The Biology of Skin Color: What Makes Us Black and White?

By | May 5, 2009

albinoJablonski, now chairman of the anthropology department at the California Academy of Sciences, begins by assuming that our earliest ancestors had fair skin just like chimpanzees, our closest biological relatives. Between 4.5 million and 2 million years ago, early humans moved from the rain forest and onto the East African savanna. Once on the savanna, they not only had to cope with more exposure to the sun, but they also had to work harder to gather food.

Mammalian brains are particularly vulnerable to overheating: A change of only five or six degrees can cause a heatstroke. So our ancestors had to develop a better cooling system. The answer was sweat, which dissipates heat through evaporation. Early humans probably had few sweat glands, like chimpanzees, and those were mainly located on the palms of their hands and the bottoms of their feet. Occasionally, however, individuals were born with more glands than usual.

The more they could sweat, the longer they could forage before the heat forced them back into the shade. The more they could forage, the better their chances of having healthy offspring and of passing on their sweat glands to future generations. A million years of natural selection later, each human has about 2 million sweat glands spread across his or her body.

Human skin, being less hairy than chimpanzee skin, “dries much quicker,” says Adrienne Zihlman, an anthropologist at the University of California at Santa Cruz. “Just think how after a bath it takes much longer for wet hair to dry.” Hairless skin, however, is particularly vulnerable to damage from sunlight. Scientists long assumed that humans evolved melanin, the main determinant of skin color, to absorb or disperse ultraviolet light. But what is it about ultraviolet light that melanin protects against? Some researchers pointed to the threat of skin cancer.

But cancer usually develops late in life, after a person has already reproduced. Others suggested that sunburned nipples would have hampered breast-feeding. But a slight tan is enough to protect mothers against that problem. During her preparation for the lecture in Australia, Jablonski found a 1978 study that examined the effects of ultraviolet light on folate, a member of the vitamin B complex. An hour of intense sunlight, the study showed, is enough to cut folate levels in half if your skin is light. Jablonski made the next, crucial connection only a few weeks later.

At a seminar on embryonic development, she heard that low folate levels are correlated with neural-tube defects such as spina bifida and anencephaly, in which infants are born without a full brain or spinal cord. Jablonski later came across three documented cases in which children’s neural-tube defects were linked to their mothers’ visits to tanning studios during early pregnancy. Moreover, she found that folate is crucial to sperm development — so much so that a folate inhibitor was developed as a male contraceptive. (“It never got anywhere,” Jablonski says. “It was so effective that it knocked out all folate in the body.”) She now had some intriguing evidence that folate might be the driving force behind the evolution of darker skin.

But why do some people have light skin? As far back as the 1960s, the biochemist W. Farnsworth Loomis had suggested that skin color is determined by the body’s need for vitamin D. The vitamin helps the body absorb calcium and deposit it in bones, an essential function, particularly in fast-growing embryos. (The need for vitamin D during pregnancy may explain why women around the globe tend to have lighter skin than men.)

Unlike folate, vitamin D depends on ultraviolet light for its production in the body. Loomis believed that people who live in the north, where daylight is weakest, evolved fair skin to help absorb more ultraviolet light and that people in the tropics evolved dark skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at high concentrations. By the time Jablonski did her research, Loomis’s hypothesis had been partially disproved. “You can never overdose on natural amounts of vitamin D,” Jablonski says. “There are only rare cases where people take too many cod-liver supplements.”

But Loomis’s insight about fair skin held up, and it made a perfect complement for Jablonski’s insight about folate and dark skin. …People in the tropics have developed dark skin to block out the sun and protect their body’s folate reserves. People far from the equator have developed fair skin to drink in the sun and produce adequate amounts of vitamin D during the long winter months.

PBS

3 thoughts on “The Biology of Skin Color: What Makes Us Black and White?

  1. Tod

    PUVA Treatment
    PUVA treatment is the usage of UVA light in combination with psoralens (drugs) to make the skin hyper-photosensitive and is effective in the treatment psoriasis. However aside from any concerns about the UVA, the number of treatments must be carefully monitored and restricted over the lifetime of a patient due to the potential of liver damage caused by the psoralens”.

    This treatment using UVA and very powerful psoralens is what the subjects of the study (Branda & Eaton1978) that Nina G Jablonski cites as evidence that folic acid is destroyed by sunlight was studying. Moreover 5-Methyltetrahydrofolate inhibits photosensitization reactions and strand breaks in DNA
    says(by the way 5-methyltetrahydrofolate is the predominant folate in plasma not folic acid).

    “In the absence of photosensitizers, 5-MTHF is directly decomposed only very slowly by UVA or UVB.[..]

    In the absence of photosensitizers, photodegradation of the natural folate 5-MTHF directly by UVA was only marginally faster than that due to autooxidation in the dark. ”

    Our results demonstrate that the photolysis of folate observed in their study was not due to the intrinsic photolability of 5-MTHF but instead may have been mediated by the methoxalen or other photosensitizers in plasma. An association between exposure to light and clinical folate deficiency has not yet been clearly established. Nonetheless, our results suggest that, depending on the presence of photosensitizers and possibly ascorbate status in the skin and/or plasma, sunlight or UVA may affect the folate pool. The hypothesis that skin color evolved to balance the need for adequate vitamin D production while minimizing not only burns as a major factor, but also destruction of folate should be expanded to consider the (5 , 11) effects of endogenous photosensitizers on 5-MTHF.

