Everywhere, back in ancient times clans were geographically isolated so everyone in a clan looked like each other, through inbreeding. If they didn't get along with each other natural selection got rid of them. Eventially other clans would come by and the less somebody looked like you the farther away their hunting grounds were, depending on whether your people said "Yay, immigrants" or "der teykin ayr jerbs" determined how many of your people would be left in twenty years to continue this philosophy. Natural selection didn't prove to promote diversity. This is why people hate immigrants, it is an evolutionary advantage. *this would also suggest that people that participate in incest have a smaller circle that they consider "us" and a bigger "them" circle. Meaning people who marry their siblings are naturally more racist.