Anthropology tells us that early modern humans slowly populated what is now sub-Saharan Africa and then began to migrate out into the rest of the world. Whether this was out of curiosity, the search for food or population pressure is unknown, but it was probably some combination of all three. The most likely route was along the western coast of the Red Sea and then into the Eastern Mediterranean. From there, humans slowly fanned out across the globe. It’s all speculation, but that’s the logical route out of Africa.
The point here is the first wave of human migration was south to north. Humans appear to have made it as far north as Siberia and Scandinavia before changes in climate forced people back south again. There are some who think that the populations to the north had acquired better cognitive and maybe even physical tools that allowed them to migrate south and displace the populations in the more temperate zones. The changes in climate had the effect of improving the human populations in Asia and Europe.
That may be debatable, but what is not debated is that large scale human migrations change both the invaded and the invader. The people being conquered are either massacred or bred out by the winning tribes. On the other hand, the invading people tend to borrow things from the conquered people, including genetics from the female line. The Irish say the Vikings took the best looking women with them to Iceland, which is why Icelandic women are some of the most beautiful in the world.
A good example of this may be Islam. Historians have assumed Islam grew out of the polytheistic religions of Arabia in the late 6th century. While the details of Mohamed’s life are not clear, it has generally been accepted that he was a real person, who lived around Mecca in the 6th century. It has been assumed that Islam was an Arab response to the growth of Judaism and various Christian sects. In time, Islam spread through the Middle East, as the Arabs conquered the region.
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