Monday, March 30, 2009
Fitting into the World
Humans are a diverse bunch. In the section Man Against the Sun, Carroll discusses skin color in the genetic sense. Why did skin color and hair color adapt the way it did to help Europeans, Africans, Indians, Asians, and Hispanics in their respective climates? Explain what helps absorb light rays, relating back to hormones and the endocrine system. What other features could be explained by adaption to the climate? Do these also have a genetic backing? Explain.
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First of all, we can separate Europeans, Africans, Indans, Asians, and Hispanics in to separate climate zones. Europeans, they belong to Northern hemisphere where its cold and get small amount of sunlight, Africans, Hispanics and Indians belong right on the equator, where they get most sunlight, and finally Asians where they are located right above the equator.
ReplyDeleteFrom this classification, we can notice the pattern. More sunlight people get, more darker skin they have. This is consistent with what Sean Carroll said ;"UV light induces the synthesis of MC1R and aMSH, which in turn leads to the productio nof increased levels of melanin"(171). Produciont of melanin is basic mechanism for humans to prtect the body against harmful UV light. Therefore, Africans, Indians, and Hispanics, who are more exposed to strong Sun light, have developed the darker skin to survive and reproduce.
Interesting fact about the melanin is that melanin production is closely related to hormones and the endocrine system. The melanocortin-1 receptor reacts to the melanocyte -stimulating hormone which is produced from the pituitary gland. people who have red hair or who don't get tan well, they have variants in melanocortin receptor so their melanin is not produced well. When we get a sun burn, another hormones help to regulate our body to seduce the pain. For example, ACTH is released to the Adrenal cortex and it releases glucocorticoids to reduce the pain in response to sunburn.
Another feature that people developled while adapting to the climate is the ability to live in a high altitude with little oxygen. For example, we wonder why so many people from Kenya win the marathon. A possible explanation is that Kenyans have adapted to their environment which is highlands. Since Oxygen is scare in their environment, they are able to run better than anybody else at the normal altitude. This shows that people evolve through natural selection and that people get affected by its envrionmental factors just like skin colors.
As the previous comment stated, skin color changes within Europeans, Africans, Indians, Asians and Hispanics because of different climates. Regions that get more direct UV light tend to have darker skin pigmentation than those in regions with less UV light. Most research leans toward the argument that this is the cause of evolution and selective advantages. The darker pigmented skin found in places like Africa have a lot of direct sunlight and their skin has a lot of melatonin. Regions like northern Europe have a lot less melatonin and their skin therefore appears much lighter and almost reddish because the blood vessels are visible through the skin. Melatonin is a protective agent against harmful UV rays. Darker pigmentation is necessary for more protection from the sun.
ReplyDeleteHowever, the selective advantage of having different skin pigmentations for different regions is not a severe rule. Over years and years, regions have generally developed towards a color of skin. However, there are plenty of Caucasian Europeans that have survived in Africa on the equator without having the dark skin that is dominant in that region.
Some regions have adaptations, other than skin pigmentation, that have been selective advantages towards the environment. Often, hair pigmentation is linked closely with skin pigmentation. Therefore, darker pigmentation of the skin because of the large amounts of UV light will also cause darker hair. On the other hand, fair skin often goes with fair hair.
The main cause for dark pigmentation is protection from the sun. Therefore, fair skinned people are more susceptible to the harmful UV rays. This harmful light causes skin cancer and fair skinned people are 10 times more likely to get this cancer than darker skinned people from around the equator. Dark skin prevents the radiation from harming folic acid which is derived from vitamin B.
The other comment talked about oxygen intake at higher altitudes. Also, lighter skin pigmentation is beneficial here because it allows an increased ability to absorb sunlight. Some sunlight is necessary and causes an increased production of vitamin D3. This is needed for calcium absorption and bone growth. The absence of melatonin causes a condition called melatonin. This means very light skin, hair and eyes. It is caused by the inability to synthesize tyrosine.
Since Sammy and Meagan has wonderfully described the big picture behind variation of human skin color for adaptation in different regions of the world, I’ll try to go more specific and talk about melanin, a class of compounds found in plants, animals, and even protests, where it serves mainly as a pigmentation compound.
ReplyDeleteSkin color is due primarily to the presence of a pigment called melanin. Both light and dark complexioned people have this pigment. However, two forms are produced; pheomelanin, which is red to yellow in color, and eumelanin, which is dark brown to black. People with light complexioned skin mostly produce pheomelanin, while those with dark colored skin mostly produce eumelanin. In addition, individuals differ in the number and size of melanin particles. The latter two variables are more important in determining skin color than the percentages of the different kinds of melanin. In lighter skin, color is also affected by red cells in blood flowing close to the skin. To a lesser extent, the color is affected by the presence of fat under the skin and carotene, a reddish-orange pigment in the skin.
