Gene Expression

Skin color & Vitamin D & folate

i-e3de6b461c00b1e249947100ade5a354-footcolor.jpgRecently I had some blood work done for a routine check up and it turned out that I had vitamin D deficiency. The doctor explained to me that this is common amongst darker skinned people who live at high latitudes, especially in areas where cloudiness is the norm. That would fit the bill for my own ecological surroundings for these past few years, but I never believed that I would be susceptible to vitamin D deficiency because I perceived myself as someone who got sufficient sun. To the left you see a photo of my foot. I normally wear sandals, so please note the contrast between my exposed and unexposed skin in terms of pallor. But I was obviously fooling myself, my vitamin D levels were half that of the lowest acceptable values. Needless to say I’m now taking supplements to bring my nutrition back into its natural balance. Now, I do find it interesting that of the few sources for vitamin D fish ranks high, and I do consume more fish than the typical American (who turns down salmon?). It also brought home to me the fact that biology and genetics aren’t just theoretical, they’re personal. On the flip side a friend of mine who was vacationing in the Maldives told me how he decided to go shirtless on the beach when he saw the locals swaggering about. My pale skinned friend was suitably fried pink despite his protective lotion.

I’ve talked a fair amount about the evolutionary genetics and history of skin color on this weblog. To review, it seems to be a polygenic trait whose variation world wide tends to be controlled by 4-6 loci of large effect. Not only do older quantitative genetic methods of inquiry come to this conclusion, but new data from genomics confirms this general picture. In terms of the history of the characteristic the general outline seems to be that our lineage started out light skinned, lost its fur a few million years ago and developed dark skin, and then diversified in complexion as we spread across the globe. I haven’t focused much on the details of why skin color varies because there are several competing hypotheses which some of you are likely familiar with, and I wanted to shine the spotlight on the power of convergent evolution and its relative speed. But sometimes to understand the big picture you need to focus on the details.

i-1343eec6e4529f2ee53ce581f0545d4d-lightpenetration.jpgThe chart to the left illustrates the two primary biochemical dynamics which serve as oppositional selective forces upon the color of human skin; which itself is just a reflection of melanin density and size. Though popular reports focus on the increased cancer rates of light skinned individuals, it is important to remember that cancers often occur late in life and so their reproductive impact is diminished. Nina Jablonski has put forward a more reproductively salient selective pressure: the interference with folic acid synthesis which occurs when excessive UV radiation penetrates deep into the dermis. The end result of this is reduced folate levels, which in pregnant females often causes neural tube abnormalities. Any impact on pregnancy success is an extremely powerful selective force. In this model the dark skin of humans naturally arose because women who were darker skinned carried more normal fetuses to term than those who were light skinned. In dark skinned populations the MC1R locus is extremely conserved, suggesting powerful selective constraints which prevent sequence variation. No matter the phylogenetic relationships between dark skinned populations there is a consensus sequence which seems to have been selected for deep in the human past which remains the norm across these groups (i.e., though Melanesians and Africans are both very distantly related their dark skin is the end product of the same genetic architecture). But counterbalancing the need to block sunlight due to reduction in folate levels is the fact that vitamin D synthesis requires a minimum level of radiation to be catalyzed. Reduced vitamin D levels not only result in bone deformations (i.e., rickets), but a heightened sensitivity to a host of diseases.

i-18cdf75afd67f2081c66f8734d7e2e19-skinColor.jpgThe map to the left is a rough sketch of variation in skin color throughout the world. It shouldn’t surprise anyone, there is the general trend of populations becoming lighter as one moves away from the tropics. In Jablonski’s model this is comprehensible insofar as one moves north the the UV radiation drops. This means that melanin pigement concentrations need not be as dense to protect the folic acid synthesis pathway from degradation. And, the reduced density of melanin maybe necessary for vitamin D levels to be maintained due to the essential role of radiation in catalyzing its production. In short, as one moves away from the tropics the selective constraint to maintain high folate levels by blocking UV via dark skin is removed, while a countervailing pressure to reduce the density of melanin so as to allow enough radiation to stimulate vitamin D synthesis emerges. But there’s a problem with this narrative: the lightest populations in the world are not from the farthest north. In Scandinavia the Sami are darker than the peoples to the south, and the Inuit are generally a light brown skinned people. It seems that the fairest peoples in the world reside around the shores of the Baltic sea, not in the circumpolar regions.

