Even with our species flourishing, the chances of any one of us being born are still remote. Think of all the slight variations in human migration patterns, for example, that could have kept your great-great-grandparents from ever crossing paths. If you have any European ancestry, someone in your lineage had to survive the black death in the fourteenth century, which killed more than half the people on the continent. If you have any Native American heritage, somehow your forebears managed to pass their genes on to you, despite the fact that only 10 or 20 percent survived the microbes and
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