Common Humanity: More Than a Dream  

We belong to an extended family in which all of our planetary co-habitants are cousins, at one remove or another. How right we are, then, to dream of the warm embrace of our more immediate human family.

By: Dr. David L. Katz*

Martin Luther King Jr. dreamed of a society affirming equality, embracing a common humanity. He would have been pleased to know that science was on his side all along.

In 2005, scientists at Pennsylvania State University reported in the prestigious journal Science the discovery of a genetic mutation responsible for white skin. The report suggested that among our distant ancestors on a northward journey out of Africa, one was 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. Absent vitamin D in childhood, bone will not grow, resulting in a stunting and crippling condition called Rickets. Once epidemic in industrialized countries, Rickets is 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 what happened. 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 an isolated 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. Consequently some of us – in common with our mammalian cousins – are lactose intolerant as adults. The rest of us are dependent on a mutation that allows us to enjoy milkshakes.
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!
How misleading our superficial differences! We belong to an extended family in which all of our planetary co-habitants are cousins, at one remove or another. How right we are, then, to dream of the warm embrace of our more immediate, human family.
In 1953, James Watson and Francis Crick discovered that the recipe of life resides in the double helical strands of DNA. We have been beneficiaries of accelerating genetic insights ever since.
The knowledge we need to see past our superficial differences and claim our kinship is increasingly available to us, yet all too often ignored in this divisive world. Our very DNA is a spiral staircase we might all ascend to understand ourselves, and one another, better.
Science supports King’s dream, and the aspirations of all who perceive a common family of humankind. Perhaps what we know about skin pigment 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.

* David L. Katz, MD, MPH, FACPM, FACP; Director, Prevention Research Center, Yale University School of Medicine and medical contributor for ABC News. He may be reached at www.davidkatzmd.com.)