The fact that I eat a relatively rich diet and do expose myself to the sun when I can, but still exhibit vitamin D deficiency, brought home to me the dependence upon nutritional parameters as well as skin color. In terms of nutrition the last 10,000 years has not, on average, been “good” for humanity. Though agricultural peoples are efficient at reproduction and natural increase because of the high yields they can extract from the land via intensive farming, they have generally had to deal with the trade off that their diet became reliant on starches which were poor in many vital nutrients as well as proteins. Human have become smaller over the last 10,000 years, and the contrast between the relatively healthy skeletons of hunter-gatherers and the physiological stress exhibited by the farmers which succeeded them have long been noted. Recently work in genomics also suggests genes implicated in various metabolic functions have been under powerful selection over the past 10,000 years as the agricultural lifestyle has spread. The deleterious consequences of switching many non-agricultural populations to the starch rich diet are well known (obesity, diabetes, etc.). Selection happens, and it seems likely that a genetic revolution was ushered in by the radically altered nutritional universe of the farmer. Which brings me to Europeans and why they might be so light. Frank W. Sweet published an essay in 2002 which offered that the feasibility of a farming lifestyle at very high latitudes in Europe due to peculiar climatological conditions served to drive Europeans to develop light skins over the last 10,000 years. In short, Sweet argues that the diets of pre-farming peoples were richer in meats and fish which provided sufficient Vitamin D so that skin color was likely light brown as opposed to pink. But with the spread of agriculture Vitamin D disappeared from the diets of northern European peoples and so only by reducing their melanin levels could they produce sufficient amounts of this nutrient to keep at bay the deleterious consequences of deficiencies. This explains why the Sami, who never adopted agriculture, remained darker. One could hypothesize that the relative swarthiness of groups like the Welsh might be due to greater reliance on fish and game as opposed to agriculture, but the point is not to explain every last detail but to clarify the overall trend.

Sweet’s essay was written in 2002. In 2005 a gene, SLC24A5, was implicated in explaining a large proportion (25-38%) of the between population difference in skin color for Europeans and Africans. It seems that on this locus the two populations were disjoint, they exhibited no substantial overlap. In European it seems that 6 to 10 thousand years ago a new variant arose which subsequently swept to fixation. In the model above it seems likely that the mutation was just there at “the right place and right time.” Interestingly in East Asians SLC24A5 exhibits the same sequence as it does in Africans. But, it seems that other loci are responsible for the lightening of the skin of East Asians recently as well, though not to the same extent as Europeans. The reason for this is likely the fact that temperate East Asia as at a far lower latitude than Europe.

Of course, there are other anomalies. Sweet points out that South Americans are far paler than they should be if Old World populations are to be any judge. His explanation is simple: light skin evolves quickly via loss of function mutations while the original settlers of the Americas did not carry the fully complement of alleles necessary for black skin. Though Sweet doesn’t say it in so many words he is basically suggesting that gain of function mutations are rare relative to loss of function. There are two parameters which might explain the relative lightness or darkness (and overall homogeneity) of New World populations. First, settlement was relatively recent, so there hasn’t been enough time for evolution to work. Of course the copious evidence of recent human evolution suggests that this isn’t really a major issue. Rather, the bigger problem was likely that extant genetic variation was reduced by a population bottleneck. Over time this variation would be replenished by mutation, but this is a slow process. Tropical populations of the New World are not only lighter than one would expect, they also display a far stockier and “northern” build than they should if form followed from function. This suggests evolutionary lag on many characteristics due to the low genetic diversity combined with the particular time frame over which evolution could occur (but even here, do note that there is a skin color gradation from the tropics to the temperate zones in the New World among indigenous peoples).

I don’t expect this model to explain all the details. Some theorists have offered other hypotheses. For example, Peter Frost puts the focus on sexual selection. Peter’s thesis is basically that particular ecological conditions in Ice Age Europe, the low-latitude continental tundra, resulted in specific social patterns which forced females to compete for the attention of males (there was a shortage of males, but the constraints of the ecosystem limited mating patterns to monogamy). All things equal I tend to favor more ecological resolutions to questions of phenotypic evolution than sexual selective ones because the evolutionary dynamics of the latter seem to be so chaotic and difficult to characterize. Judith Rich Harris has a different spin insofar as she believes that mothers selected lighter skinned infants because they found them beautiful. I am generally skeptical of this explanation simply because beauty does not emerge from a vacuum. Of course these factors are not eliminated by the selective forces sketched out above, and it seems likely that evolutionary forces are multivalent (Frost in particular focuses upon hair color as opposed to skin color for his frequency dependent sexual selection model).

