Many (most?) sharp racial boundaries are not found along major geographic fault lines, but appear because of recent massive migrations which obliterated "intermediate" groups.
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For example, 4,000 years ago there was a smooth cline of Bushmen ancestry stretching from S. Africa to Ethiopia. Bantu and Middle Eastern migrants moved in and today's East Africans are 0% Bushman.pic.twitter.com/XygubetPPT
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Exactly. The cline idea works great when looking at populations East-to-West across the great landmass of Eurasia, or across the subcontinent of India. Not so great within Africa, or across places separated by sea.
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