David Reich is a geneticist at Harvard whose lab helped pioneer the field of ancient DNA — extracting and sequencing genetic material from human remains thousands of years old. What they found overturned nearly everything scientists thought they knew about where we come from. Before 2010, the story of human migration was mostly guesswork. Since then, ancient DNA has revealed that modern humans interbred with Neanderthals, that a group of steppe nomads reshaped the entire ancestry of Europe, and that in some places the population was replaced so completely that the people living there today share almost no DNA with those who lived there just a few thousand years ago. DAVID REICH:  When you can take DNA from people who lived in the past and compare it to each other, and with people living today, you see things that you just didn’t even imagine before: the steppe migrations, the expansion of Lapita pottery culture, people of East Asian ancestry moving to the Southwest Pacific.…