...it might be like incest on a grander scale....as different peoples are isolated and cut off from each other, they only do the do with their own kind...and being seperated by centuries or millenia it results in two peoples with radically different complexions/facial features etc.
...or like a few other people said it could be simple adaptation to an environment....for a different race to begin you have to a bunch of interacial screwing in a society until that society becomes racially homogenous....like making a stew.
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