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Thread: new study challenges lighter vs darker skin color story

  1. #1
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    new study challenges lighter vs darker skin color story

    This seems pretty interesting...If I understand it correctly, while skin getting lighter as humans migrated northward and needed extra help with UV light to get enough Vitamin D was still a factor, the new study says dark skins ability to retain moisture in high heat environments became less needed as humans migrated north to cooler area's and thus the body spent less energy making it...

    "In human evolution, changes in skin's barrier set northern Europeans apart"
    http://www.sciencedaily.com/releases...0630140836.htm

    The popular idea that Northern Europeans developed light skin to absorb more UV light so they could make more vitamin D – vital for healthy bones and immune function – is questioned by UC San Francisco researchers in a new study published online in the journal Evolutionary Biology.


    Ramping up the skin’s capacity to capture UV light to make vitamin D is indeed important, according to a team led by Peter Elias, MD, a UCSF professor of dermatology. However, Elias and colleagues concluded in their study that changes in the skin’s function as a barrier to the elements made a greater contribution than alterations in skin pigment in the ability of Northern Europeans to make vitamin D.
    Elias’ team concluded that genetic mutations compromising the skin’s ability to serve as a barrier allowed fair-skinned Northern Europeans to populate latitudes where too little ultraviolet B (UV light for vitamin D production penetrates the atmosphere.
    Among scientists studying human evolution, it has been almost universally assumed that the need to make more vitamin D at Northern latitudes drove genetic mutations that reduce production of the pigment melanin, the main determinant of skin tone, according to Elias.
    “At the higher latitudes of Great Britain, Scandinavia and the Baltic States, as well as Northern Germany and France, very little UVB light reaches the Earth, and it’s the key wavelength required by the skin for vitamin D generation,” Elias said.
    “While is seems logical that the loss of the pigment melanin would serve as a compensatory mechanism, allowing for more irradiation of the skin surface and therefore more vitamin D production, this hypothesis is flawed for many reasons,” he continued. “For example, recent studies show that dark-skinned humans make vitamin D after sun exposure as efficiently as lightly-pigmented humans, and osteoporosis – which can be a sign of vitamin D deficiency – is less common, rather than more common, in darkly-pigmented humans.”
    Furthermore, evidence for a south to north gradient in the prevalence of melanin mutations is weaker than for this alternative explanation explored by Elias and colleagues.
    In earlier research, Elias began studying the role of skin as a barrier to water loss. He recently has focused on a specific skin-barrier protein called filaggrin, which is broken down into a molecule called urocanic acid – the most potent absorber of UVB light in the skin, according to Elias. “It’s certainly more important than melanin in lightly-pigmented skin,” he said.
    In their new study, the researchers identified a strikingly higher prevalence of inborn mutations in the filaggrin gene among Northern European populations. Up to 10 percent of normal individuals carried mutations in the filaggrin gene in these northern nations, in contrast to much lower mutation rates in southern European, Asian and African populations.
    Moreover, higher filaggrin mutation rates, which result in a loss of urocanic acid, correlated with higher vitamin D levels in the blood. Latitude-dependent variations in melanin genes are not similarly associated with vitamin D levels, according to Elias. This evidence suggests that changes in the skin barrier played a role in Northern European’s evolutionary adaptation to Northern latitudes, the study concluded.
    Yet, there was an evolutionary tradeoff for these barrier-weakening filaggrin mutations, Elias said. Mutation bearers have a tendency for very dry skin, and are vulnerable to atopic dermatitis, asthma and food allergies. But these diseases have appeared only recently, and did not become a problem until humans began to live in densely populated urban environments, Elias said.
    The Elias lab has shown that pigmented skin provides a better skin barrier, which he says was critically important for protection against dehydration and infections among ancestral humans living in sub-Saharan Africa. But the need for pigment to provide this extra protection waned as modern human populations migrated northward over the past 60,000 years or so, Elias said, while the need to absorb UVB light became greater, particularly for those humans who migrated to the far North behind retreating glaciers less than 10,000 years ago.
    The data from the new study do not explain why Northern Europeans lost melanin. If the need to make more vitamin D did not drive pigment loss, what did? Elias speculates that, “Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation. The body will not waste precious energy and proteins to make proteins that it no longer needs.”
    For the Evolutionary Biology study, labeled a “synthesis paper” by the journal, Elias and co-author Jacob P. Thyssen, MD, a professor at the University of Copenhagen, mapped the mutation data and measured the correlations with blood levels of vitamin D. Labs throughout the world identified the mutations. Daniel Bikle, MD, PhD, a UCSF professor of medicine, provided expertise on vitamin D metabolism.



    What do you guys think?

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    Quote Originally Posted by MikeWhalen View Post
    What do you guys think?
    I think I need to do a wholesale re-write of my paragraphs on this topic! I'd had a bit of a clue a few months ago from something in a TV programme. This takes things a lot further.

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    This doesn't explain why Both the Denisova and Mesolithic hunter-gatherers were dark skinned .

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    Quote Originally Posted by MikeWhalen View Post
    “For example, recent studies show that dark-skinned humans make vitamin D after sun exposure as efficiently as lightly-pigmented humans, and osteoporosis – which can be a sign of vitamin D deficiency – is less common, rather than more common, in darkly-pigmented humans.”
    ...
    Elias speculates that, “Once human populations migrated northward, away from the tropical onslaught of UVB, pigment was gradually lost in service of metabolic conservation. The body will not waste precious energy and proteins to make proteins that it no longer needs.”
    The cited studies are very valuable contributions to the debate, but Elias' alternative explanation is extremely weak.

