Multiple geographic isolations of populations in the past, and varying dispersals of these around the planet at different times, experiencing different climates and conditions which have affected evolution of traits differently. Race is a risky term to use though as there's so many different definitions in science. We are all Homo sapiens though, which came from a small population in Africa around 200,000 years ago and colonised the World from there, although there are controversial (and likely untrue) theories that East Asian populations evolved from an ancestor which lived in the region, different from the rest of the World. It's pretty much rubbish though.
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