Melanin is normally located in the epidermis, or outer skin layer. It is produced at the base of the epidermis by specialized cells called melanocytes. Nature has selected for people with darker skin in tropical latitudes, especially in nonforested regions, where ultraviolet radiation from the sun is usually the most intense. Melanin acts as a protective biological shield against ultraviolet radiation. By doing this, it helps to prevent sunburn damage that could result in DNA changes and, subsequently, melanoma, or skin cancer. Melanoma is a serious threat to life. In the United States, approximately 54,000 people get this aggressive type of cancer every year and nearly 8,000 die from it. Those at highest risk are European Americans, who have a 10 times higher risk than African Americans (Source: http://en.wikipedia.org/wiki/Human_skin_color).
Ultraviolet radiation reaching the earth usually increases in summer and decreases in winter. The skin's ability to tan in summertime is an acclimatization to this seasonal change. Tanning is primarily an increase in the number and size of melanin granules due to the stimulation of ultraviolet radiation. While skin tanning is often most noticeable on light complexioned people, even those with very dark brown skin can tan as a result of prolonged exposure to the sun. Some Northwest Europeans have substantially lost the ability to tan as a result of relaxed natural selection. Their skin burns and peels rather than tans. This is due to the fact that they produce a defective form of a skin protein (melanocortin-1 receptor or Mc1r), which is necessary for the production of melanin (Source: http://en.wikipedia.org/wiki/Melanin). They are at a distinct disadvantage in tropical and subtropical environments. Not only do they suffer the discomfort of readily burning, but also they are at a much higher risk for skin cancer. The same is true of albinos.
It would be harmful if melanin acted as a complete shield. A certain amount of shortwave ultraviolet radiation (UVB) must penetrate the outer skin layer in order for the body to produce vitamin D. Approximately 90% of this vitamin in people normally is synthesized in their skin and the kidneys from a cholesterol-like precursor chemical with the help of ultraviolet radiation (Source: http://www.wisegeek.com/what-is-melanin.htm). The remaining 10% comes from foods such as fatty fish and egg yolks. Simple vitamin D is converted by our bodies into two sequential forms. The last form, commonly referred to as vitamin D, is needed for the intestines to absorb calcium and phosphorus from food for bone growth and repair. Calcium is also necessary in adults to maintain normal heart action, blood clotting, and a stable nervous system. Vitamin D plays an additional important role in promoting the production of cathelicidin, which apparently is an effective defender against fungal, bacterial, and viral infections, including the common flu. Too much ultraviolet radiation penetrating the skin may cause the break down of folic acid (or folate--one of the B vitamins) in the body, which can cause anemia (Source: http://www.mayoclinic.com/health/vitamin-d/NS_patient-vitamind). Pregnant women who are deficient in folic acid are at a higher risk of having miscarriages and babies with neural tube defects. Because folic acid is needed for DNA replication in dividing cells, its absence can have an effect on many body processes, including the production of sperm cells. It may be that the ability to produce melanin was selected for in our early human ancestors because it helped preserve the body's folic acid supply in addition to reducing the chances of developing skin cancer.
People who live in far northern latitudes, where solar radiation is relatively weak most of the year, have an advantage if their skin has little shielding pigmentation. Nature selects for less melanin when ultraviolet radiation is weak. In such an environment, very dark skin is a disadvantage because it can prevent people from producing enough vitamin D, potentially resulting in rickets disease in children and osteoporosis in adults. Contributing to the development of osteoporosis in older people is the fact that their skin generally loses some of its ability to produce vitamin D (Source: http://www.britannica.com/EBchecked/topic/373722/melanin). Women who had prolonged vitamin D deficiencies as girls have a higher incidence of pelvic deformities that prevent normal delivery of babies.
In conclusion, melanin is like a double edged sword. Having too little of it puts one in a greater position to get skin cancer and that person would not have much protection against the Sun's ultraviolet rays. However, having too much melanin would be harmful because it limits one's ability to use sunlight to make vitamin D. Nevertheless, natural selection has made the best possible variation in melanin where people who live near the equator have darker skin tone to protect them from the harmful ultraviolet rays while people who live farther away have a predisposition of being able to synthesize more vitamin D.