What looked like a simple story of migration turned into a record of repeated mixing, vanished populations, and scientific shocks—including the discovery that many people outside Africa carry Neanderthal DNA, and that a little girl’s pinky bone once exposed an entirely unknown human lineage. Harvard geneticist David Reich argues that every new sample forces scientists to rethink the past, and that the biggest revelations about our ancestry may still be ahead. DAVID REICH:  The world is full of secrets about the past and how people are related to each other. And all my presuppositions and all my biases are probably wrong, because every time we get data from somewhere that we didn't have data from before, we turn out to be shown to be wrong. The truth is that the world is a mystery, and that we've carved out areas of clarity. In 2007, it had just become cheap enough to generate genome scale data from lots of people living today.…