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?