dark skin

[kevinquinnyo]

I have been trying to figure this out with google, but I keep hitting a dead end, or worse, something racist or just strange.

I'm wondering, why is darker skin advantageous in sunnier regions, and lighter skin advantageous in less sunny regions.

I have found the statement probably 10 times now, that goes something like, "...because dark skin blocks UV rays."

I want to know why. I always thought dark colors absorb heat, not reflect it. (ever sat on black leather seats when you get into a car that's been in the sun? owch)

So how does dark skin block UV rays? Does it? Does dark skin absorb heat, but reflect light? Is there something to do with that aspect of light/heat that I don't understand?

[dasmiller]

I believe the melanin absorbs the UV before the UV penetrates far enough to damage the deeper skin layers. Heat isn't the issue.

[edona7]

Heat absorption has nothing to do with it. Ultraviolet light does. All people suffer ultraviolet damage to skin cell DNA. The body can repair it to a degree, but the more damage, the more likely some screwed up DNA will avoid repair. In sunnier regions, light skin suffers far more DNA damage related to the UVA/B. This increases greatly the incidence of nasty things like melanoma. In these regions, dark skin is advantageous.

Skin is dark due to the chemical melanin, which absorbs much of the ultraviolet rays, protecting the skin cells from damage. Vitamin D conversion in the skin isn't really impaired because there is so much UV light around.

In lower sunlight areas, lighter skin is advantageous for 2 reasons. Light skin can make more vitamin D because less melanin is around to absorb the UV. Also, keeping the skin dark requires constant production of melanin and melanocytes. The energy expense is not necessary, so either didn't evolve or piffled out over the millennia.

The actual mechanism involves a chemical breakdown of melanin by the ultraviolet light. This is then repaired, and more melanin is produced de novo. This mechanism keeps the incidence of melanoma and other skin cancers low compared to caucasians. Tanning protects against sunburns but not so much the long term DNA damage that triggers cancers.

[Soapy Sam]

Could you be confusing Infra red with ultraviolet?

I think the idea is that once our African ancestors lost their hairy pelt and opted for cooling via sweating, a dark skin regulated the amount of UV reaching inner cells, so cutting skin cancer risk. When we migrated to high latitudes where insolation was weaker, there was an advantage to paler skin in terms of vitamin A D * production- one reason we add vitamins to chapatti flour being to augment natural shortage in recent dark skinned immigrants- especially muslims who tend not to run around half nekkid as soon as the sun comes out.

* Fixed. I can never spell vittamminz.

[edona7]

Vitamin D is converted in our skin in response to ultraviolet light. Vitamin A is absorbed from food, primarily yellow orange foods. Beta carotene is a precursor to vitamin a and is what gives such foods their orange color.

Vitamin D is actually a complex little thing, and some people don't like it being termed a vitamin. Chemically, it functions more as a hormone than a vitamin. It's production involves the kidneys to make a prohormone, then on to the skin to be converted to it's most physiologically active form by ultraviolet light. Really nifty stuff.

[kevinquinnyo]

Thanks for the very informative answers.

I love the JREF Forum. It's like Yahoo Answers except less idiots and more actual answers

[lionking]

Thanks for the very informative answers.

I love the JREF Forum. It's like Yahoo Answers except less idiots and more actual answers
I agree. I also have wondered about this, but don't have to now.

[Naddig74]

Can we roll this thread up somehow and force it up Paul Bethke's nose? :D

[Soapy Sam]

D. I've been told that several times and always get it wrong. I think it's stuck. Fixed original post. Thanks.

[nathan]

I love the JREF Forum. It's like Yahoo Answers except less idiots and more actual answers

Which one has fewer grammar nazis?

[Shrike]

Thanks for the very informative answers.

I love the JREF Forum. It's like Yahoo Answers except less idiots and more actual answers

I think the hip term to use now is QFT.

