Seems like basic evolution to me.
People who evolved in the tropical zones never had to develop certain technologies to survive--technologies involving shelter, clothing and cultivation that the Europeans and Asians had to develop in order to live through the colder months.
The Africans developed longer, more powerful muscles for hunting and chasing animals, where as others worked and toiled with tools, were forced to master their environment, deal with its more severe cycles. Of course, those in the warmer regions had to develop technologies of their own, but nearly to the extent of those in the harsher latitudes.
Different aspects of the human body were emphasized, as each of these races favored their own approach to survival. One emphasizes the physical while the other emphasizes problem solving. Eventually, as technology progressed, mere survival turned into expansion for those with the technological background... And so while the Europeans and Asians built ships, modern armies and developed sanitation for their giant cities, those from the warmer climates stayed largely the same--call it environmentally-imposed complancency.
Today, when we measure intelligence, we value problem solving. Hundreds, even thousands of generations of evolution can certainly account for whatever deviations there are in the statistics.
