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Evolution of Skin Colour

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Aman B

on 29 November 2012

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Transcript of Evolution of Skin Colour

Evolution of Skin Colour What Causes Skin Cancer? UV Light - Harmful to living organisms because it changes the DNA sequence thus causes mutations Environment - The ozone layer in the Earth's atmosphere is reducing therefore the intensity of UV light coming into the Earth is higher than it was 50-100 years ago. As a result, with a less atmospheric ozone layer, a higher level of UV light is able to reach onto Earth's surface. Elevation - UV light is stronger as elevation increases. This is due to the thinning atmosphere at higher altitudes thus it cannot filter UV light as effectively than it can at sea levels. Latitude - the sun's rays are the strongest near the equator Genetics - This determines your chances of developing skin cancer. If its common in the family then you may also be more prone to developing skin cancer. Why are Caucasians more at risk of skin cancer than other populations? -Dark skinned people tend to have a larger and more numerous melanin-containing particles in their skin which provide them protection from the sun's UV rays What is Melanin? -Melanin is a pigment produced by skin cells that helps protect our body from the harsh rays of the sun; this acts like a natural sunscreen
-The melanin is able to absorb the energy of UV light and shields cells from the radiations harmful effects Therefore, the amount of melanin produced in Caucasians does not protect them from the sun's rays At what age does skin cancer typically occur? Is the incidence of skin cancer greater in youth or old age? -At the age of 50, typically skin cancer occurs therefore your chances of developing skin cancer are greater in old age. Skin cancer is more frequent to take place in old age because the body loses its ability to replace old cells with new cells. How strong a selective pressure do you expect skin cancer (UV-induced mutations) to exert on reproductive success? -The selective pressure of skin cancer on the reproductive success is not that strong because skin cancer typically occurs over the age of 50, which by then many do not reproduce anymore therefore skin cancer does not lead into their offspring. What is the apparent effect of UV light exposure on blood folate levels? - Blood folate levels decrease after UV light exposure which causes folate deficiencies Folate deficiencies during pregnancy lead to anemia in mother and malformations of nervous system, gastrointestinal system, aorta, kidney and skeletal system in fetus which means there is a higher rate of miscarriages. Plus, spermatogenesis defects occur in males which is the inability to form sperm (Prud’homme-Genereux, The Evolution of Human Skin Colour). How is folate linked to natural selection? Since folate is related to pregnancy, natural selection favours people with dark pigmentation because they have higher folate levels therefore they are able to produce more/healthier offspring

-A lack of folate reduces sperm production + DNA synthesis therefore elimination of fair skinned people in the warmer areas

-Because folate levels decrease when exposed to high levels of UV light; thus individuals that can adapt to this were naturally selected. All other things being equal, which skin tone would you expect to be correlated with higher levels of folate? -Dark skin tone would be correlated with high levels of folate because they can maintain these levels since they are less affected by the UV light due to the high amounts of melanin in their body
-Since they can be protected more from UV light they can maintain their folate levels better Predict the skin tones expected at different latitudes, taking folate needs into consideration. Distribution of UV Light -UV light exposure higher near the equator than the polar ends
-Latitudes near the equator are lower, and UV light index is greater Melanin and UV light - Melanin is the pigment that prevents cells from getting damaged by the UV light

- Darker skin has more and bigger melanin particles (Prud’homme-Genereux, The Evolution of Human Skin Colour)

- UV light index is greater near the equator, this means that the production of melanin must be higher here Why do people of the same population have different skin colours? -Mutations in their DNA-Genes play a role in determining skin colour-Diseases or infection Why different skin colours evolved Hypothesis (Location):

