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quote:Originally posted by Carol-AMelanin protects us, to some degree from the damaging effects of the sun, and so people livingclose to the equator will have dark skin and eyes, to prevent too much uv damage. The downside of a lot of melanin is that it reduces the amount of vitamin d you can make. Near to the equator, where sun is not a problem, vitamin d is not a problem, but closer to the poles, where sun is limited, a dark skin could result in ricketts, so closer to the poles, there has been an advantage to reducing the level of melanin. Melanin is what causes hair and eye colour too. Skin and hair pigment is made up of different types of melanin. There are two broad groups of melanin, eumelanin, which is brown, and phaeomelanin, which is red. If you have high levels of melanin, the hair and eyes will basically look black, but as the total level of melanin falls, differences in hair and eye colour will show. It is not fully understood why there are some people with very dark hair but fair skin, but people are working on it!
quote:Originally posted by neilepThis is all very interesting....I am one of those very fair skinned people with dark hair and blue eyes, I only have to be in the sun for a few minutes before turning pink !!.
quote:THE Rh-NEGATIVE POPULATIONThe first mesolithic people from the Basque peninsula were without doubt the most experienced sailors of the Atlantic. These people who populated the northwest coast of Europe have a very special blood peculiarty which thier descendants are still living today. Dr.Luigi Cavalli-Sforza published a map of the populations with the highest percentage of thier members with Rh-negative blood. He wrote:"Rh-negative genes are frequent in Europe, infrequent in Africa and West Asia, and virtually absent in East Asia and among the aboriginal populations of America and Australia. One can estimate degrees of relatedness by subtracting the percentage of Rh-negative individuals among, say, the English (16%) from that of the Basques (25%) to find a difference of nine percentage points.The highest percentage of people with rh- blood is found in the Atlas mountains of Morocco(40%). The next highest are the Basques, reported in different publications as having 25 and 32%, depending on location. The people of northwest Ireland, the Highland Scots and the western islanders of Norway all have between 16 and 25%, while the Lapps of Norway and Finland have between 5 and 7%
quote:2002 October 10 ThursdayMutation for redheads is of recent origin In evolutionary terms a mutation that arose only 20,000 years ago is pretty recent. If this is correct then the pheomelanin variation of melanin that produces red hair is pretty new.quote:According to the most recent estimates, the first red hair sprouted just 20,000 years ago, long after the advent of modern homo sapiens.
quote:According to the most recent estimates, the first red hair sprouted just 20,000 years ago, long after the advent of modern homo sapiens.
quote:Originally posted by geckoif this is true, why arent dark haired and dark eyed people dying from vitamin D depletion all through russia and northern europe? is it because nutrition is more adequate?
quote:Rickets was a common disease afflicting children in the eighteenth century. However, the cause for it was not well understood, and many children died because there was no cure (see Tracing the Cause of Disease). As physicians began investigating other diseases such as beriberi, they realized that there were factors in food other than proteins and salts which were essential to health. Research into these “accessory food factors” led scientists to demonstrate the existence of vitamins (see "…a substance different from protein and salts..."). As scientists turned their attention to rickets again, they found that exposure to sunlight seemed to be an effective treatment. Physicians also were attempting to isolate nutrients in food that might help and found that an unknown nutrient in cod liver oil was effective against rickets. Following the designation of vitamins in alphabetic order, they dubbed this new nutrient vitamin D. One of the reasons vitamin D was a puzzle to scientists for so many years was that it was initially misidentified as a true vitamin, that is an essential substance that our bodies cannot manufacture and which therefore, can only be obtained from our food. But, unlike essential dietary trace elements, such as vitamins A, B, and C, which humans must get directly from food, vitamin D can be produced in the body through a photosynthetic reaction when the skin is exposed to sunlight. The resulting substance is only a precursor, however, which must then undergo two transformations--first in the liver and then in the kidney--to become the biologically active substance the body uses. This active form of vitamin D is a hormone, chemically akin to familiar steroid hormones, such as the sex regulators testosterone and estrogen and the stress regulator cortisol. Independently, the two teams of researchers irradiated excised skin as well as such food substances as vegetable oils, egg yolk, milk, lettuce, or rat chow and found that irradiation produced a substance that seemed to work on rickets much as the vitamin D in cod-liver oil did. Rats that were fed irradiated foods or irradiated skin were protected against rickets, whereas those fed unirradiated foods or skin were not. Recognizing that simply irradiating certain foods that were common in most people's diets could spare large numbers of children from the bone disease, Steenbock patented the food irradiation process using ultraviolet light in 1924, donating all future proceeds to support research at the University of Wisconsin. Because of these discoveries, it became possible to synthesize the vitamin in large quantities. Synthesizing the vitamin costs a fraction of what it costs to irradiate foods and does not destroy or change food flavors, as irradiation sometimes does. Synthesized vitamin D provided the capstone of the public health campaign to eradicate rickets. For his "research into the constitution of the sterols and their connection with the vitamins," Windaus was awarded the Nobel Prize for Chemistry in 1928.
quote:Folic acid addition to food is of greatest benefit to light-skinned children. Women of all races strike a very fine balance between being too fair for their environment and being too dark. Fair skin allows in enough sunlight to permit Vitamin D production even in areas that are usually overcast, like northern Europe or northeast Asia. Skin that is too dark won’t permit this, so milk is fortified with Vitamin D. Dark skin on the other hand does not appear to have evolved as protection against skin cancer; it takes several decades to develop skin cancer, by which time someone will already have produced offspring, so evolution is essentially done with them. The dark skin is to prevent the UV in sunlight from breaking down folic acid in pregnant women. A folate shortage during pregnancy results in a baby with spina bifida.
quote:Another complicating factor for folate may be sunlight. A recent article in Scientific American cited evidence that ultra-violet radiation can actually penetrate the skin and destroy folate in the bloodstream, especially in fair-skinned people. This is another reason to avoid sunbathing.