All living human populations belong to a single
biological species (Homo sapiens) within a larger group or
genus (Homo). Within the human species, a large number of
populations may be differentiated genetically through readily
observable charasteristics ( e.g., skin, hair, and face and
body proportions) and through less obvious but more distinctive
biological traits, such as blood type. These biological groupings
within species are commonly called races, in humans as well
as in other living forms.
Black skin (people from Africa, Native Australians,
African-Americans, Caribaeans), is darker than northern European
skin because of the increased amount of melanin in the skin.
Long before the microscope made it possible to count melanin
granules in skin sections, naturalists had come to appreciate
the importance of differences in skin (and eye) pigmentation.
They observed a direct relationship between sunlight levels
and skin pigmentation, at least in Europe and Africa. Later
it was detemined that the pigment granules serve as a natural
sunscreen, protecting the deeper layers of skin as harmful
ultraviolet wavelengths in sunlight are absorbed, and that
there is correlation between higher melanin content and slightly
lower sweating thresholds.Conversely, low levels of melanin
in the skin appear to be an advantage in parts of the world
where cloudy conditions are common or where (as in the polar
regions) there are prolonged periods of limited sunlight.
Lighter-skinned individuals are more able to convert vitamin
D precursors in the skin to usable vitamin D . Disorders caused
by vitamin D deficiency, are more common among dark-skinned
children living in northern Europe, with its generally cloudier
climate and shortened hours of sunlight in winter. It has
been demonstrated that the rate of skin cancer rises sharply
as pigment densities go down. Light-skinned individuals may
generate considerable melanin, with continued exposure to
sunlight (and so tan), but this does not occur until after
damage to the skin has been done. Those who have less melanin
in their irises are also more likely to develop cataracts
from long-term exposure to ultraviolet radiation.
does hair turn grey?
|The color of a person's hair too is due to melanin
and how it is distributed. When the melanocytes
(the cells that produce melanin) cease to function,
the hair loses its color. In fact, there is nothing
known as grey hair, it's only a term commonly used.
In fact, the hair turns white. The intermingling
of white hairs with those still retaining their
color causes the grey appearance.