But the degree of isolation needed to keep biological features distinct genetically vanished when the first human crossed a river or climbed a hill. Contemporary geographers reject the entire biological basis of classifying humans into a handful of races, because their features are not rooted in specific places. Geographers stress the heterogeneity of the human population, and an examination of biological differences among people does not explain why people live as they do. The distribution of persons of color matters to geographers because it is the most fundamental basis by which people in many societies sort out where they live, go to school, recreate, and perform many other activities of daily life. The indigenous people here have dark skins but they are not genetically closely related.
Melanin is the pigment that causes dark skin.
Americans arrived from 30,000 to 13,000 years ago and that's recent compared to the length of time humans have been in tropical Africa and Asia. They've been in Australia for about 50,000 years.