Overall, this a good time to be interested in questions about normal human phenotypic variation. The tools are manifold and triangulating to the most plausible explanation is far easier than in the past simply because various methods are on hand. Me, I’m taking my Vitamin D pills, and perhaps I’ll get me some cod oil.

Comments

  1. #1 cuchulkhan
    July 8, 2007

    In the extreme north snow-reflection might provide sufficient sunlight, moreso than around middle Europe. Could be why darkish skin endured. If light skin did begin to emerge 10,000 years ago in Europe then the end of the ice age might have been a factor – decreasing snow reflection decreased the need for darkish skin.

  2. #2 razib
    July 8, 2007

    In the extreme north snow-reflection might provide sufficient sunlight, moreso than around middle Europe.

    think this through. do you think that sami and inuit are walking around with skin exposed when there’s snow on the ground?

  3. #3 Caledonian
    July 8, 2007

    Like vitamins C and A, I always assumed that far-northern people got their vitamin D from the fresh meat and fish they ate. With all of the clothes they have to wear, their faces likely couldn’t absord enough UV to produce it themselves.

    Perhaps that’s one of the reasons why their skin color doesn’t seem to have lightened much – no selection pressure, since they didn’t produce their own.

  4. #4 razib
    July 8, 2007

    I always assumed that far-northern people got their vitamin D from the fresh meat and fish they ate.

    put the emphasis on fish.

    Cod liver oil, 1 Tablespoon 1,360 340% daily
    Salmon, cooked, 3½ ounces 360 90% daily
    Mackerel, cooked, 3½ ounces 345 90% daily
    Tuna fish, canned in oil, 3 ounces 200 50% daily
    Sardines, canned in oil, drained, 1¾ ounces 250 70% daily
    Milk, nonfat, reduced fat, and whole, vitamin D fortified, 1 cup 98 25% daily
    Margarine, fortified, 1 Tablespoon 60 15% daily
    Pudding, prepared from mix and made with vitamin D fortified milk, ½ cup 50 10% daily
    Ready-to-eat cereals fortified with 10% of the DV for vitamin D, ¾ cup to 1 cup servings (servings vary according to the brand) 40 10% daily
    Egg, 1 whole (vitamin D is found in egg yolk) 20 6
    Liver, beef, cooked, 3½ ounces 15 4% daily
    Cheese, Swiss, 1 ounce 12 4% daily

  5. #5 Agnostic
    July 8, 2007

    One could hypothesize that the relative swarthiness of groups like the Welsh might be due to greater reliance on fish and game as opposed to agriculture

    I was thinking of the Dark Irish / Scottish when you mentioned the Sami. I don’t know jack about ancient history, let alone ancient nutritional history in sufficient detail, but Greg says ancient people called the Picts a Mediterranean race.

    Re: sexual selection, forgetting larger scale between-group differences (like Italian vs Lithuanian), look just within the British Isles. Which groups are more lusted over, pretty much regardless of who’s judging: Celtic or English? That’s true for males as well as females. And it usually isn’t the pale, blue-eyed, red-haired ones: it’s Sean Connery, Pierce Brosnan, Catherine Zeta-Jones, Lara Flynn Boyle, etc. (Jennifer Connelly and Sherilyn Fenn are halfies or quarteries, iirc.)

  6. #6 Torbjörn Larsson, OM
    July 8, 2007

    Interesting for me, that lives around these population groups, and have luckily picked up some dark skin protection against sun burn in my ancestry – I can get fairly dark fast when it is needed.

    As you know, european populations have moved about to a fair extent, which should have implications for the original skin colors. Sami have been tracked genetically to Iberian and Berber populations, while Fins language group seems to be of central european origin. So if some groups of them have managed to keep their original color variations, it may not be surprising that some of their individuals are fairly dark.

