As you may have heard, the most popular thesis on where the humans came from is the "out of Africa" theory. Wheather this makes any sense or not, can be argued about, as even scientists don't agree on it yet. Fact is, that either as the humans got farther north, or as the human race was "born" in the north, their skin was brighter the farther north they were.
This has a simple reason. Folic acid runs through our blood and is a very important nutritive substance. A lack of folic acid causes a slower production of red blood cells and so there aren't enough of them left. The red blood cells are responsible for transporting oxigen to all parts of the body. Besides during pregnancy folic acid is especially important to make sure the baby will be healthy. This acid is destroyed by sunlight, which means that in a hot country a person with light skin will keep less of their folic acid than a person with darker skin.
Another vitamin is necesary for us, it is the one the sunlight itself gives us, Vitamin D. Too dark skin prevents the vitamin from getting into the body and the lack of vitamin D can cause all sorts of depressions and illnesses.
Thus a person with dark skin can't be perfectly healthy in a country with not much sun. Just as a person with fair skin can't be healthy in a country with a lot of hot sunlight.
The result of this is, that to be healthy every person must stay in their country, where the dosis of sunlight perfectly matches their skincolour. Only that way you can stay healthy, have a healthy family and make sure your people stays healthy, too.
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