It was asked
> Yes, obviously genetic but what I was really wanting to know was
why
> certain populations,such as the Guanches, northern Europeans and
> even some Australian Aboriginies have a tendency towards lighter
> hair or eye coloration. Is it just a genetic accident or was there
> some environmental factor that changed the appearance of man as he
> spread around the world?
> Thanks for your useful replies to my other questions.
Blond hair colouration was originally a cause for looking at
Caucasoid connections. Thus the Aboriginal Australoid, has been
called "Paleo-Caucasoid", and the presence of fiar hair amongst
certain Amerind groups (Mandan, Cheyenne and in Peru), has led to
certain theories about early pre-Columban migrations across the
Atlantic (eg. Heyerdahl's "Ra", the stories of St Brendan etc).
These theories have now been thoroughly discredited by the Human
Genome Diversity Project (HGDP).
Lighter hair colouration between Europeans and the rest of the world
seems due to a number of factors. Firstly there seems that there is
a close corelation between light hair and light skin, that is not
observed amongst Aboriginal people where light hair colouring comes
independent of a light skin colouration. This is confirmed by the
human genome diversity project which has shown that hair colouration
in Europeans and the rest of the world is determined by a different
set of genes (albeit having the same end result - fair hair).
Whether the difference in skin colouration is seen amongst the
Cheyenne in the USA or amongst the Aymara and Quechucha of Peru
(peoples who also show a tendency towards fairness of hair on
occasions), I cannot find out, as the data does not show.
Firstly there is a clear association between climate and hair
colouration, that seems, in the European case, to be linked with skin
colour. The absence of melanin in the skin also leads to the absence
of melanin in the eyes (blue eyes) and an absence of melanin in the
hair follicles (leading to blond hair). There are a number of
theories as to why skin colour is darkest in the tropics - and these
have been confirmed that melanin offers protection against
ultraviolet rays of the sun. Dark skin reduces the likelihood of
skin cancer. It is shown, for instance that one of the mutations
regarding European hair colour, leading melanin to "clump" and be
totally absent from neighbotring parts of the hair and skin -
producing "freckles" - is especially prone to skin cancers. Thus we
find the population most prone to skin cancer are eople with red hair
and green eyes (these are also traits caused by the "clumping" of
melanic cells), who also tend to be the people who suffer most from
freckles. Such freckling is not just a European trait, but also
appears eleswhere in the world as a recessive mutation.
That Europeans seem to have most concentration of a genetic
characteristic that is harmfull (absence of melanin) suggests that
there has been a secondary factor which has led to its appearance, a
factor which has enabled it to have some sellection advantage, which
has masked the disadvantage found elsewhere. This disadvantage seems
to have been agriculture.
Humans get vitamin D either by producing it in their skin (from
ultraviolet light) or from their diet. Absence of vitamin D shows up
in the condition of rickets. The shift to a farming way of life and
the movement of the first (presumably dark haired) farmers, increased
the prevailance of rickets as contemporary graves showed. Diets were
deficinet in Vitamin D compared to that of the original hunter
gatherers, and their melanin rich dark hair and dusty skins prevented
vitamin D being synthesised in their skin. Evolution would favour
mutatons which would reduce skin melanin - and two became fixed
within the European population, one which promoted melanic clumping
(red hair) and the other that reduced melanin in the skin altogether
(blind hair).
Amongst the Aboriginal people of Australia, a completely different
pathway led to the "fair hair" genes appearing. These are people who
live in some of the hottest places on Earth. Here dark hair greatly
adds to the risks of heat stroke, especially if vegetation cover is
rare. Human evolution here would also promote the appearance of fair
hair, but not fairness of skin, which would be prevented due to
increased cancer risks. This is exactly what one would observe.
Now regarding the fairness of Berbers. It has been in the past
believed that the fairness of Berbers has been due to the immigration
of Nordics - eg. Mycenaean Greeks through Cyrenica, or perhaps
Gaiserics Vandals in North Africa. The Guanche situation shows both
these theories to be incorrect. Nevertheless, the Berbers like the
Australian Aboriginal populations also originally come from one of
the hottest and driest environments in the world. It would appear
that the explanation for fairness amongst these people may have more
in common with the Aboriginal people than it has with the European.
Similarly the fairness of Peruvian and northern Chilean Amerindians
also seems linked with inhabiting a desert environment, where black
hair increases likeliness of heat stroke.
Final comments regarding hair colour. Fair hair is also sometimes
seen in North China. This seems to be the one case where a migration
from Europe may have resulted in fair colouraton alleles in the
population, asit has been shown that fair-haired Iranian populations
inhabited the Kansu corridor down to historic times. The expansion
of dark haired Turkic and Mongolian peoples on the steppe has seen a
darker colouring replace the fair-haired Indo Iranians, although it
is sometimes said that Ghenghis Khan had red hair.
Research is continuing in Britain to see if the association of red
hair and firey temperament is more than a myth, and also to check to
see if, given the proneness to cancer, red hair gives any other
advantages.
Hope this helps
Regards
John