  7. #7 razib
    July 8, 2007

    Sami have been tracked genetically to Iberian and Berber populations, while Fins language group seems to be of central european origin.

    that’s a lineage connection on one locus (mtDNA). you can’t project from this to the whole population history. the sami are a complex case and seem relatively isolated, but please note the details from wikipedia:
    Sami genetic history has been of great interest because of their large genetic distance to other European populations including their closest neighbours. It is mainly the north Sami and east Sami that have been investigated. There is considerable genetic variation between the different Sami groups but they all share a common ancestry. Female mtDNA especially has been investigated, but also Y chromosomes and classical autosomal markers. The research indicates that 95.6% of Saami mtDNA originated in the Iberia refugia while only 4.4% is of Siberian-Asiatic origin (Tambets 2004). The age of the two main mtDNA haplogroups are estimated to be 5 500 and 7 500 YBP (Ingman 2006, Delghandi 1998). A genetic link has been established between the Sami and the Berbers of North Africa going back 9000 years (Achilli 2005).

    Sami Y chromosomes indicate that 29.8% originated in the Iberia refugia and 58.2% originated in Eastern Europe (Tambets 2004). The autosomal classic markers shows that the Sami have no close relatives in any population including their closest linguistic relatives but are in general more closely related to Europeans than people of other continents. The closest of the distant relatives are Finnish people, but this is probably due to more recent immigration of Finnish people into the Sami areas, and the assimilation of the Sami population into the mainstream population in today’s Finland (Meinila 2001).

    the sami relationship to the iberian refugia is on the order of 5-10 K BP. this is enough time for selection to work, especially since i’m positing here that 5-10 K BP europeans on the whole were far darker anyhow. please be careful of projecting from phylogeny. the fact that some sami are stereotypically scandinavian in appearance (i.e., nordic) suggests that admixture has been sufficient for phenotypic replacement to have occurred if it had been selected for.

  8. #8 razib
    July 8, 2007

    but Greg says ancient people called the Picts a Mediterranean race.

    hm. i’m sure about this, but i do know that roman observers were surprised to find that the people of wales looked like the iberians.

  9. #9 windy
    July 8, 2007

    …while Fins language group seems to be of central european origin.

    Aargh! You disappoint me, Torbjörn :) Central Russia or West Siberia, more likely.

    “95.6% of Saami mtDNA originated in the Iberia refugia” sounds a bit off. Tambets et al only state that 95.6% of their mtDNA is West European, and the second of the common western haplogroups came to the Saami via Eastern Europe.

  10. #10 razib
    July 8, 2007

    Aargh! You disappoint me, Torbjörn :) Central Russia or West Siberia, more likely.

    i think he is conflating origins with the fact that the hungarian language has a very distant relationship with finnish (i.e., finno-“ugric”).

  11. #11 razib
    July 8, 2007

    And it usually isn’t the pale, blue-eyed, red-haired ones: it’s Sean Connery, Pierce Brosnan, Catherine Zeta-Jones, Lara Flynn Boyle, etc. (Jennifer Connelly and Sherilyn Fenn are halfies or quarteries, iirc.)

    come now, that’s a small N isn’t it? in any case, i refute you thus: i like it pale.

  12. #12 Brian
    July 8, 2007

    Re: South Americans, my impression is that quite a lot of the indigenous ones lived in rain forests, which would seem to help limit their exposure to the sun as compared to their co-latitudinal friends in Africa.

  13. #13 Vorpal Blade
    July 8, 2007

    Harrumph! As a rule, with few exceptions, a good looking woman has dark eyes, dark hair, and enough coloring in her complexion to qualify as having a complexion.

    The world over, fishbelly white is not an attractive color for skin to have. (Why else would palefaces spend so much money and take such stupid risks getting their skin tanned?) Blue or green eyes may be striking to look at, but the eyes you get lost in are a deep dark brown. Nearly all the women in the world are brunettes.

    If you were to pick at random a million women from around the world, and then picked the best looking thousand or hundred, almost every one would be dark-dark-and-dark.

  14. #14 razib
    July 8, 2007

    my impression is that quite a lot of the indigenous ones lived in rain forests, which would seem to help limit their exposure to the sun as compared to their co-latitudinal friends in Africa.

    pygmies live in the rain forest. likely for quite a long time in comparison to amazonians.

