End of the rainbow?

How long will it take for humanity to blend into a single race? How many years? Or... Could new "races" develop? I'm thinking at least 400 years for an ethnic mixdown. But what if the "opposite" were to happen? What if war or some natural disaster cut off isolated pockets of people? From that could there emerge "new" races? Keep in mind that despite our vast differences in appearance, genetic markers indicate we all came from one original ancestral family. Changes came with migration over extended periods of time. Comments?

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