One can imagine two separate groups who had maintained their genetic distance for a long time suddenly deciding that enough was enough and starting to mix. But it is more difficult to visualize groups that had already been mixing waking up one day and deciding to put a stop to it, and creating barriers to continued intermixing. The genetic study says that this is exactly what happened. It was as if around 100 CE a new ideology, which had gained ground and power, imposed on the society new social restrictions and a new way of life. It was social engineering on a scale never attempted before or
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