exceptions, tends to be
continuous (Figures 12.2 and 12.3). The exceptions have
to do with the historic movement of certain populations

Lightest

Medium light

Medium

Medium dark

Darkest

Figure 12.2 This map illustrates the distribution of dark and light human skin pigmentation
before 1492. Medium-light skin color in Southeast Asia reflects the spread into that region of
people from southern China, whereas the medium darkness of people native to southern Australia
is a consequence of their tropical Southeast Asian ancestry. Lack of dark skin pigmentation among
tropical populations of Native Americans reflects their more recent ancestry in northeastern Asia a
mere 20,000 or so years ago.

(^17) Neer, R. M. (1975). The evolutionary significance of vitamin D, skin pig-
ment, and ultraviolet light. American Journal of Physical Anthropology 43,
409–416.
(^18) Branda, R. F., & Eatoil, J. W. (1978). Skin color and photolysis: An evolu-
tionary hypothesis. Science 201, 625–626.