Survival of the Sickest: The Surprising Connections Between Disease and Longevity. Jonathan Prince

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СКАЧАТЬ circumstances, almost as soon as you are exposed to the sun, your pituitary gland produces hormones that act as boosters for your melanocytes, and your melanocytes start producing melanin on overdrive. Unfortunately, it’s very easy to disrupt that process. The pituitary gland gets its information from the optic nerve – when the optic nerve senses sunlight, it signals the pituitary gland to kick-start the melanocytes. Guess what happens when you’re wearing sunglasses? Much less sunlight reaches the optic nerve, much less warning is sent to the pituitary gland, much less melanocyte – stimulating hormone is released, much less melanin is produced – and much more sunburn results. If you’re reading this on the beach with your Ray-Bans on, do your skin a favor – take them off.

      Tanning helps people cope with seasonal differences in sunlight in their ancestral climate; it’s not enough protection for a Scandinavian at the equator. Someone like that – with very little natural ability to tan and regular, unprotected exposure to tropical sun – is vulnerable to severe burning, premature aging, and skin cancer, as well as folic acid deficiency and all its associated problems. And the consequences can be deadly. More than 60,000 Americans are diagnosed with melanoma – an especially aggressive type of skin cancer – every year. European Americans are ten to forty times as likely to get melanoma as African Americans.

      As humanity was evolving, we probably had pretty light skin too, underneath a similar coat of coarse, dark hair. As we lost hair, the increased exposure of our skin to ultraviolet rays from the strong African sun threatened the stores of folate we need to produce healthy babies. And that created an evolutionary preference for darker skin, full of light – absorbing, folate-protecting melanin.

      As some population groups moved northward, where sunlight was less frequent and less strong, that dark skin – “designed” to block UVB absorption – worked too well. Now, instead of protecting against the loss of folate, it was preventing the creation of vitamin D. And so the need to maximize the use of available sunlight in order to create sufficient vitamin D created a new evolutionary pressure, this time for lighter skin. Recent scientific sleuthing reported in the prestigious journal Science goes so far as to say that white-skinned people are actually black-skinned mutants who lost the ability to produce significant amounts of eumelanin.

      Redheads, with their characteristic milky white skin and freckles, may be a further mutation along the same lines. In order to survive in places with infrequent and weak sunlight, such as in parts of the U.K., they may have evolved in a way that almost completely knocked out their body’s ability to produce eumelanin, the brown or black pigment.

      In 2000, an anthropologist named Nina G. Jablonski and a geographic computer specialist named George Chaplin combined their scientific disciplines (after already combining their lives in marriage) to chart the connection between skin color and sunlight. The results were as clear as the sky on a cloudless day – there was a near-constant correlation between skin color and sunlight exposure in populations that had remained in the same area for 500 years or more. They even produced an equation to express the relationship between a given population’s skin color and its annual exposure to ultraviolet rays. (If you’re feeling adventurous, the equation is W = 70 – AUV/10. W represents relative whiteness and AUV represents annual ultraviolet exposure. The 70 is based on research that indicates that the whitest possible skin – the result of a population that received zero exposure to UV – would reflect about 70 percent of the light directed at it.)

      Interestingly, their research also proposes that we carry sufficient genes within our gene pool to ensure that, within 1,000 years of a population’s migration from one climate to another, its descendants would have skin color dark enough to protect folate or light enough to maximize vitamin D production.

      There is one notable exception to Jablonski and Chaplin’s equation – and it’s the exception that proves the rule. The Inuit – the indigenous people of the subarctic – are dark-skinned, despite the limited sunlight of their home. If you think something fishy’s going on here, you’re right. But the reason they don’t need to evolve the lighter skin necessary to ensure sufficient vitamin D production is refreshingly simple. Their diet is full of fatty fish – which just happens to be one of the only foods in nature that is chock-full of vitamin D. They eat vitamin D for breakfast, lunch, and dinner, so they don’t need to make it. If you ever had an enthusiastically caring grandmother try to force cod liver oil down your throat, she was onto something for the same reason – since it’s full of vitamin D, cod liver oil was one of the best ways to prevent rickets, especially before milk was routinely fortified with it.

      If you’re wondering how people who have dark skin make enough vitamin D despite the fact that their skin blocks all those ultraviolet rays, you’re asking the right questions. Remember, ultraviolet rays that penetrate the skin destroy folate – and ultraviolet rays that penetrate the skin are necessary to create vitamin D. Dark skin evolved to protect folate, but it didn’t evolve with a switch – you can’t turn it off when you need to whip up a batch of vitamin D. So that would seem to create a new problem for people with dark skin – even if they lived in a sunny climate – because even though they received plenty of exposure to ultraviolet rays, the skin color that protected their supply of folate would prevent them from stocking up on vitamin D.

      It’s a good thing evolution’s such a clever sort, because it took that into account – it kept room for a little guy called apolipoprotein E (ApoE4) in the gene pool of dark-skinned population groups. And guess what ApoE4 does? It ensures that the amount of cholesterol flowing through your blood is cranked up. With more cholesterol available for conversion, dark-skinned people can maximize the use of whatever sunlight penetrates their skin.

      Much farther to the north, without a similar adaptation, the light-skinned people of Europe would face a similar problem. There, instead of plenty of sunlight that was largely blocked by dark skin, they had to deal with too little sunlight to make enough vitamin D even with the benefit of their light skin. And sure enough, ApoE4 is also common throughout Northern Europe. The farther north you go up the continent, the more you’ll find it. As it does in Africans, the ApoE4 gene keeps cholesterol levels cranked up, allowing its carriers to compensate for limited ultraviolet exposure by maximizing the cholesterol available for conversion to vitamin D.

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