The notion of a pure race is one thing and imagining isolated populations another. Human populations must have been somewhat isolated for us to have developed distinct physical characteristics.
My known heritage is Italian, German, Irish and French so I have no particular racialist ax to grind.
All my grandparents came from Sweden but I don’t feel all that pure ggg.
Even the English are "mutts": Celts, Angles, Saxons, Jutes, Normans, Romans, etc.