    As a rule, with few exceptions, a good looking woman has dark eyes, dark hair, and enough coloring in her complexion to qualify as having a complexion.

    whose rule? yours? mine? there is a lot of difference of opinion. check out peter frost’s work for data on the prevalence of within population fairness in females cross-culturally.

  15. #15 Luke
    July 9, 2007

    “As a rule, with few exceptions, a good looking woman has dark eyes, dark hair, and enough coloring in her complexion to qualify as having a complexion.”

    I’m not a historian, but I’ve often heard that beauty used to be more associated with pale skin, because it was a sign that you didn’t have to work out in the sun. Now tan skin may be more preferred, because it is a sign that you can afford to vacation in the sun.

    Razib — As for your suggestion that northern European’s fair skin is associated with the advancement of agriculture and diets high in starch, wouldn’t there also be an increase in milk consumption from dairy farming? If northern Europeans started getting more vitamin D from milk, the selection pressure against melanin production would decrease.

  16. #16 razib
    July 9, 2007

    luke,

    even one cup of vitamin d fortified milk is 25% of one’s daily needs. some of the shortfall would have been made up surely, but not all it seems.

  17. #17 Torbjörn Larsson, OM
    July 9, 2007

    razib:

    that’s a lineage connection on one locus (mtDNA). you can’t project from this to the whole population history.

    I understand (but as your reference says, it is also a Y chromosome connection), but that is what the data so far implies.

    the fact that some sami are stereotypically scandinavian in appearance (i.e., nordic) suggests that admixture has been sufficient for phenotypic replacement to have occurred if it had been selected for.

    Okay, I don’t follow what you are trying to imply. Your post discuss that it is easier to loose an ability than to acquire it. All I proposed (I hope) is that the original Sami population could have been well suited with the ability. I was careful to suggest “variations”.

    i think he is conflating origins with the fact that the hungarian language has a very distant relationship with finnish (i.e., finno-“ugric”).

    Are you perhaps overlooking or overinterpreting that I specifically mentioned that it was a language group, to conclude conflation? Not that I know much about languages (nothing, really :-), but they spread in several ways, and one of them should be population movements. I note that genetics as described by windy support such a movement, btw.

    Aargh! You disappoint me, Torbjörn :) Central Russia or West Siberia, more likely.

    Oh, I’m sorry. I must confess to lack of knowledge here as well. But I like the implication, it is even more exotic.

  18. #18 Torbjörn Larsson, OM
    July 9, 2007

    Oops, seems something got cut in final edit. The last part of the comment was:

    windy:

    Aargh! You disappoint me, Torbjörn :) Central Russia or West Siberia, more likely.

    Oh, I’m sorry. I must confess to lacking knowledge here. But I like the implication, it is even more exotic.

  19. #19 Torbjörn Larsson, OM
    July 9, 2007

    “I note that genetics as described by windy support such a movement, btw.”

    Oh, bad brain day! There was no source given, so that should be “I note that the description by windy support such a movement, btw.”

  20. #20 Torbjörn Larsson, OM
    July 9, 2007

    Are you perhaps overlooking or overinterpreting that I specifically mentioned that it was a language group, to conclude conflation?

    Oh, finally I believe I understand, you may have thought it was a mistake of believing that the language split off from the existing area of ugric use, similar to complaining that “I don’t come from no chimp”. Sorry, I *am* slow today.

    No, my fleeting memory was that finnish-ugrish was somehow pinned to down to a central european area. (Which factoid perhaps originated in the above mistake.)

    But it wasn’t essential, I was looking for signs of population movement. Which is perhaps why I didn’t check the factoid. :-)

  21. #21 razib
    July 9, 2007

    I understand (but as your reference says, it is also a Y chromosome connection), but that is what the data so far implies.

    that piece of data. the Y tends toward more non-iberian refugia. the autosomal does not point to this from what i recall (cavalli-sforza in history and geography of human genes).

    Okay, I don’t follow what you are trying to imply. Your post discuss that it is easier to loose an ability than to acquire it. All I proposed (I hope) is that the original Sami population could have been well suited with the ability. I was careful to suggest “variations”.

    your comment implied that you believed sami darkness was a function of their phylogeny. that is, their relationship to relatively dark southwest europeans. but

    1) we know there has been genetic exchange with fair groups to the south (this is evident in the genes).

    2) if selection occurred for light skin in the north then it would have been operative for the sami. even if they started out dark even small amounts of intermarriage would have lead to the spread of these genes.

