How did some asian people come to develop their present day physical features?

I'm talking about people in the eastern parts of asia and their distinguishing looks, more so. Like people of China, Korea, Laos, Japan, and those areas. For example: people in north europe having fair skin from being further from the equator for many years. But I don't know how people got really distinguishing features over time, like some asians having more narrow eyes (I'm not meaning any offense). I figured it was some genetic trait that just lasted for a good purpose at the time and created the differences in modern humans. But that's just my guess and I'm not a professional. So, if someone (biologist, evolution specialist, etc) could say something scientific about it and add some detail to my quesswork, or correct me, that would be cool.