[Paleopsych] SW: On Geography and Skin Color

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On Geography and Skin Color

    The following points are made by Jared Diamond (Nature 2005 435:283):
    1) The most obvious -- and most discussed -- aspect of human
    geographical variability is skin color. Most people would say that
    skin color becomes darker towards the Equator to give more protection
    against tropical sunlight. But that claimed correlation of skin color
    with latitude is riddled with exceptions, and that functional
    interpretation of the correlation is debated. Most scientists shy away
    from the whole subject because it so interests racists, and the
    motives of scientists studying it become suspect.
    2) Jablonski and Chaplin [1-3] have brought order to this confused
    field, starting with quantitative measurements of skin color and
    sunlight. By convincingly identifying the strongest correlate of skin
    color, they open the door for anthropologists to explore other
    correlates and exceptions.
    3) Skin color was formerly described qualitatively by matching it
    against colored tablets, but Jablonski and Chaplin tabulate numerical
    values, obtained by skin reflectance spectrophotometry. And instead of
    using latitude as a proxy for sunlight, Jablonski and Chaplin tabulate
    ultraviolet radiation (UVR) itself at the Earth's surface. UVR does
    decrease with latitude, because at high latitudes the oblique angle at
    which sunlight falls on the atmosphere results in a longer atmospheric
    path, and hence more absorption and scattering of UVR. But the
    correlation of UVR with latitude is imperfect: UVR also increases with
    altitude owing to atmospheric thinning (for example, UVR is high on
    the Tibetan and Andean altiplanos); it also decreases with atmospheric
    water vapour in the form of rain, clouds or humidity (UVR is higher in
    the Atacama Desert, southwestern United States, and the Horn of
    Africa, than in adjacent, wetter areas to the west or east).
    4) In this quantitative database, variation in UVR proves to be the
    strongest predictor of skin reflectance, explaining 77% (Northern
    Hemisphere) or 70% (Southern Hemisphere) of its variation. The causes
    of this correlation have been the subject of many theories, such as
    protection against skin cancer, protection against overproduction of
    vitamin D, and camouflage in tropical jungles.
    5) Jablonski and Chaplin prefer a combination of two selective factors
    involving several costs and one benefit of UVR. The costs involve the
    destructive photolysis of many compounds, of which Jablonski and
    Chaplin attach particular importance to the B vitamin folate.
    Everybody requires folate, so everybody would have dark skins (to
    screen out UVR and reduce photolysis) if there were no other selective
    factors. However, UVR also provides a benefit: catalysing the
    synthesis of vitamin D. Hence skin color evolves as a compromise
    between skins light enough to permit UVR penetration for vitamin D
    synthesis, but dark enough to reduce folate photolysis.[4,5]
    References (abridged):
    1. Jablonski, N. G. Annu. Rev. Anthropol. 33, 585-623 (2004)
    2. Chaplin, G. Am. J. Phys. Anthropol. 125, 292-302 (2004)
    3. Jablonski, N. G. & Chaplin, G. J. Hum. Evol. 39, 57-106 (2000)
    4. Hawkes, K. et al. Am. J. Hum. Biol. 15, 380-400 (2003)
    5. Diamond, J. Nature 410, 521 (2001)
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by D.A. Hughes et al (Current Biology
    2004 14:R367):
    1) Systematists have not defined a "type specimen" for humans, in
    contrast to other species. Recent attempts to provide a definition for
    our species, so-called "anatomically modern humans", have suffered
    from the embarrassment that exceptions to such definitions inevitably
    arise -- so are these exceptional people then not "human"? Anyway, in
    comparison with our closest-living relatives, chimpanzees, and in
    light of the fossil record, the following trends have been discerned
    in the evolution of modern humans: increase in brain size; decrease in
    skeletal robusticity; decrease in size of dentition; a shift to
    bipedal locomotion; a longer period of childhood growth and
    dependency; increase in lifespan; and increase in reliance on culture
    and technology.
    2) The traditional classification of humans as Homo sapiens, with our
    very own separate family (Hominidae) goes back to Carolus Linnaeus
    (1707-1778). Recently, the controversial suggestion has been made of
    lumping humans and chimpanzees together into at least the same family,
    if not the same genus, based on the fact that they are 98-99%
    identical at the nucleotide sequence level. DNA sequence similarity is
    not the only basis for classification, however: it has also been
    proposed that, in a classification based on cognitive/mental
    abilities, humans would merit their own separate kingdom, the
    Psychozoa (which does have a nice ring to it).
    3) As for sub-categories, or "races", of humans, in his Systema
    Naturae of 1758 Linnaeus recognized four principal geographic
    varieties or subspecies of humans: Americanus, Europaeus, Asiaticus,
    and Afer (Africans). He defined two other categories: Monstrosus,
    mostly hairy men with tails and other fanciful creatures, but also
    including some existing groups such as Patagonians; and Ferus, or
    "wild boys", thought to be raised by animals, but actually retarded or
    mentally ill children that had been abandoned by their parents. In his
    scheme of 1795, Johann Blumenbach (1752-1840) added a fifth category,
    Malay, including Polynesians, Melanesians and Australians.
    4) Blumenbach is also responsible for using the term "Caucasian" to
    refer in general to Europeans, which he chose on the basis of physical
    appearance. He thought Europeans had the greatest physical beauty of
    all humans -- not surprising, as he was of course European himself --
    and amongst Europeans he thought those from around Mount Caucasus the
    most beautiful. Hence, he named the "most beautiful race" of people
    after their supposedly most beautiful variety -- a good reason to
    avoid using the term "Caucasian" to refer to people of generic
    European origin (another is to avoid confusion with the specific
    meaning of "Caucasian", namely people from the Caucasus).
    5) The extent to which racial classifications of humans reflect any
    underlying biological reality is highly controversial; proponents of
    racial classification schemes have been unable to agree on the number
    of races (proposals range from 3 to more than 100), let alone how
    specific populations should be classified, which would seem to greatly
    undermine the utility of any such racial classification. Moreover, the
    apparent goal of investigating human biological diversity is to ask
    how such diversity is patterned and how it came to be the way that it
    is, rather than how to classify populations into discrete
    1. Nature Encyclopedia of the Human Genome. (2003). Cooper, D. ed.
    (Nature Publishing Group),
    2. Fowler, C.W. and Hobbs, L. (2003). Is humanity sustainable?. Proc.
    R. Soc. Lond. B. Biol. Sci. 270, 2579-2583
    3. Encyclopedia of Human Evolution and Prehistory. (1988). Tattersall,
    I., Delson, E., and Van Couvering, J. eds. (Garland Publishing)
    4. World Health Organization Website http://www.who.int
    Current Biology http://www.current-biology.com

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