[jasonpatterson]

Dark skin color definitely helps avoid skin damage from the sun, but it has a much more important reproductive purpose. Sunlight does two things to vitamins in our bodies, first it helps make vitamin D, as previously described, second, it destroys folate.

Folate is vital for cell division, and a deficiency in it during pregnancy can lead to neural tube defects in newborns.

Skin color is a balance between producing enough vitamin D given available UV (lighter preferred) and protecting the body's supply of folate (darker preferred.) People who have migrated from high UV to low UV areas (dark skinned people in high latitudes) tend to have more problems with vitamin D deficiency, people who have migrated from low UV to high UV areas (light skinned people in low latitudes) tend to have more problems with folate deficiency.

[rjh01]

There is also one other issue not discussed in this thread. Our bodies are a heat producing system. We need to get rid of this heat. Near the poles this is not a problem. The problem is trying not to lose heat. In the tropics the reverse is true. We need to lose heat as fast as possible. The colour of the skin helps this process. Dark skin will radiate heat much better than light skin.

If that is confusing to you, here is a thought experiment. Imagine two similar objects in a room. The only difference between the two is the colour. One is light and the other dark. The room is at a constant temperature. What is the temperature of the two objects?
a. Dark objects radiate and absorb heat better than light objects. So both objects are the same temperature as the room.
b. The light colored object reflects the heat of the room. The dark object will absorb the heat of the room so the dark object will be warmer than the light object.

If the answer is b then we could extract the heat from the dark object and convert it to electricity and so solve all our energy problems. Since this does not happen the answer must be a.


Edit. This answer is an expansion of the second last paragraph of the OP. Their is nothing in any post that says why skin that is good at producing Vitamin D has to be dark or the other way round.

[Skeptic Ginger]

Exposure to direct Sun produces vitamin D and skin cancer. Evolution of skin color is all about balancing those two competing problems.

Evolution is slow. Culture is faster. Thus we have vitamin D supplements, hats and sunscreen.

Yes, Virginia, it is that simple. :D

[Uncayimmy]

There is also one other issue not discussed in this thread. Our bodies are a heat producing system. We need to get rid of this heat. Near the poles this is not a problem. The problem is trying not to lose heat. In the tropics the reverse is true. We need to lose heat as fast as possible. The colour of the skin helps this process. Dark skin will radiate heat much better than light skin.

If that is confusing to you, here is a thought experiment. Imagine two similar objects in a room. The only difference between the two is the colour. One is light and the other dark. The room is at a constant temperature. What is the temperature of the two objects?
a. Dark objects radiate and absorb heat better than light objects. So both objects are the same temperature as the room.
b. The light colored object reflects the heat of the room. The dark object will absorb the heat of the room so the dark object will be warmer than the light object.

If the answer is b then we could extract the heat from the dark object and convert it to electricity and so solve all our energy problems. Since this does not happen the answer must be a.


Edit. This answer is an expansion of the second last paragraph of the OP. Their is nothing in any post that says why skin that is good at producing Vitamin D has to be dark or the other way round.

I found your explanation to be, I dunno, odd. Anyway, according this source:

http://www.ndt-ed.org/EducationResources/CommunityCollege/Other%20Methods/IRT/IR_Science.htm
Human skin is nearly a perfect blackbody as it has an emissivity of 0.98, regardless of actual skin color.

Other sources seem to concur, so that theory doesn't work. As for what you were attempting to answer, I have no idea what you're driving at or what your Edit section means.

[Kumar]

People with iron overload may have change in skin color to a slate-gray or bronze color.

[nathan]

Evolution is slow. Culture is faster.

I thought the skin colour adaptation occurred fairly rapidly in evolutionary terms (thousands or tens of thousands of years?). Unfortunately google's not helping me find a reference.

[Skeptic Ginger]

I thought the skin colour adaptation occurred fairly rapidly in evolutionary terms (thousands or tens of thousands of years?). Unfortunately google's not helping me find a reference.Thousands of years is relatively slower than the tens of years it took Caucasians moving to Australia to adopt hats and sunscreen to prevent skin cancer.

