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Of Skin and Kin at Christmas

How misleading our superficial differences! How right we are to think of a human family, and indeed, an extended family in which all of our planetary cohabitants are cousins, at one remove or another.
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In 2005, scientists at Penn State University reported in the prestigious journal Science the discovery of a genetic mutation responsible for white skin. The report suggested that one among our distant ancestors on a northward journey out of Africa, was simply, suddenly born pale due to a genetic error. Natural selection favors the propagation of such errors when they confer a survival advantage, and this one likely did.

We all rely on the action of sunlight on skin to generate vitamin D. Vitamin D, in turn, is essential for the absorption of calcium from food, and the growth and maintenance of bone -- among many other functions. Absent vitamin D in childhood, bone will not grow, resulting in a stunting and crippling condition called rickets, once epidemic in industrialized countries -- and the very reason why milk is now routinely vitamin D fortified. Vitamin D deficiency in adulthood also leads to disabling bone disease.

The dark pigment in skin that protects tropical peoples from the sun's intensity attenuates the manufacture of vitamin D. With intense sun exposure, this is not a problem, as the quantity of sunlight compensates perfectly for the relative inefficiencies of vitamin D production, leading to just the right levels. But when dark-skinned people migrated to more temperate climes, the costs of dark skin would have started to outweigh the benefits.

On such a trek, a fair-skinned "mutant" would prosper. That, apparently, is just what happened. And we see the culmination of this trend in familiar demographic patterns. The fairest skinned peoples, such as Scandinavians and Irish, live far from the equator and/or under frequent overcast.

While most of us already know that the cradle of humanity was in Africa, and that we all trace our ancestry there, the genetic discovery about skin tone adds an important footnote to the epic tale of human dispersion to different corners of the globe, and different cultures. But for a mutation, we are all black.

While thinking of mutation as abnormality is questionable in the context of natural selection, that is nonetheless the common implication. If we choose to invoke that vernacular, and accept a mutation as a deviation from normal, then black skin is normal and white skin an aberration. Some of us are mutants with a pallor problem. How might the history of humankind have differed had this genetic insight long prevailed?

Nor is this an isolated revelation about our common bonds to one another, and indeed, to other species, too. We refer to "lactose intolerance" when adults have trouble digesting lactose, the complex sugar in milk. But, in fact, lactose "tolerance" is the abnormality.

In nature, mammals have access to milk only in infancy. Genes that produce the enzyme "lactase," needed to digest milk sugar, shut off in early childhood in all mammals. But for some human cultures, with a long history of dairying, a survival advantage was conferred when a mutation prevented these genes from shutting down. So some of us, in common with all of our mammalian cousins, are lactose intolerant as adults. The rest of us are dependent on a mutation that lets us enjoy milkshakes.

And at the risk of offending any strict biblical sensibilities, it's only fair to note that science tells a tale of the sexes a bit at odds with scripture. All human embryos develop female features at first. Then, the action of hormones called androgens, under the guidance of genes on a Y chromosome, convert this template to male in some of us. But for the action of these hormones, we are all female. And, indeed, there is a medical condition -- testicular feminization -- in which androgen receptors are insensitive, and a person genetically programmed to be male, with XY chromosomes, is for all the world female, albeit sterile. So but for a mutation or two, we are all both black and female.

In a characteristically masterful approach to the complexities of evolution, Richard Dawkins, formerly of Oxford University, compares the development of biodiversity over eons to a pilgrimage through time in The Ancestor's Tale (Houghton Mifflin, 2005). Along with pointing out the connections among all species, increasingly demonstrable with ironclad molecular techniques that are to fossils what space travel is to a rickshaw, Dawkins explains how trivially appearance reflects true biological diversity. Experts agree that there is more fundamental genetic diversity among different types of bacteria, than among all other species put together. Yet those other species look as different from one another as penguins from pine trees, while to our eyes, bacteria are relatively indistinguishable, microscopic specks.

How misleading our superficial differences! How right we are to think of a human family, and indeed, an extended family in which all of our planetary cohabitants are cousins, at one remove or another.

And so it is we find ourselves at Christmas, or as it is known more generically, the "holiday season." Whatever one's faith, this is a season of gently fervent hope, and soaring ideals. Such elusive goals as "peace on earth and good will toward men" canter from our lips, infuse our greetings, and ride the melodies of long-familiar holiday tunes. Is there any reason to think that this year our actions are more likely than in years past to follow this rhetoric toward a better future for us all?

Probably not, though it's a consummation devoutly to be wished. Any year we choose, such hopes could climb the spiral staircase of our DNA to understand ourselves, and one another, better. From the height of that stair, it is abundantly clear that we are all kin, our divisions of little more consequence than variations in skin pigment.

Science supports the aspirations of this season to place us all in a common family of man, a family of humankind. Perhaps what scientists have learned about skin will help us see past it, and other such veneers, to the deeper truths, the common bonds, and the claims to a better destiny -- that coil within.

Please accept my best holiday wishes for you and your loved ones. After all -- we are related.


Dr. Katz' new book, DISEASE PROOF, is available in bookstores nationwide and at:

Dr. David L. Katz;

N.B.- this column was adapted from an article published in the New Haven Register in 2005

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