Gene Expression

White skin & wheat?

i-369b8d938e2a8e3c891d39d927499e16-wonder_bread.JPGA few days ago I put up a post, Why red Indians aren’t white, where I offered up a rough & ready model for why the indigenous peoples of the New World are relatively swarthy at the same latitudes as Europe compared to Europeans. Regular readers of this weblog know that I have somewhat of an obsession with skin color genomics, and am puzzled by some issues, both empirical and theoretical, and have been attempting to generate plausible explanatory scenarios for what we know, and what we expect. But in the process I assume a lot, so I’m going to hit the primary background assumptions in this post, since I’ll be posting the topic a fair amount for the near future.


First, there’s the theory and expectation. Vitamin D deficiency isn’t good, not only does it cause rickets, but it suppresses the immune system. The standard model is that northern Eurasian peoples became pale in response to lower radiation levels, because those lower radiation levels resulted in reduced endogenous production of vitamin D in the skin. Light skin is multiplicatively more productive in generating vitamin D than dark skin.

Until recently we didn’t know much about skin color genetics, so this was all theoretical. Now we know a fair amount. Ancient DNA extraction leads us to the likely inference that Neandertals were light-skinned. Since it seems likely that we are not descended from the Neandertals by and large humans who migrated north out of Africa within the last 50,000 years evolved this trait independently. It turns out that East Asians and Europeans tend to have different genetic architectures that produce similar lightnesses of skin. So we know that light skin evolved three times independent, once in Neandertals and twice in modern humans.

We also know the specific genes and have a general idea of how recently they flipped from the dark skinned ancestral variant to the lighter skinned derived variants. In Europeans, two genes, SLC24A5 and OCA2 seem to have been subject to very recent selection events, likely on the order of (or less than) 10,000 years. Similar dynamics seem operative in East Asia, where some pigmentation genes have been selected very recently. That’s rather perplexing, because Eurasians have been extant at high latitudes for the past 40,000 years or so. Though northern Europe was uninhabited for much of that time, it is important to note that genes such as SLC24A5 seem to be subject to recent selection across the Middle East, North Africa and even into South Asia.

Empirically we know that not all extreme northern peoples are pale; the circumpolar peoples for example are relatively swarthy by Northern European standards. Even within Scandinavia the Sami, the indigenous people of the far north, are darker, on average, in coloring (manifest in eye and hair, though these have some correlation with skin reflectance as there are common genes underlying various pigmentation traits). The data from SLC24A5, which explains around 1/3 of the skin color difference trait value between Africans and Europeans, suggests that until recently many more northern people were relatively dark. Why?

L. L. Cavalli-Sforza offered up a model decades ago, long before we had data about recent evolution on these skin color loci, that the transition to agriculture in Europe would have resulted in a great deal of change because of the dietary shift. In short, farmers have very little diversity in their nutrients, cereals have a fair amount of calories, but those are mostly in the form of carbohydrates. We know empirically that it seems with the spread of the Neolithic agricultural traditions into parts of Northern Europe people switched from a diverse hunter-gatherer menu, often supplemented with a great deal of marine organisms, to a more monotonous cereal diet. Marine organisms, and to a lesser extent meat and milk, are relatively enriched for vitamin D in comparison to cereals. One of the explanations for why circumpolar peoples can be relatively swarthy is that their diet is relatively rich in vitamin D from marine organisms (this is particular true of the Inuit).

There are many details which need to be fleshed out for this model. Additionally, there are other alternatives such as sexual selection. But I believe that the genomic data are evidence which demand hypotheses, and so I’m trying to fill the breach, or, more precisely recycle older ideas and add some twists.

Related: My skin color posts.

Comments

  1. #1 agnostic
    January 23, 2008

    Wonder bread, hahahahhaahaha

  2. #2 deang
    January 25, 2008

    As you mentioned in your previous post on this topic, there were agricultural areas in the Americas, agricultural areas responsible for much of the food we eat today. You wondered in the last post if maybe Native Americans remained dark skinned because they didn’t move as exclusively to a plant-based diet as Europeans presumably did. I know that in the main centers of American plant domestication, Mesoamerica and the Andes, a vast array of plant types were grown, a variety of plants which I suspect dwarfs that of the Fertile Crescent and Europe. This could have contributed to better nutrition. Outside of those areas, there was not a clear switch to exclusive reliance on agriculture. In almost all regions of the Americas where there was agriculture, it was supplemental, with wild plant gathering and hunting continuing even in the Mesoamerican and Andean cradles of American plant husbandry, even when populations were large. This is partly because of cultural differences from Indoeuropeans that resulted in preservation of much of the natural environment (I realize that Native Americans didn’t leave their environment undisturbed, but we can’t ignore that a great deal more of the natural environment was preserved by them than by Europeans in their home territory, even where population sizes were equivalent).

    What about high latitudes of Australia? They are equivalent to southern Europe, where people are typically of a European lightness, yet aboriginal Australians are as dark as Africans and they’ve been there for 40,000+ years. And what about high altitudes throughout the world, many of whose indigenous populations remained dark skinned? Are issues of altitude similar to issues of latitude on this topic?

