In our cells, we possess Mitochondrial DNA. This DNA is different from standard DNA as it is passed only through the maternal line. Ergo, no matter who the father might be, if the children have the same mother, their mtDNA will be the same. Of course, over time, with the passage of generations, mutations can occur within the mtDNA to allow some variation. My mtDNA is not the exact same as that of my great-great-great-great grandmother. If you go back far enough, one finds that all humans can trace their mtDNA to a single parent: Mitochondrial Eve.
Of course, we all look different, but it is possible that we all have a common ancestor. This idea can then be combined with that of allopatric speciation in which a population splits and finds itself in different geographic areas. This leads to limited genetic drift as those sub-populations is forced to mate within themselves. (Limited due to the fact that the result in this situation didn't not lead to a new species but to greater variation within the species.) The figure below show possible historic allopatric speciation patterns in humans.
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