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Thread: Dark skin-best UV protection?

  1. #1 Dark skin-best UV protection? 
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    There is theory that black skin evolved as protection against UV radiation.To protect DNA from mutations.
    http://en.wikipedia.org/wiki/Human_s...mental_factors
    I would be glad to know how this theory explains dark hair color, that many people have? I never heard about such thing as "cancer of hair".Most of solar's radiation energy (95%) lays in infrared and visible part of spectrum.Therefore it would be better to have white hair and skin as protection against of Sun.And why DNA cannot be protected by some kind of white particles or envelope which do not develop cancer itself?Do exist some color which is able to reflect all parts of EM spectrum?


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  3. #2  
    Time Lord
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    I thought it was more likely the reverse. Why paler folk are found in thin atmosphere e.g. mountains.

    As for hair, theories about parasite suppression are popular. Maybe if the lice are mousy-brown, it is advantageous to have any colour hair but that.


    A pong by any other name is still a pong. -williampinn
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    I am afraid it has to do with the capability of certain pigments to absorb incoming UV-radiation and transform it, by a process known as “ultrafast conversion” into heat. As it turns out, melanin (a dark coloured pigment) is capable of dissipating more than 99.9% of the absorbed UV radiation as heat. The ability of melanin to transform a potentially harmful source (for DNA) of energy into a harmless one makes it a ubiquitous pigment that is used throughout the animal kingdom.

    Given its photoprotective abilities, high concentrations of melanin in skin will shield organisms from the higher incidence of dangerous UV radiation in the southern hemisphere, while organisms in more Northern regions will no longer need the same protection against UV radiation. Additionally, organisms in Northern regions rapidly shed their melanin pigmentation (over generations) as a means of absorbing more sunlight—in sun deficient regions—allowing then to produce more vitamin D. Interestingly, there is a high correlation between the tone of human skin of indigenous peoples and the average annual ultraviolet radiation available for skin exposure where the indigenous peoples live.......On a tangent....this led to the formation of the infamous “white people”, whose most striking distinction is simply down to a lower concentration of skin melanin than “black people”, and upon which lies centuries of slavery and racial inequality.

    And now moving on to hair colour: hair follicles are pigmented by two types of melanin: eumelanin and pheomelanin. As more melanin is added, hair colour gets typically darker. Different concentrations of one or more of these pigments would lead to different coloured hair......But forget your socially created assumptions about malignant cancer. Filaments of protein, of which hair is essentially made of, are inanimate entities and are therefore fearless in the face of cancer.

    Several theories exist as to why and how light hair pigmentations came to spread—as beings originated from humans with black hair. The consensus explanation for the evolution of light hair is again related to the requirement for vitamin D synthesis and northern Europe's seasonal deficiency of sunlight. In this way, high frequencies of light hair in northern latitudes are a result of the light skin adaptation to lower levels of sunlight..... Or maybe it was an initial mutation, followed by sexual selection that initiated the spread of light hair. ....Other theories are out there though.
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    One such theory comes from the observation that the blonde hair/blue eyes trait is very recent - the last few thousand years. Since people have lived under low UV intensities for tens of thousands of years, it seems unlikely that the fair hair trait has anything to do with UV.

    Instead, it has been proposed that this recent mutation has spread through the human gene pool due to sexual selection. Gentlemen prefer blondes!!
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  6. #5  
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    Thank you dupont. I can see the need for vitamin D overriding UV protection. Canadians fly south in winter primarily to soak up the sun, which we're as greedy for as fresh oranges in January.

    This made me wonder if shelter played a role. Traditional northern lifestyle often had people spending most of their hours under cover - in some cultures women basically lived indoors. Would that be good enough reason to abandon UV protection in order to maximize D exposure?

    Then since yes I think you're correct that melanin loss is rapid (generations) what of lifestyle? I'm thinking of royal families. The association of relatively pale skin tone with upper class is common worldwide (or so it was traditionally). Current explanation holds this as a phenotype distinction only i.e. nobles don't work in the fields so they don't tan. I suggest that privileged families regardless of climate adapted to opportunize D.

