Posted on 02/28/2011 12:05:32 PM PST by SeekAndFind
Variations in skin color provide one of the best examples of evolution by natural selection acting on the human body and should be used to teach evolution in schools, according to a Penn State anthropologist.
"There is an inherent level of interest in skin color and for teachers, that is a great bonus -- kids want to know," said Nina Jablonski, professor and head, Department of Anthropology, Penn State. "The mechanism of evolution can be completely understood from skin color."
Scientists have understood for years that evolutionary selection of skin pigmentation was caused by the sun. As human ancestors gradually lost their pelts to allow evaporative cooling through sweating, their naked skin was directly exposed to sunlight. In the tropics, natural selection created darkly pigmented individuals to protect against the sun.
Ultraviolet B radiation produces vitamin D in human skin, but can destroy folate. Folate is important for the rapid growth of cells, especially during pregnancy, when its deficiency can cause neural tube defects. Destruction of folate and deficiencies in vitamin D are evolutionary factors because folate-deficient mothers produce fewer children who survive, and vitamin D-deficient women are less fertile than healthy women.
Dark skin pigmentation in the tropics protects people from folate destruction, allowing normal reproduction. However, because levels of ultraviolet B are high year round, the body can still produce sufficient vitamin D. As humans moved out of Africa, they moved into the subtropics and eventually inhabited areas up to the Arctic Circle. North or south of 46 degrees latitude -- Canada, Russia, Scandinavia, Western Europe and Mongolia -- dark-skinned people could not produce enough vitamin D, while lighter-skinned people could and thrived. Natural selection of light skin occurred.
The differences between light-skinned and dark-skinned people are more interesting than studying changes in the wing color of moths or, the most commonly used evolutionary example, bacterial colonies, according to Jablonski. Adaptation to the environment through evolutionary change becomes even more interesting when looking at the mechanism of tanning.
"In the middle latitudes tanning evolved multiple times as a mechanism to partly protect humans from harmful effect of the sun," Jablonski told attendees at the annual meeting of the American Association for the Advancement of Science today (Feb. 20) in Washington, D.C.
Tanning evolved for humans so that when ultraviolet B radiation increases in early spring, the skin gradually darkens. As the sun becomes stronger, the tan deepens. During the winter, as ultraviolet B wanes, so does the tan, allowing appropriate protection against folate destruction but sufficient vitamin D production. Tanning evolved in North Africa, South America, the Mediterranean and most of China.
Natural variation in skin color due to natural selection can be seen in nearly every classroom in the U.S. because humans now move around the globe far faster than evolution can adjust for the sun. The idea that variation in skin color is due to where someone's ancestors originated and how strong the sun was in those locations is inherently interesting, Jablonski noted.
"People are really socially aware of skin color, intensely self-conscious about it," she said. "The nice thing about skin color is that we can teach the principles of evolution using an example on our own bodies and relieve a lot of social stress about personal skin color at the same time."
Jablonski noted that the ability to tan developed in a wide variety of peoples and while the outcome, tanablity, is the same, the underlying genetic mechanisms are not necessarily identical. She also noted that depigmentated skin also developed at least three times through different genetic mechanisms. Students who never tan, will also understand why they do not and that they never will.
Provided by Pennsylvania State University
More to it than just that. Blacks in northern climates who supplement their diet with vitamin D are under no selective constraints to have lighter skin. Similarly, whites in equatorial climates who wear covering and sun screen are also under no selective pressure to have darker skin.
But if there is another mechanism, other than evolution through natural selection of genetic variation, that explains why different human populations look differently and are locally adapted to environmental conditions - I would certainly like to hear it.
So, if my family had moved to florida at one point, I or some of my heirs might be black??
Each of Noah’s son’s looked differently. light, dark, yellow. They went off to different parts of the world after the flood.
It is more evolutionary tripe. Why are the Eskimos not white? Why are the Mongols not white? Why are the Aztecs decedents not black?
Intersting. I would have guessed that in the tropics natural selection would have FAVORED darkly pigmented individuals. I had NO idea that a theory could be imbued with the power of creation.
One of the worst sunburns I've ever gotten on my face occurred in the dead of winter, on a ski slope. This is not uncommon. "Tanning" also occurs due to wind and cold.
Either someone is not very good at explaining themselves, or the theory needs work.
Apparently it’s the new religion.
How did Noah end up with three sons that looked so different? What was the mechanism? Dark skin was the curse set upon Ham
Why did these differences accumulate instead of abrogate?
How could you get these three distinct genetic types all from one set of parent?
And how did these differences accumulate such that three who were brothers gave rise to populations that looks so distinct?
They's some of them there creationists and don't know no better.
14,000 or so years (approximately 700 generations) might not be enough time.
No........but if they moved there about 11,500 years ago, nobody would mistake you for an Irishman.
Wind and cold?
LOL!
Try “the reflection of the sun off the snow” and you will be closing in on the truth.
I take it you’ve never experienced windburn. It’s distinct from sunburn.
I believe we once had a more diverse gene pool and we would have children that often didn’t look like us. Similar to cat or puppy litters. In time, those who looked similar were more familiar or attracted to each other and those traits became dominate and repeated, distinctive groups formed.
There is some circumstantial evidence in the Bible where parents would have several children and one would be in the same image.
Eskimos are not white because their diet historically has been rich in organ meats as they used to live an almost totally carnivorous lifestyle. Organ meats and fish are very high invitamin D so there was no selective pressure to develop a light skin color.
Windburn is from chafing.
Sunburn is from sun.
Tanning is caused by exposure to sunlight - not from exposure to wind or cold.
Sunburn is common among the light skinned in the dead of winter when they ski on a sunny day because they are getting the reflection of the sun off the snow, effectively almost twice the amount they had become used to - and usually all day - and without much protection.
Why are the Eskimos not white?
They get most of their vitamin D from sea food and or they migrated there relatively recently.
This pigment "science" is the same thing.
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