maybe i'm speculating but i feel like human subspecies or races were a lot more distinct and isolated in the past and over time more or less every group has become mixed. totally guessing but it's how it seems when you look at a population with so many different traits, and accross racial "borders" between europe and asia (as an example) you can see clear traces of mixing between more caucasian groups and more mongol groups. and even in europe you have so many different traits that seem mixed together in populations.