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I understand that darker skin tone is caused by melanin which helps to protect against the extra UV radiation that comes from more direct sunlight. It evolved in Africa before we migrated away from Africa and as I understand it was later evolved away from in areas further from the equator, where less sun meant less need for UV protection, because less sun also made it harder to get sufficient Vitamin D. Thus those near the equator tend to have darker skin and the further from the equator one goes the lighter the skin.

There are parts of South America which reside right on the equator, where melanin would provide important UV protection and where vitamin D deficiency was not a concern. I would expect them to have black skin, as dark as someone from Northern Africa or the Indonesians has, due to the extra UV exposure associated with the equator.

However, natives of Brazil and other equatorial parts of South America don't seem to have the same skin pigmentation as I see in the other equatorial areas. They have skin darker then Europeans and other northern natives sure, but not to nearly the degree of other equatorial natives. They clearly have the mutation for melanin, since that existed before we left Africa and they do have partial dark skin, which suggests that for some reason there was an evolutionary advantage to having skin as dark as theirs, but not darker.

Can anyone explain to me why South Americans wouldn't have evolved (or not evolved away from, depending on what part of the world they originated from) the almost-black level of dark skin the other equatorial natives possess?

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    $\begingroup$ This question really needs to be pruned down a lot. Focus on the core question (Why are indigenous people in South America not as dark as sub-Saharan Africans living at the same latitude, with the same intensity of sun exposure?) and get rid of all the fluff, background, and other unnecessary material. $\endgroup$ – MattDMo Aug 22 '15 at 1:50
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    $\begingroup$ The current prevailing wisdom on human migration holds that the Americas were populated last and from groups that migrated from Northeastern Asia. Evolutionary time was likely not long enough and difference in environment, Rainforest or high mountains versus savanna could have contributed. Also it has been shown genetically that H.sapiens that migrated out of Africa came in contact with and integrated with other Homo species that left Africa several hundred thousand years before who had the traits of lighter skin, straighter hair etc associated with Europeans and Asians.... $\endgroup$ – AMR Aug 22 '15 at 2:14
  • $\begingroup$ That these genes survived in H.sapien today likely has to do with environmental selection that you mention. $\endgroup$ – AMR Aug 22 '15 at 2:15
  • $\begingroup$ @AMR Your answer is the correct one. Why don't you post it as an answer instead of a comment? $\endgroup$ – Leonardo Castro Jul 5 '17 at 14:58
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    $\begingroup$ There hasn't been that much time for it to happen, and it may not be that much of a strong selective force. The main Amerindian peoples apparently arrived 14-15 thousand years ago, at the height of the ice age. They were from cold climates, Siberians, travelling a bit like Eskimos in the snows of Alaska. Indeed, they may have not evolved light skin after coming out of Africa 50mn years ago, but they may have acquired light skin from Neanderthals and Denisovans who had lived in cold climates for over 1/2 a million years. $\endgroup$ – aliential Sep 27 '18 at 19:47
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Could not fit in a comment....

Northern Africans and Indonesians have pretty much the same skin color (to my first impression). Sub-Saharan African (that you don't talk about) are darker. And Northern Africa in not an equatorial region (not more than Los Angeles) as it is right under the tropic of cancer.

Also I am a little confused about who you call "we" and who call "they". I think that by "we", you mean any non-sub-Saharan African human but I am not sure. If this is the case just naming sounds more appropriate that to make a big category that you call "we" to exclude some ethnic groups :)

I think that when you are talking about Brazilians (and other people in South America living close to the equator such as Ecuadorians), you are mostly talking about people from Spanish and Portuguese descent. So let's talk about the great migrations

The Indonesians arrived in their lands about 70,000 years ago. The indigenous South-Americans arrived in South America about 10,000 years ago. The Spanish and Portuguese arrived in South America about 500 years ago.

I think you might want to read a bit more about the geography of those big migrations.

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  • $\begingroup$ Your paragraph about time of arrival should be expanded into a "real" answer (and the numbers verified), as this is most likely the exact reason that indigenous South Americans (and North Americans as well, really) are not darker than they are - they didn't arrive that long ago. Here is a good place to start. $\endgroup$ – MattDMo Aug 22 '15 at 1:45
  • $\begingroup$ They have been pushing back the dates of the migrations into the Americas with new archeological findings... They are looking at closer to 15k years now. Also with the work of Svante Paabo and others, it may very well be that the mutations that we associate with Europeans and Asians arose in non Sapien humans and that modern humans inherited those traits from breeding between the groups. $\endgroup$ – AMR Aug 22 '15 at 2:22

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