    How explain Serum folate levels after UVA exposure: a two-group parallel randomised controlled trial

    Our data suggest that both single and serial UVA exposures do not significantly influence serum folate levels of healthy subjects. Therefore, neural tube defects claimed to occur after periconceptual UVA exposure are probably not due to UVA induced folate deficiency

    About Vitamin D
    Rickets is a nutritional disease, which Nigerian children get by the way.

    So this is rickets that has nothing to do with vitamin D deficiency, but with mineral deficiency?

    Loren Cordain: No, both. Cereal grains seem to have a simultaneous influence on vitamin D and Ca metabolism.

    Robert Crayhon: How do they alter vitamin D metabolism?

    Loren Cordain: Epidemiological studies of populations consuming high levels of unleavened whole grain breads show vitamin D deficiency to be widespread. A study of radio-labelled 25 hydroxyvitamin D3 (25(OH)D3) in humans consuming 60g of wheat bran daily for 30 days clearly demonstrated an enhanced elimination of 25(OH)D3 in the intestinal lumen. The mechanism by which cereal grain consumption influences vitamin D is unclear. Some investigators have suggested that cereal grains may interfere with the enterohepatic circulation of vitamin D or its metabolites, whereas others have shown that calcium deficiency increases that rate of inactivation of vitamin D in the liver. This effect is mediated by 1,25 dihydroxyvitamin D (1,25(OH)2D) produced in response to secondary hyperparathyroidism, which promotes hepatic conversion of vitamin D to polar inactivation products which are excreted in bile. Consequently, the low Ca/P ratio of cereal grains has the ability to elevate PTH which in turn stimulates increased production of (1,25(OH)2D) which causes an accelerated loss of 25 hydroxy vitamin D.

    Which makes this less surprising
    Know Your Pathology: Osteomalacia and Rickets

    Evidence has been found in Neolithic skeletons from Denmark and Norway, and more plentiful evidence comes from Hungary in the Roman period. However, the rarity of this disease in the past is attested by the few cases described even in exhaustive studies of human remains prior to the Medieval period.

    Modern humans entered Northen Europe over thirty thousand years ago the European specific alleles for skin lightening do not date to that time, one that goes with red hair is about 80,000 years old the others are estimated at about 15,000-3000 years old.

    According to Veith ‘The Pharmacology of Vitamin D, Including Fortification Strategies’

    During summer, we accumulate vitamin D3 and store it, so that supplies for vitamin D do not become completely depleted during the winter months. Within three days of a dose of vitamin D3, very little of the original vitamin D is detectable in plasma of rats (101) or humans (102). Most vitamin D entering the circulation appears to be excreted unmetabolized into the bile.[…]

    If 30,000 years after entering northern Europe natural selection hasn’t changed this (as it easilly could). I wonder if the reason isn’t that the Vitamin D requrements are met, and then some, by North European sunshine.

    If one looks at the system of vitamin D metabolism in Figure 2 from the perspective of a system
    designed to control something, it becomes clear that this is a system better designed to cope with an abundance of supply, not a lack of it. The flow of vitamin D toward 25(OH)D is remarkably inefficient, with most bypassing it. Furthermore, there is no way to correct for deficiency of vitamin D, other than to redirect utilization of 25(OH)D toward 1,25(OH)2D production, which is the pathway most acutely important for life. That is, when supplies of
    vitamin D are severely restricted, its metabolism is directed only toward the maintenance of calcium homeostasis. To expand on the point that the system of vitamin D metabolism is effectively designed for adjusting for higher inputs, not lower inputs, I offer the example of an air-conditioner system. Air conditioners are designed to compensate for excessive heat, but they are a useless way to compensate for a cold environment.

    This points towards a natural homeostasis of vitamin D synthesis and storage in northern Europe that has not modified from the one evolved in Africa because it’s still dealing with an excess.

    Vitamin D Council

    Studies show that if you go out in the summer sun in your bathing suit until your skin just begins to turn pink, you make between 10,000 and 50,000 units of cholecalciferol in your skin. Professor Michael Holick of Boston University School of Medicine has studied this extensively and believes a reasonable average of all the studies is 20,000 units. That means a few minutes in the summer sun produces 100 times more vitamin D than the government says you need! As discussed in other pages, this is the single most important fact about vitamin D.

    The skin does another amazing thing with cholecalciferol. It prevents vitamin D toxicity. Once you make about 20,000 units, the same ultraviolet light that created cholecalciferol, begins to degrade it. The more you make, the more is destroyed. So a steady state is reached that prevents the skin from making too much cholecalciferol. This is why no one has ever been reported to develop vitamin D toxicity from the sun, though it is possible when taken orally.

    If the synthesis of Vitamin D was so difficult due to low levels of UVB in northern Europe why would there be a mechanism to restrict it after only 20 minutes. Well UVB may be most intense near the equator but clearly it’s intense enough in the northern European summer to synthesize and store so much that the limiting mechanism and systen of metabolism is orientated to cope with excess, not scarcity.

    What Makes Us Black and White? SEXUAL SELECTION AND HUMAN GEOGRAPHIC VARIATION

  2. ryan @ vitamin d benefits

    It is useful to know that consuming Vitamin D is useful for treating chronic pains. Since a lot of diseases cause pain and in some way or the other, are triggered off by inadequate supply of vitamin D in the body, increased intake of vitamin D can help ease the pain.

Leave a Reply