Small group? It went down to estimated 1500 worldwide. That is plenty. Note that last common ancestor of the most differing human ethnics is something like 18

Small group? It went down to estimated 1500 worldwide. That is plenty. Note that last common ancestor of the most differing human ethnics is something like 180-200k years ago. Furthermore, with the advent of denser living and agriculture, evolution sped up quite a lot.