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This question already has an answer here:

According to Physics, Black Body absorb all incoming light reflect nothing when compared to White body. This phenomenon is called Black-body radiation. So Melanin should turn your skin into white for protect the body from harmful rays in sunnier regions, that is not happening in Biology of Human Beings.

This is why Most of School Uniforms are specifically white on Saturdays (Because of intensity of sunlight is more on that day)

Then How Dark skin protects humans in sunnier climates?

My Question is different from this question because I mentioned physics subject also in my question.

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marked as duplicate by another 'Homo sapien', Bryan Krause, canadianer, AliceD, James Mar 22 '17 at 10:21

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ "This phenomenon is called Black-body radiation". No, Black-body radiation has nothing to do with this. Black-body radiation is the electromagnetic radiation emitted by a black body when heated to a certain temperature, which depends only on the temperature itself and not on the molecular composition of the black body. Planck's explanation of the black-body radiation, proposing the "quanta" of energy, initiated what is now called quantum physics. $\endgroup$ – user24284 Mar 21 '17 at 4:51
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    $\begingroup$ Also, Saturdays are in no way any different from any other day of the week in terms of average sun intensity. Where did you even hear this??? $\endgroup$ – theforestecologist Mar 21 '17 at 14:00
  • $\begingroup$ I suggest that you do some more of your own research on the topic to learn more about the issues/errors in your arguments that led you to ask this question in the first place. Then, you can try to edit this question to improve it's quality to avoid having it closed. $\endgroup$ – theforestecologist Mar 21 '17 at 14:03
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Your question is based in a wrong assumption: that the epidermis should be white or that a white epidermis would reflect electromagnetic radiation and, therefore, protect the DNA against the ionizing portion of that radiation.

Before anything else: human epidermis without pigments (like melanin) has no color, that is, it's transparent. The white color we see in people with low melanin concentration is mostly due to the collagen in the underlying connective tissue1.

So, we have two possibilities:

  1. A transparent epidermis, which allows ionizing radiation reaching the living layers of epidermis (stratum granulosum and below, see the image) and dermis;
  2. An epidermis with a pigment that somehow "blocks" (absorbing or reflecting) the ionizing radiation.

enter image description here

Of course, it's not a good idea allowing all that ionizing radiation getting to the DNA, and because of that possibility #2 was the selected one. Therefore, there is production/accumulation of a pigment in the epidermis.

But which one, a white one or a black one?

Two important pieces of information may help you to understand this point.

First, the white pigment assumption doesn't mean the pigment need to be white: the pigment only have to reflect the ionizing (UV) section of the spectrum. It could be of any color, even almost black. Actually, melanin does reflects some UV radiation2.

Second, and the most important: if a given pigment just reflects the radiation, this radiation can still go to other nuclei and reach DNA, just like the scattered blue light in the sky goes everywhere, up and down. It's way more logical (I'm not implying that evolution operates logically, I'm just addressing your question) having an pigment that absorbs the ionizing radiation, which is the case of melanin.

By absorbing ionizing radiation, melanin (partially) avoids this ionizing radiation reaching the DNA.

References:

  1. Skin: A Natural History; Nina G. Jablonski, ISBN: 9780520275898, February 2013

  2. Brenner M, Hearing VJ. The Protective Role of Melanin Against UV Damage in Human Skin. Photochemistry and photobiology. 2008;84(3):539-549. doi:10.1111/j.1751-1097.2007.00226.x.

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    $\begingroup$ Nice answer, +1! I've added a references section so that readers know which articles you're citing, without opening them. Feel free to roll back :) $\endgroup$ – another 'Homo sapien' Mar 21 '17 at 8:18
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    $\begingroup$ It's way better now, thanks!. I'm still editing just to improve the grammar/wording. $\endgroup$ – user24284 Mar 21 '17 at 8:19
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  1. Melanin is an antioxidant and absorbs free radicals associated with UV rays, iron, metals etc. This explains why anemia correlates with melanin.

  2. Wetter climates have more diseases and free radicals, which is why moister climates are melanizing as well.

  3. Melanin can potentially crowd in the body's own oxidants, such as testosterone, because it removes external oxidants.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ Thanks for your contribution...can you add some references to your answer? We always appreciate cited references for everything on the site - it helps individuals who are less familiar with the subject area read more about the question/answer. $\endgroup$ – Vance L Albaugh Mar 21 '17 at 2:08

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