    3) there is a non-trivial circumstantial evidence to suggest that 5-10 thousand years ago all european populations were relatively dark. 5-10 thousand years is also the time frame for the sami separation from southwest european peoples (assuming that marker as totally valid). so phylogeny is not a compelling explanation.

    Are you perhaps overlooking or overinterpreting that I specifically mentioned that it was a language group, to conclude conflation? Not that I know much about languages (nothing, really :-), but they spread in several ways, and one of them should be population movements.

    there is zero evidence that the finnish language is from central europe. there is a possible distant relative of finnish (very distant) in hungary. but, this language is exogenous to central europe, brought by magyar nomands from the lower volga region a bit over 1,000 years ago. finnish is part of the uralic language family. control-f uralic and look at the map:
    http://webspace.ship.edu/cgboer/languagefamilies.html

    you will see quite clearly why windy said what he said about a siberian origin for finnish. additionally, there is some genetic evidence (tat-c haplotype) which relates finns to siberian peoples (though the relationship could be inverted in that northwest eurasia could be a source for the east eurasian linages).

  22. #22 c23
    July 9, 2007

    Judith Rich Harris has a different spin insofar as she believes that mothers selected lighter skinned infants because they found them beautiful. I am generally skeptical of this explanation simply because beauty does not emerge from a vacuum.

    I would assume that the preference for light skinned babies would be caused by the tendency of children to have lighter skin than adults, which would lead to humans responding to “light” as “cute.” This is analogous to the tendency of women to have lighter skin than men as Frost points out – except that kids are lighter than women.

    Whether Harris is on the mark or not, I’m sure that infanticide was a powerful selective force in the past for something, because it was so prevalent.

  23. #23 c23
    July 9, 2007

    Also, what about pleiotropic effects? Melanin’s synthetic pathway involves lots of other chemicals, which affect more than just skin color. We see that albinos are different than other organisms in more ways than just having light complexions. So could light skin be a result of selection for something totally different?

  24. #24 razib
    July 9, 2007

    So could light skin be a result of selection for something totally different?

    yes. do you have any ideas?

  25. #25 Torbjörn Larsson, OM
    July 10, 2007

    razib:

    your comment implied that you believed sami darkness

    I insist that I was discussing range (“some of their individuals”). After all, you implied that “Sweet doesn’t say it in so many words he is basically suggesting that gain of function mutations are rare relative to loss of function”.

    If you are claiming that 10 000 years will eradicate that, I understand. I wasn’t even aware that the Sami was migrating so early, since it was just at the end of the latest ice age. Opportunistic.

    there is zero evidence that the finnish language is from central europe.

    Sure, I have explained in an earlier comment why that was an unsupported factoid.

    Actually, I note in passing that while your source is clearly more illustrative than the references in Wikipedia, the later seems very authorative. It wants to place the most likely birthplace within Europe and discredit the alternatives.

    Again, this is language, not populations, so I don’t want to argue outside the original indication of population movement. And your claim on time frame made the discussion, ehrm, “academic”.

    Btw, I read your follow-up on eye color, and it was a striking consequence/tentative support. Sexual selection suddenly felt moot.

  26. #26 jaim klein
    September 14, 2007

    When calculating water demand for irrigation, you have this factor of insolation, a measure of solar radiation arriving to the plant. It is related to latitude, but cloudy, misty climates have low evapotranspiration and low water demand. (I am a professional water engineer, take it on my authority). Europe’s Atlantic board – from Galicia to the Baltic countries, gets very little sun even in summer, and the natives are most pallid. The Arabian desert gets most sunlight but local people’s skin is carefully covered and they also show strong preference toward lightskinned females and children.

  27. #27 Tom Bri
    September 15, 2007

    I have been around the world quite a bit, lived in Central America and Japan, traveled in Europe a fair bit, grew up in the US.

    One thing I have noted is that women in every non-white population I know of personally generally prefer to be more light-skinned. And the men strongly prefer light skinned women.

    Some of this must be cultural, at least in countries like Mexico where the elite are mainly of European descent, or at least prefer to claim it.

    But the Chinese and Japanese and Thais are quite proud of their native cultures, and the Thais and Japanese were never colonies of Europe.

    This includes Koreans, Japanese, Chinese,Indians, Thais and all Latin Americans.