    Hair, I think some funny things were up with that. Much theory deals with parasite mitigation... I wonder if shelter/exposure also played a role? Consider the ideal bride. In most cultures the length of her hair largely determined her value. Additionally note the girl was expected to live shut away (preferably indoors) in her parent's home, and have minimal social contacts. Meanwhile young men freely exchanged lice and cropped their hair to combat infestation. So what does a bride's long hair prove to the world? The length is a record proving her years - years! - of cloistered celibacy. If some lousy cad gave her a roll in the hay, she'd wear the evidence.
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  7. #6  
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    Pong

    I agree. Shelter definitely plays a role. But the body has developed an infinitely more ingenious strategy to neutralise any excess or deficiency of UV radiation in the environment—influx in UV radiation triggers melanin synthesis in melanocyte cells, while low incidence of UV radiation will ensure modest manufacture of melanin (giving skin its distinctive pale colour), to maximise vitamin D production.

    That’s true. In the past, social distinctions existed between those of tanned complexion and those without. So much so that the upper-class used to cover their faces with a lead-based cream to augment their paleness, and consequently died (served them right) from lead poisoning (they haven’t changed, plastic surgery, cosmetics, etc...). More symptomatically, liver and kidney failure was extremely prevalent, as was rickets and osteomalacia, all due to vitamin D deficiency.

    However, was it purely an epigenetic trait (UV radiation incidence), or could it have been amplified through sexual selection—once it became part of cultural reality and associated with the upper-class system, could it have been amplified by interbreeding between families of a similar social status? Or maybe long before that. Is sexual preference for paleness in women by men, or vice-versa, common in all cultures? Probably not, but it opens up questions.

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  8. #7  
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    While relative skin shades within an isolated local population are actionable indicators of social class, colonialism (by foreign population averaging lighter or darker) raises an additional discrimination. Traditional ideals may be confused and irreconcilable with ethnic worth.

    Anecdotally I've noticed a common preference for relatively pale brides in pre-colonial art & folklore. This is starkly incongruous with modern values backed by science. Well, I think investigation by way of social anthropology is going to make a lot of people uncomfortable.



    My speculations about "bride hair" are pretty tangential to melanin/A. I guess we could draw it back by asking how hair colour & texture further relates to parasites (lice), bearing in mind that hair vulnerable to infestation may actually be a sexually selected trait because it proves a lady's quality.
    A pong by any other name is still a pong. -williampinn
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    Gloger’s Rule (1833)

    Within the same species of warm-blooded animals, individuals from populations living nearer to the equator tend to have darker, more heavily pigmented skin, and individuals living farther from the equator tend to have lighter pigmented skin because of the selective advantage that it provides. Heavy pigmentation protects from skin cancer caused by ultraviolet radiation from the sun. However, too much pigmentation can significantly reduce the skin’s ability to produce vitamin D, which is necessary for the absorption of calcium and phosphorus from food. The equator typically experiences high amounts of intense ultraviolet radiation, and temperate and arctic regions relatively little. Subsequently, heavy skin pigmentation is an advantage near the equator, and light skin pigmentation is an advantage farther away from it.

    Gloger's Rule is just one of several "terrestrial ecogeographical rules" devised by scientists to explain these sorts of phenomena. It was named after German zoologist and ornithologist Constantin Wilhelm Lambert Gloger, who was also the first person to install artificial bat boxes.
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  10. #9  
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    I am still waiting for a nice product but someone suggest me this products checkout for know better...
    http://allnutri.com/pid36798/skin+defense+complex.aspx
    Thanks for sharing with us vital information and Gloger’s Rule is great for my new interest and I will bookmark this topic for know better.>!!
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  11. #10  
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    FYI, other terrestrial and marine ecogeographical "rules" exist:
    - Bergmann’s Rule (Body size), 1847
    - Allen’s Rule (Appendage size), 1870’s
    - Buffon’s Law (Different regions and species)
    - Jordan’s Rule (Marine fish vertebrae number)
    - Thorson’s Rule (Marine invertebrate development mode)
    - Rapoport’s rule (Range of marine species)
    - Vermeij’s hypothesis (Marine predation-morphology)
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  12. #11  
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    Quote Originally Posted by Pong

    This made me wonder if shelter played a role. Traditional northern lifestyle often had people spending most of their hours under cover - in some cultures women basically lived indoors. Would that be good enough reason to abandon UV protection in order to maximize D exposure?

    Then since yes I think you're correct that melanin loss is rapid (generations) what of lifestyle? I'm thinking of royal families. The association of relatively pale skin tone with upper class is common worldwide (or so it was traditionally). Current explanation holds this as a phenotype distinction only i.e. nobles don't work in the fields so they don't tan. I suggest that privileged families regardless of climate adapted to opportunize D.
    It would be interesting if the recent development in this direction were associated with the rise of class structure and agriculture. My theory of beauty is simply that everyone wants to be whatever the nobles are. (Or in the modern world, whatever the celebrities are.... which is basically the same thing.)

