From what I understand, the evoluntionary pressure for light or dark skin is thus:
In hot climates, the light-skinned people get skin cancer and die. Eventually, the majority of the population is dark-skinned.
In cold climates, dark-skinned people get Vitamin D deficiancies and die. Eventually, the majority of the population is light-colored.
Of course, I’m oversimplifying here, but let’s go with the cancer vesus D thing right now.
So, my questions (or should I say debates):
Is the process still going on today, albeit much slower, or have the inventions of things like sunscreen and vitamin pills obviated the Darwinian selection of skin color?
How long does the process take? If you could magically switch a group of Eskimos and Maasai, how long would it take for the Kenyan Eskimos to become black, and the Alaskan Maasai to become white? (Pre-sunscreen, vitamins, etc).
I’ve been wondering about this for a while, and I feel this is the place to ask.
Yes, the process of natural selection is still in operation. Vitamin pills, sunscreen, and timely cancer detection and surgery are possibly slowing it down, but many poor people in the world do not have access to any of the above and are still subjected to medical problems that would otherwise be alleviated.
However, swapping the Masai for the Inuit may not result in the Masai becoming lighter in skin color if they adopt the traditional Inuit diet, which is high in vitamin D from seafood sources. In other words, Inuit did not depend on sunlight for vitamin D and this may account for why they are relatively dark skinned compared to, say, Lapps or Finns or other northern peoples who are light-skinned. In the case of the seafood-eating Inuit dark skin did not interfere with vitamin D acquisition and thus was not selected against. Masai who eat a lot of seafood may likewise remain dark skinned at no penalty regardless of where they live.
This topic was discussed on NPR recently and was featured on their On Science podcast that was released yesterday. Robert Krulwich interviewed Nina Jablonski, an anthropologist from Penn State. She estimated the change in color from dark to light — or the other way around in some cases — takes place in 50 to 100 generations. The pigmentation is related more to UV rays than to temperature. She also notes that this change in skin color is slowing, which she attributes to wearing clothes.
Arabs live in some of the most sunny places on earth and yet they are on the lighter end of the spectrum, while many people in equitorial places live in jungles and get a lot less sunlight and are darker.
I suspect that you might be surprised how much sunlight actually reaches the people on the forest floor of a “jungle,” (setting aside, for the moment, the imprecise nature of the word, “jungle,” itself).
The OP appears to have a factual answer, with no more actual contention than one would expect to find in any discussion of a complex topic, so I am going to send it over to General Questions where it might catch the eye of a few more posters to provide good information.
It’s not true in general that equatorial peoples live inside jungles. Many sub-Saharan Africans, such as Masai, historically lived in open savanna country. Most other sub-Saharan Africans are agriculturalists, and are in the sun most of the time. The only groups that really lives “in jungle” are the pygmies.
I used the “jungle” to illustrate a point, that we have people who live in some very sunny climes, who are generally on the lighter end of the spectrum. That seems to go against the theory about; more sun; dark. Yes Arabs are darker than NW Europeans, but why are they lighter than sub-saharan Africans (despite getting the same amount of sun) and many say Inuits who get vey little?
While there is clearly a correlation between skin color and the degree and intensity of sunshine a population experiences, that correlation is just as clearly imperfect, as you point out. Selection for this characteristic, although present, may be rather diffuse, and take a long time to alter skin coloration in large populations. The current discrepancies between skin color and insolation are likely caused by migrations between different areas. I recall having seen an estimate, I think in Scientific American, of the time lag between migration to an area and adaptation to its climate; it is much longer than the “50 to 100 generations” mentioned in Julius Henry’s post.
Take as an example New World populations. The ancestors of these populations migrated from Siberia between 20,000 and 15,000 years ago. In the time since, there has been some local adaptation to insolation. Compare for example these Greenland Inuit with these Kuna from Panama. While the Inuit are darker than northern Europeans, and the Kuna are much lighter than equatorial Africans, the populations differ in skin tone. It is possible that Amerindians may have lost the genetic variants for the darkest skin tones in their travels across northern Asia.
Actually, they are probably not getting “the same amount of sun.” One factor of sun exposure is the angle in which it strikes the earth–an angle that varies proportionally the farther one moves from the Equator. When we think of “Middle Eastern Desert,” we tend to be talking about Syria, Iraq, Iran, and into Afghanistan–an area that is wholly North of the Tropic of Cancer.
I went looking for an on-line version of Biasutti’s map of skin tone and found one on Wikipedia. (It is not the one with which I am familiar, so I am not sure which of them was the actual representation from Biasutti.) The map is linked from the Wikipedia article on Human skin color, which also provides additional information.
Note that the darkest skin is found in central Africa, the Southern tip of the Arabian Peninsula, the Southern tip of India, and then New Guinea and Australia. My WAG, based on the map, and presuming that skin color changes require some number of generations while humans keep moving around, would be that the dark skin in Africa was the “original” tone, (following the pre-Homo Sapiens shift from light to dark as proto-humans lost their hair), and that migrations within Africa have tended to keep those tones “refreshed” across that continent while invasions from lighter toned peoples later in history have pushed into the Malaysian peninsula. (The area in Southern Africa that appears to retain darker skin actually corresponds to a migration from Central Africa that happened in (geologically) recent times.) South America might present a problem with its lack of very dark tones, but that might be covered by the fact that the Equator does not pass through a heavily populated region of South America, that there has been less time for darker skin to re-assert itself in South America since the human migrations*, or that Biasutti did not have good access to darker skinned people in the Amazon basin when he was developing his map.
The idea that the Americas were only settled 15,000 - 11,000 years ago is being challenged, but even earlier proposed dates do not leave any given population on the Equatorial line long enough to re-develop darker skin.