    I suspect that there is strong selection pressure towards lighter skin, held in check by counter selection against it wherever people are exposed to strong sunlight.

    The pressure is communication. Lighter skin communicates emotions much more quickly and accurately than dark. Blushes, the pallor of rage, all important cues to a partners state of mind.

  28. #28 Tod
    October 4, 2008

    I would be careful about intake of cod liver oil it contains Vitamin A and it is relatively low in D. In obtaining a good amount of Vit.D from cod liver oil large amounts of Vitamin A would also be consumed, bearing the reported risks of beta carotene (which is converted)to A in mind this is best avoided. There is an issue with pollutants in fish oil One manufacturer, I think it’s Seven Seas has a patented extraction process to get around this, So the extra strength type if taking it for DHA/EPO. I do not recomend ingestion of cheap fish oil. Vitamin D is good for prevention of muscle wasting, limiting calories also helps.

  29. #29 Tod
    January 14, 2009

    Rickets is really a nutritional disease stemming from cereal agriculture Cereal grains

    Unfortunately for Sweet’s theory African populations who have been agriculturist for several thousand years get it A Comparison of Calcium, Vitamin D, or Both for Nutritional Rickets in Nigerian Children
    As the treatment for rickets includes Vitamin D one might expect African agriculturists to have become lighter skinned than African hunter gatherers. They are not, quite the opposite in fact.
    Most of the diseases vitamin D protects against are not relevant to evolutionary time they tend to be ones related to immune over activation.

    Folic Acid deficiency should have been common in those whites sunbathing or just going about in a T shirt in Australia or South Africa. The theory would predict a pandemic of pregnancy problems; that hasn’t come to pass.
    5-Methyltetrahydrofolate inhibits photosensitization reactions and strand breaks in DNA
    Branda and Eaton reported a significant decrease of folate in plasma from psoriasis patients treated with methoxalen phototherapy. They also found loss of folate in plasma exposed ex vivo to UVA. They suggested that “Prevention of ultraviolet photolysis of folate and other light sensitive nutrients by dark skin may be sufficient explanation for maintenance of this characteristic in human groups indigenous to regions of intense solar radiation.” (11) . 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…/
    We demonstrate that 5-methyltetrahydrofolate (5-MTHF, the predominant folate in plasma) is also a potent, near diffusion limited, scavenger of singlet oxygen and quencher of excited photosensitizers. Both pathways result in decomposition of 5-MTHF, although ascorbate can protect against this loss. In the absence of photosensitizers, 5-MTHF is directly decomposed only very slowly by UVA or UVB. Although synthetic folic acid can promote DNA damage by UVA, submicromolar 5-MTHF inhibits photosensitization-induced strand breaks. These observations suggest a new role for reduced folate in protection from ultraviolet damage and have bearing on the hypothesis that folate photodegradation influenced the evolution of human skin color.—

  30. #31 Tod
    January 14, 2009

    Association between quantitative measures of skin color and plasma 25-hydroxyvitamin D

    Plasma 25OHD was determined and sun-induced (outer fore-arm) and constitutive (upper inner-arm) measured by reflectance colorimetry.
    Results Mean (SD) 25OHD was significantly higher in Europeans than Pacific People, 88 (31) nmol/L vs. 75 (34) nmol/L, respectively. Based on constitutive skin color, 35% of participants were very light, 45% light, 16% intermediate, 4% tanned, and 0% brown or dark. Skin color at the forearm but not constitutive skin color was a significant predictor of 25OHD. Each 10° lower skin color value at the forearm (more tanning) was associated with a 5 nmol/L higher 25OHD (P <  0.001).
    Conclusions Tanning but not natural skin color was an important determinant of 25OHD. Further study is needed in a population with a higher proportion of darker skin people.

  31. #32 DDeden
    January 14, 2009

    Pre-agriculture settlement, camouflage would have been the major factor, ochre, tatoos, charcoal, chalk, henna, leaves are ancient coverings. Consider infant coloration, the most vulnerable of the tribe. People of sandy areas tended to be sand colored, of rainforest to be dark colored, of semi-annual snow & gray-brown foliage (lat. & alt.) to be light colored. Northern peoples wore light toned hooded garments with fur (innuit) or colored trim (saami) and generally ate fish or marine mammals, dark eyes protected against snow blindness.

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