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Question: Netflix has recently produced a two-part miniseries, The Evolution of US, that examines the evolution of "man". The topic of of human skin colour was covered and it was stated that 'lighter skin pigmentation was a preferred natural selection for early humans living in the "northern" regions'. The idea being that lighter skin could "absorb" sun-rays; which apparently is necessary for activating vitamin-D. Is this a true statement ? And where might I find good sources of literature to read on this topic ?

Now this particular series does not define "northern" and to be fair neither was the term "lighter skin" . My motivation in asking the question is driven by my own travels in far eastern Russia, Mongolia, Alaska, and parts of Peru and Ecuador. In each of those locations I observed numerous groups of indigenous people with dark brown skin and pitch black straight hair; which seems to contradict the statement in the series.

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  • $\begingroup$ No "apparently" about sunlight being needed for the formation of Vitamin D. See e.g. [this article]. As regards how much sunlight (UV-B) penetrates the skins of people of different ethnic groups, that I don't know. $\endgroup$
    – David
    Nov 25, 2018 at 18:09
  • $\begingroup$ @David ; which article are you referring to? Are you sure the link you provided is correct ? $\endgroup$
    – user3195446
    Nov 25, 2018 at 19:47
  • $\begingroup$ I do apologize — somehow the link never got pasted. Here is is: a review in less-specialized language available from NCBI entitled Vitamin D: The Sunshine Vitamin. $\endgroup$
    – David
    Nov 25, 2018 at 20:21
  • $\begingroup$ Rather large nitpick here: Equador and Peru are not "northern" (or more accurately, polewards) regions. In fact (as you might have guessed from the name :-)) the equator passes right through Equador. As for the indigenous populations of northern Asia & North America, you might ask whether they've lived there long enough to diverge from their ancestral populations. Evolution takes time. I might also wonder if the people you saw spent a lot of time outdoors. Despite a mostly northern European ancestry, I get pretty brown in the summer :-) $\endgroup$
    – jamesqf
    Nov 26, 2018 at 5:05

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Melanin reduces the skin's ability to make Vit D due to reduced UV penetration. This is a disadvantage in northern areas where people get less sun light, and also need to wear heavy cloths to keep warm which adds to the problem. People in Sunny regions have the opposite need as there is more than enough UV light exposure, but they run the risk of developing skin cancer, which is why having melanin provides an evolutionary advantage in these regions.

Interestingly, in the Arabian Gulf region, there is currently an epidemic of Vit D deficiency especially among those with dark skin. Although there is plenty of sun light, people tend to avoid the harsh sun almost completely by commuting strictly in cars and avoiding walking outdoors during the day. Car windows (and glass in general) are opaque to UV rays, therefor reducing the skin's ability to produce Vit D, and forcing many locals who adopt such lifestyle to take oral Vit D supplements.

Here is a link with some references to confirm the medical link between Vit D deficiency and lack of sunlight exposure:

https://www.webmd.com/food-recipes/qa/how-can-dark-skin-lead-to-vitamin-d-deficiency

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  • $\begingroup$ as a follow up question how might we exlain my observation - the existence of various groups of people living near the arctic regions or at high altitudes and whom have demostrably dark skin colours. $\endgroup$
    – user3195446
    Nov 25, 2018 at 21:13
  • $\begingroup$ I do not know the specific answer in relation to each of these different groups of people. However I know that in relation to Eskimos, they get their Vit D through diet rather than skin production. Their diet of whale, seal, trout and walrus blubber are all rich in Vit D. I expect you will find the other groups you mentioned have a similar affinity to Vit D rich diets. $\endgroup$
    – Matt Dee
    Nov 26, 2018 at 20:48

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