    If you look at the rest of the world outside of Europe, it seems most people's colors are like camoflauge. Only white skinned soldiers need to smear camo makeup all over their faces in order to sneak around at night.
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  13. #12  
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    Adaptation to low UV and sexual selection are not mutually exclusive - the one leading to tall, healthy, straight-limbed individuals with robust physiques and good teeth and greater reproductive potential, the other selecting them as mates.

    One tipping point might have been at the dawn of agriculture in any given region - that moved people inland and fixed them in places depleted of dietary Vitamin D all year 'round, while creating the possibility of prolific reproduction and expansion of the genetic combination field.
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  14. #13  
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    Eye and hair colour seems to be linked to mutations in Eastern Europe during the Ice Age 20k years ago, the predominant hypothesis that I'm familiar with is that limited resources resulted in heavier sexual selection because males could only afford to support one female. That's why we find lighter eye colour more in the northern reaches of Europe, although the traits have gained some footholds in central Asia and some parts of India.

    http://cogweb.ucla.edu/ep/Frost_06.html

    Dark hair colour seems to be prevalent amongst other apes too, gorillas and chimps. It's entirely likely that early hominids would have had dark hair too. Light hair colour in humans is a relatively recent occurrence, and only managed to take hold in Europe and amongst Australian aboriginals.

    Skin colour would similarly be effected by the strong role of sexual selection in Europe during the last ice age. The problem is that other northern ethnic groups like the Inuit seem to be lighter skinned than related ethnic groups South of them, but they aren't significantly darker than East Asians. The Inuit are also darker than Europeans.

    What's interesting is that it seems dark skin in Australian aboriginal groups are genetically very different from Africans, despite similar skin colour, and they are quite far from the equator. We can't even be sure people in Africa were dark when humans moved out of Africa.
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  15. #14  
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    Tired and sleepy said

    "limited resources resulted in heavier sexual selection because males could only afford to support one female."

    I seriously doubt that was the case. Monogamy occurs for the very simple reason that there is roughly one male for every female. Of course, that does not guarantee fidelity, but that is another story.

    If you had, in a tribe, a bunch of guys with 2, 3 or more women each, and a bunch of other guys with no woman, guess what happens? I will give you a clue. It involves the polygamists bleeding heavily from the throat. And that is only from what the women did to them!

    About Australian aboriginees.

    It is reasonable to suggest this was a tropical people with dark skins when they first arrived in Australia. You may not know this, but Australia has a very high ultra violet flux in summer, even well south. This would work against any tendency to evolve fair skin, as happened in the north of Europe, with much lower UV.
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  16. #15  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    This isn't about the evolution of monogomy, it's about gender ratio disparity. What you need is an excess of females, an environment that causes higher death in males (i.e. constant interfighting between tribal groups, and harsh hunting conditions for a nomadic people) and a resource restriction which results in many undesirable women not mating successfully. The last Ice Age in Europe provided the perfect environment for this kind of selection to take place. Moreover, in smaller population groups, disparity in gender ratio can easily occur. Note that this wouldn't have to be consistent over long periods, but just require that the gender ratio get skewed towards women relatively frequently.

    In fact if you look at populations in Europe (especially Eastern Europe), you'll find there is still a skewed gender ratio caused largely by male war deaths, and to a small extent greater female longevity.

    Edit: I don't disagree that higher UV likely does select for darker skin. It's likely a combination of sexual selection with a lack of negative selection because of lower UV levels that allows lighter skin to spread in Northern regions.
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  17. #16  
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    Re skewed gender ratios.

    Anthropologists have found this more in tropical areas, such as the Amazon and New Guinea. It seems to be a result of continued inter-tribal warfare, which kills off lots of males.
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  18. #17  
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    An interesting take on the evolution of skin colour was published recently in PNAS:

    Jablonski, N. G. and G. Chaplin (2010)
    Human skin pigmentation as an adaptation to UV radiation.
    Proc. Nat. Acad. Sci. USA 107(supp.):8962-8968. (direct link to full pdf)

    The paper has also been discussed on Jerry Coynes's blog recently: Why are we different colors?

    In summary: the UV-DNA damage hypothesis is rejected (no evidence that it is true, apparently). Instead, they propose that the primary selective pressure for dark skin is to protect cutaneous folate from UV photolysis. The authors of the paper are also critical of the vitamin D story too.

    It'll be worthwhile following this story to see how it pans out in the long run. In the meantime, it's something to have a little think about.
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