Your ancestors aren't who you think they are | David Reich: Full Interview

1:07 – 10:16

The technology that turned speculation into evidence

A pinky bone from a little girl in Siberia changed the map of humanity. Reich describes how, before 2010, ancient DNA was basically unusable for answering big historical questions. Researchers had bones, tools, burial sites, and theories, but not the kind of data that could directly compare a person who lived 50,000 years ago with people alive now. The default story was tidy: humans left Africa 50,000 to 70,000 years ago, spread outward, and mostly stayed put. Then the data arrived and the tidy story broke immediately. Reich had entered the field through medical genetics and work on recent mixtures in African American and Latino populations, but being invited onto the Neanderthal genome project changed everything. He calls it "the best data in the world." The team expected little or no mixing between modern humans and archaic humans. In fact, that had been close to the orthodoxy. But when they compared genomes, the Neanderthal sequences from Europe were clearly closer to present-day non-Africans than to Africans. They tried hard to kill the result. Reich says they kept trying to make it "go away," testing alternative explanations from multiple angles. It only got stronger. The conclusion was unavoidable: as modern humans moved out of Africa, they mixed with Neanderthals, and about 2% of the DNA of non-Africans today comes from those archaic humans. That was only the opening shock. In the same year, a genome from that Siberian pinky bone revealed a previously unknown human group, now called Denisovans. They were neither Neanderthals nor modern humans, and they too had mixed with us. Reich gives the concrete number that people in New Guinea derive maybe 4% of their DNA from Denisovans. This is his larger point about ancient DNA as a scientific instrument. It is like the telescope or microscope: not just a better version of older methods, but a device that lets you see things no one had imagined. Malawi 3,000 years ago, Cameroon, Libya 7,000 years ago, the Southwest Pacific, each new site keeps producing populations unlike the ones living there now. The world, in Reich's phrase, is still "a mystery," with only islands of clarity carved out of it.