Your timeline is correct, but we are defining rapidly differently. Actually, black skin evolved more than once. As humans migrated north, skin color lightened. Then as people migrated back to the tropics, dark skin reappeared anew.

[fuelair]

Dark skin color definitely helps avoid skin damage from the sun, but it has a much more important reproductive purpose. Sunlight does two things to vitamins in our bodies, first it helps make vitamin D, as previously described, second, it destroys folate.

Folate is vital for cell division, and a deficiency in it during pregnancy can lead to neural tube defects in newborns.

Skin color is a balance between producing enough vitamin D given available UV (lighter preferred) and protecting the body's supply of folate (darker preferred.) People who have migrated from high UV to low UV areas (dark skinned people in high latitudes) tend to have more problems with vitamin D deficiency, people who have migrated from low UV to high UV areas (light skinned people in low latitudes) tend to have more problems with folate deficiency.

THIS IS THE REALLY IMPORTANT ONE - KNOWN ABOUT 11 YEARS NOW!!!!!
It is the basis of race and the reason(well, one of them) that racism is ignorant, stupid, asinine, etc. Race is basically about keeping enough of both folate and Vit. D. and that depends on sunlight level and UV level specifically.

[Skeptic Ginger]

People with iron overload may have change in skin color to a slate-gray or bronze color.They can have hyperpigmentation (regular freckles, more of them) but you may be mixing up the "slate gray" skin color with the people who OD on colloidal silver.

[Skeptic Ginger]

THIS IS THE REALLY IMPORTANT ONE - KNOWN ABOUT 11 YEARS NOW!!!!!
It is the basis of race and the reason(well, one of them) that racism is ignorant, stupid, asinine, etc. Race is basically about keeping enough of both folate and Vit. D. and that depends on sunlight level and UV level specifically.Or you could say race is about arbitrary differences amounting to about 3-4% of the human genome responsible for outward appearance while other genetic categories like blood type could just as easily define race divisions if race divisions were actually real. :)

[fuelair]

Or you could say race is about arbitrary differences amounting to about 3-4% of the human genome responsible for outward appearance while other genetic categories like blood type could just as easily define race divisions if race divisions were actually real. :)And, frankly, we could just put big quote marks around the word "race" for all the importance and meaning it ought to have!:):):):)

[Kumar]

They can have hyperpigmentation (regular freckles, more of them) but you may be mixing up the "slate gray" skin color with the people who OD on colloidal silver.

How people developed black colour of skin where other pale or white?

[JoeyDonuts]

How people developed black colour of skin where other pale or white?

That's a good question, one that brings up the salient subpoints "how is babby formed?" and "how girl get pragnent?"

Personally, I think they should do way instain mother, but YMMV, particularly WRT this issue.

[Tatyana]

How people developed black colour of skin where other pale or white?
Read the thread.

[GlennB]

Thanks for the very informative answers.

I love the JREF Forum. It's like Yahoo Answers except less idiots and more actual answers

Which one has fewer grammar nazis?

Well done.
Many would make the mistake of saying "less grammar nazis" :D

[neutrino_cannon]

I thought the skin colour adaptation occurred fairly rapidly in evolutionary terms (thousands or tens of thousands of years?). Unfortunately google's not helping me find a reference.

Jared Diamond mentions in... one of his books, I want to say The Third Chimpanzee that skin tone and sun exposure aren't particularly well correlated, most likely due to recent human migration. Ethnic Tasmanians, for example, were far darker than Koreans or Maori, who were both the same distance from the equator. Scandanavians tend to be far lighter than their Native American counterparts at comparable latitudes; this despite the fact that the Americans have been living at that latitude for longer.

How people developed black colour of skin where other pale or white?

Reasonably sure that dark skin is plesiomorphic for humans; so what you should be asking is 'how people developed pale or white skin where other black colour'.