What reasons led to the emergence of different human races within the Homo sapiens species?

What reasons led to the emergence of different human races within the Homo sapiens species? What proves the unity of the origin of the human races?

In the process of migration representatives of the species Homo sapiens occupied new continents. People faced new environmental conditions and adapted to them, gradually changing their physical appearance, this is a natural process of natural selection inherent in any kind of living creatures. The unity of origin of races is proved by the same number of chromosomes, full-fledged offspring from representatives of different races, and anthropological findings.

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