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    I once read another theory that European light skin has to do with diet.

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    Quote Originally Posted by Stellaritic View Post
    This doesn't explain why Both the Denisova and Mesolithic hunter-gatherers were dark skinned .
    Dark skin is just the standard model for Homo, presumably since our ancestors lost their fur. If a creature does not have fur to protect its skin from ultra-violet light, it needs melanin to do the job. What geneticists are aiming to explain is the gradual change to paler skin in regions further from the equator. A crucial word is 'gradual'. Natural selection works slowly, over thousands of years.
    Last edited by Jean M; 07-12-2014 at 10:03 AM.

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    Quote Originally Posted by Jean M View Post
    What geneticists are aiming to explain is the gradual change to paler skin in regions further from the equator. A crucial word is 'gradual'. Natural selection works slowly, over thousands of years.
    As we have discussed earlier, "natural" (biological) selection operates over tens of thousands of years. It cannot possibly explain the rapid change in European skin color that we now know occurred within the span of a few thousand years at most. Here is a typical popular science article on the issue:
    ---
    The analysis of the man, who lived in modern-day Spain only about 7,000 years ago, shows light-skin genes in Europeans evolved much more recently than previously thought.
    ...
    Many scientists have believed that lighter skin gradually arose in Europeans starting around 40,000 years ago, soon after people left tropical Africa for Europe's higher latitudes. The hunter-gatherer's dark skin pushes this date forward to only 7,000 years ago, suggesting that at least some humans lived considerably longer than thought in Europe before losing the dark pigmentation that evolved under Africa's sun.
    ...
    But the new discovery shows that latitude alone didn't drive the evolution of Europeans' light skin. If it had, light skin would have become widespread in Europeans millennia earlier, Lalueza-Fox said.
    ...
    The finding implies that for most of their evolutionary history, Europeans were not what many people today would call 'Caucasian', said Guido Barbujani, president of the Associazione Genetica Italiana in Ferrara, Italy, who was not involved in the study.
    ---

    In contrast, a Neanderthal introgression may play a role in the skin color of some East Asians, particularly Taiwanese aborigines:
    ---
    We further discovered that all of the putative Neanderthal introgressive haplotypes carry the Val92Met variant, a loss-of-function variant in MC1R that is associated with multiple dermatological traits including skin color and photoaging. Frequency of this Neanderthal introgression is low in Europeans (~5%), moderate in continental East Asians (~30%), and high in Taiwanese aborigines (60-70%).
    ---
    Last edited by lgmayka; 07-12-2014 at 01:26 PM.

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    Quote Originally Posted by lgmayka View Post
    As we have discussed earlier, "natural" (biological) selection operates over tens of thousands of years. It cannot possibly explain the rapid change in European skin color that we now know occurred within the span of a few thousand years at most.
    A highly beneficial gene could sweep through a population much more quickly, right? Sexual/cultural selection could also cause a more rapid population transformation than natural selection.

    There is also quite a bit of variation in skin colour within Africa. Is there an explanation for lighter skin color among Khosians? The melanin/vitamin D trade-off seems like a possible evolutionary mechanism. Skin moisture does not seem like a plausible explanation for Khosians.

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    Quote Originally Posted by GailT View Post
    A highly beneficial gene could sweep through a population much more quickly, right?
    In extreme cases, yes:

    - In a shrinking population (i.e., during a bottleneck period), an allele with a very high survival rate may effectively sweep through the population rapidly. For example, an allele that confers immunity to an otherwise common and often fatal illness.

    - In a growing population, an allele with a very high reproduction rate may effectively sweep through the population rapidly. For example, an allele that extends fertility an extra couple decades.

    But otherwise, humans are really not analogous to fruit flies. Their generations are so long, and their survival and reproduction is subject to so many different factors, that it is difficult to imagine a single allele sweeping through a human population rapidly by biological means alone.
    Quote Originally Posted by GailT View Post
    Sexual/cultural selection could also cause a more rapid population transformation than natural selection.
    Yes, exactly. That was my unspoken point. In general, human survival and reproduction is more strongly influenced by nonbiological factors such as power, wealth, status, and (perceived) beauty. Any of these can drive a swift selective sweep. Even if an allele has some biological value, its sweep through a human population is likely to be primarily social (if the biological value is observable).
    Last edited by lgmayka; 07-12-2014 at 06:46 PM.

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    Quote Originally Posted by lgmayka View Post
    Many scientists have believed that lighter skin gradually arose in Europeans starting around 40,000 years ago, soon after people left tropical Africa for Europe's higher latitudes. The hunter-gatherer's dark skin pushes this date forward to only 7,000 years ago ....
    Not as far as I know. Pigmentation is influenced by more than one gene. The KITLG mutation seems to the earliest outside Africa, as it is shared by Western Eurasians and east Asians. One estimate puts the KITLG change at 30,000 years ago.

    The Western Eurasian-type alleles at TYRP1, SLC24A5, and SLC45A2 appear to have arisen much later - all within the last 11,000-19,000 years. Given that the incoming farmers were SLC24A5 derived, that particular mutation could have arisen in the Near East 10,000 years ago or more, and been subject to selection to the point that it was fixed before arrival in Europe.

    I haven't had time to dredge through the aDNA papers to check exactly which SNPs were tested in each case, though I made a note in some cases. http://www.ancestraljourneys.org/autosomaladna.shtml. I suppose we should really find out about KITLG.
    Last edited by Jean M; 07-12-2014 at 06:57 PM.

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