• The location of individuals affects the skin colour evolution.
•Skin reflectance depends on latitude, and at different latitudes, there is different UV radiation
•UV radiation is most likely to exert a selective pressure on skin colour. Source: Figure 1 from Branda, R.F., and Eaton, J.W.(1978). Skin color and nutrient photolysis: An evolutionary hypothesis. Science 201: 625–626. Reprinted wih permission from AAAS. This figure and Table 1 may be used for non-commercial and classroom purposes only. Any other uses require the prior written permission from AAAS. •At latitudes that are closer to the equator, skin colour would be more darker than at areas farther away from the equator. •more intense UV radiation can degrade folate more quickly •at higher latitudes, lighter skin colours can be tolerated in the environment since the weaker sun rays indicate less intense UV rays. As such, lighter skin colours of humans are more frequent at higher latitudes. Can folate explain the variation and distribution of light- and dark-skinned individuals around the world? 1,2,3 •Folate can partly explain the variation and distribution of skin colour around the world because many studies have indicated that it can be degraded from exposure to UV light. •However, folate by itself does not explain why lighter skin in humans developed, even though this would make the individual more susceptible to folate degradation1. 1,2,3,5 Http://www.grida.no/graphicslib/detail/the-global-solar-uv-index_1394# How is vitamin D linked to natural selection? •Vitamin D is a fat soluble vitamin present in very few foods, found in supplements and added to others4. Generally, a diet high in oily fish prevents vitamin D deficiency. 6 Which skin tone allows someone to maintain the recommended level of vitamin D? -The skin tone that allows someone to maintain the recommended level of vitamin D depends on many factors, not just sunlight or latitude by itself. 6 -In cloudy climates
-Lifestyle choice
-Diet -Based on this new information, the variation and distribution of human skin colour is dependent on many factors. -For example, folate acts as a selection pressure -On the other hand, the need for vitamin D causes the skin to become lighter in order to absorb more sun and synthesize vitamin D4. •At higher latitudes, the sun is more weaker, meaning that not a lot of UVB rays penetrate into the skin4. As such, at higher latitudes the populations will have lighter skin tones (assuming that all the populations receive the same percentage of vitamin D from their diets). •At lower latitudes, a lot of UVB rays penetrate into the skin. As such, these people generally receive adequate amounts of vitamin D and darker skin is more favoured at lower latitude areas Can vitamin D alone explain the current world distribution of skin color? -Skin color is determined by a main factor our “Genetic codes”.

-The determination of skin tone is not directly related to vitamin D.

- Vitamin D is a product from absorbing UV light under the skin. Why do we have different skin colours? Genes level of melanin production in our skin.

Why People along equator are usually darker?
-Stronger sunlight (higher rates of UVB)
-Higher concentration of eumelanin
-More Protection.
-Lower Vitamin D UV light, vitamin D and folate needs CAN to explain the current world distribution of skin color. Main factor : Genetic code
High correlation with UV light ( the locations receive )
Lighter skin tone more UV rays absorbed more Vitamin D
Darker skin tone less UV rays absorbed less vitamin D and more protection for folate. Hypothesis for that Inuit’s, living at northern latitudes, are relatively dark-skinned (much more so than expected for their latitude). Constant and heavy snowfalls and rich source of vitamin D diet affect the evolution of the Inuit’s darker skin tone for survival. Hypothesis of Northern Europeans are slightly lighter-skinned than expected for their latitude Weather conditions such as cloudy days and shorter days in winter affects the evolution of lighter skin tone in Northern Europe. Research by Nina Jablonski and George Chaplin suggest that women generally produce 3–4% less melanin in their skin than do men in all populations of the world. Sexual Selection
Women with lighter skin tone are usually preferred in a general moral sense. As for most of the cultures, white represents purity, clean and innocent which most men desire.

Natural Selection
it could be that women need more calcium than men do because sufficient calcium will be required to give birth to healthy babies. Revised Hypothesis (Natural Selection):

•Natural selection affects skin colour evolution.
•Passed down to the offspring
•The skin colour that has a benefit will evolve, for instance lightly skin coloured individuals are more prone to skin cancer Revised Hypothesis (Folate Levels):

•Folate levels in the skin affect skin colour evolution.
•Folate levels are beneficial because during pregnancy more folate levels are needed for DNA synthesis
•Higher levels of flolate are favoured, and people with higher levels of folate are usually darker skin coloured. What is Folate? Folate or folic acid is one of the B Vitamins and is an essential nutrient for DNA synthesis Vitamin D is linked to natural because it plays an important role in the growth of bones. References 1. Borradale DC., Kimlin MG. Folate degradation due to ultraviolet radiontion: possible implications for humans health and nutrition. International Life Sciences Institute 7, 414-422 (2012).

2. Johan M., Kristian P.N., Asta J. Immediate pigment darkening: its evolutionary roles may include protection against folate photosensitization. The journal of the federation of American Socities for Expiremental Biology 26, 971-975 (2012).

3. Paul Lips, M.D., Ph.D., Afdeling E., Vrije U., Vitamin D Deficiency and Secondary Hyperparathyroidism in the Elderly: Consequences for Bone Loss and Fractures and Therapeutic Implications. http://edrv.endojournals.org/content/22/4/477.short (2011).

4. Nina G., George C. The evolution of humanskin coloration. “Journal of Human Evolution” 1, 57-106 (2000).

5. Osteomalacia. http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001414/. (2012).

6. Michael F. Holick, M.D., Ph.D. Vitamin D Deficiency. “The New England Journal of Medicine” 10, 266-281 (2007).
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