    The timing-of-human-hair-loss post you linked to in a recent post also raises an issue. The human populations with the most body hair are Europeans/Indoeuropeans/Semitic peoples and Japan’s indigenous Ainu, yet Europeans are some of the lightest-skinned people of all. Are there theories about this?

    I’m not a geneticist so my thoughts may not be worth much here, but I always think that light skins developed in Europeans/Indoeuropeans and some East Asians as aberrations, with little relation to latitude or diet. They survived because they weren’t harmful (and in the case of Europeans/Indoeuropeans, because their bearers were quite aggressive) but otherwise they’re merely limited regional variations. I’m hoping to learn more, though.

  3. #3 razib
    January 25, 2008

    What about high latitudes of Australia? They are equivalent to southern Europe, where people are typically of a European lightness, yet aboriginal Australians are as dark as Africans and they’ve been there for 40,000+ years. And what about high altitudes throughout the world, many of whose indigenous populations remained dark skinned? Are issues of altitude similar to issues of latitude on this topic?

    southern aborigines aren’t quite as dark-skinned as africans from what i know. in any case, it’s a good point, and argues for the importance of agriculture. do note that even tasmania though is only at the latitude of spain, so ‘southern australia’ is at the latitude of the sahara.

    They survived because they weren’t harmful (and in the case of Europeans/Indoeuropeans, because their bearers were quite aggressive) but otherwise they’re merely limited regional variations.

    no, the skin color genes have been under very powerful directional selection. some of these genes show among the strongest signatures in the tests for natural selection.

  4. #4 tom bri
    January 26, 2008

    Interesting you lead with a picture of bread made from wheat. Northern Europeans may have been eating cereals, but not wheat. Even now wheat is not much grown in the more northerly areas of Europe. Rye and oats and barley were the main cereal crops, being more resistant to wet and cold.

    In the Americas agriculture was confined to the south until fairly recently, meaning hundreds, not thousands of years ago. The strong sunlight would have prevented any pressure towards lighter skin. In the north agriculture was more limited. I have read that the Mississipian cultures were heavily dependant on corn, but they were wiped out by disease soon after Europeans appeared, so we know little about them, and nothing about their skin color.

    As for the first comment, that Europeans were more aggressive than others. Well, look at history. Only in the last few centuries have Europeans been pushing east. The push was mainly from the East to the West in most of known history, Alexander the Great excepted, and he managed only one short-lived invasian of India. Think Huns, Turks etc. Indo-European-speaking peoples were present in China at one time, but disappeared.

    All human groups seem aggressive when given the opportunity.

  5. #5 razib
    January 26, 2008

    Well, look at history. Only in the last few centuries have Europeans been pushing east. The push was mainly from the East to the West in most of known history,

    that’s not really true. the original push along the eurasian temperate axis was probably west to east, as evidenced by the tocharian presence on the borderlands of china. sometime around 0 AD this process reversed itself and europoid groups like tocharians were slowly absorbed by a migratory event out of the turko-mongol homelands which eventually sent the pusles to the west that you note. the toolkit that these nomads used originated from the ukrainian step. granted, the movement of kurgans (and successors like the scythians and sarmatians) was also to the west, but they started from the eastern border of the west…. (i.e., the scythians spoke an ‘iranian’ language, but they were probably never resident in iran, they simply were part of the branch of indo-european dialects which became associated with iran in later periods because aside from ossetian all the extra-iranian dialects died out).

  6. #6 Mikie
    January 29, 2008

    the only comment I have to make,
    a clarification really, is that it does seem to me that neanderthals and homosapiens interbred. It is more likely that the MC1R gene was passed rather then evolved over such a short duration in homosapiens. It is too cooincidental that a gene that is so very advantageous and necessary just spontaneously appeared during a period that both lived simultaneously in the same geographic area. You add this to the fact that MC1R is nearly identical to that of Neanderthals and…I think you gete my point

  7. #7 R Young
    February 22, 2009

    I am a year late on this, but I wanted to note that early explorers and trappers going up the Missouri from St. Louis reported encountering a community of white Indians. I think they were late wiped out by smallpox, but it would be interesting to see what remains of their DNA (if any can be found) would reveal. Apparently not all Indians were copper colored.

    It’s well known that the Indians of meso-America had a legend of white gods who had helped them to civilization. That legend could have been based on pure imagination, or an earlier contact with Europeans, or, maybe, another group of white Indians. Just a wild guess.

  8. #8 U Saxena
    February 24, 2009

    Hey Razib, a lot of the studies are about development of light skin, but I am yet to find out anything about the development of dark skin amongst us. When did the gene for melanin production evolve? Did our pre-homo-sapien-sapien hominid ancestors already have the melanin, and is that why we always presume a journey of dark to light in terms of evolution? To me it seems like the jet-black skin of the sub-Saharan Africans and many throughout the Indian subcontinent happened later from an originally lighter/brownish stock.

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