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JUDY WOODRUFF: Now: who we are and how we got here. Jeffrey Brown takes a time traveling look at how modern researchers are using the latest DNA sequencing technology to understand the movements and interactions of very ancient humans. It is the latest in our weekly science series, the Leading Edge.
JEFFREY BROWN: It's a trip into the deep human past in a lab at the Harvard Medical School, disposable Tyvek suits, gloves, headgear, all required to avoid contamination of the ancient bones being studied here.
DAVID REICH, Harvard Medical School: The bones that we're looking at right now are about 5,000- or 6,000-year-old samples from Italy. And we're trying to understand population transformations in Italy over time.
JEFFREY BROWN: David Reich, who heads this lab, is at the forefront of a revolution in DNA studies, now providing new insight into human history as old as 40,000 years.
DAVID REICH: We can open up an ancient skeleton from 10,000 years ago, sequence its genome, have as much information from that ancient individual as we would have from a person living today.
JEFFREY BROWN: And tell a story about them and their movement, and their relations to others?
DAVID REICH: Exactly. And the power of this information is evident from the fact that the stories are always so surprising.
JEFFREY BROWN: Finding our roots is all the rage these days. There's a deep human interest in where we come from. Reich and his colleagues go deeper, much deeper into the past, powered by enormous advances in sequencing technology in the last decade. He's using it to answer very big questions. The title of his new book, "Who We Are and How We Got Here."
DAVID REICH: We're looking at the history of humans and how we got to all the different places we are in the world today, and it's not something that's been possible to look at before this technology. And, really, what the ancient DNA has done, and the ability to look with high resolution at human variation has done, is, it's opened up a whole Pandora's box of archaic humans, and ancient mixtures, that we didn't know about before, but that we already can see some of them.
JEFFREY BROWN: Reich and his team work on bones collected around the world, brought to them by archaeologists and museums.
WOMAN: I think that might be an ossicle.
JEFFREY BROWN: In the so-called clean lab, we watched as an ancient skull fragment was sandblasted to isolate the cochlea, or inner ear. The petrous bone surrounding this area can retain traces of DNA for thousands of years. In another room, we met Harold the robot.
WOMAN: He prepares all of our libraries for us and our DNA so we can sequence it.
JEFFREY BROWN: Reich was part of a group of scientists which confirmed that ancient humans and Neanderthals mixed and mated until some 40,000 years ago, and that some living humans today still carry traces of Neanderthal DNA. A cave in Siberia produced another surprise, a species later dubbed Denisovans.
DAVID REICH: My colleagues obtained DNA from a finger bone from Siberia that they thought was a modern human. But when they sequenced its DNA, it was from a population that was neither Neanderthal nor modern human. So this was an incredible revelation to all of us.
JEFFREY BROWN: That suggests and that's what I wanted to ask you that there are still yet to be found, other kinds of archaic humans?
DAVID REICH: That's right. You know, I think we're alone on the planet now, but 50,000 years ago, it would have been much like the scene in Star Wars, with many, many different humans, all similar to each other and comprehensible to each other in some ways, many of them as big-brained as us, but much more different from each other than people who live today.
JEFFREY BROWN: Perhaps the biggest surprise in the ancient DNA research, though, is in more recent human history. We may think mass migration and mixing of cultures is a modern phenomenon, but it turns out to be the story of our species. We have always moved and always mixed.
DAVID REICH: The idea that human populations today might correspond to age-old separations, tens of thousands of years old, that have existed from time immemorial, has now been profoundly undermined by genetics. What the genetic data shows is that groups that we see today, and that we recognize, in fact are the results of profound mixtures, and that none of these groups are pure in any sense at all.
JEFFREY BROWN: Original populations in most regions of the world have been replaced, sometimes several times over. Europe, for example, saw what Reich calls a collision of three very different populations over the last 9,000 years, the last of them part of a great migration that began far to the east, in what we now call the Russian Steppes. To make this more concrete, Reich points to the iconic site of Stonehenge, which reached its final form around 4,500 years ago, constructed by people who descended by Europe's first farmers.
DAVID REICH: But within 100 years or 200 years, Stonehenge was taken over by these new people who were not the same people genetically.
JEFFREY BROWN: So if you're living in Britain today and you're thinking, my ancestors built Stonehenge, you're wrong?
DAVID REICH: You're basically wrong, or maybe only 10 percent of your ancestors or fewer did. And so I think that this is sort of an example of this point, which is that people today are almost never directly descended from the people who first lived in those places. There's waves and waves of population replacement, and that we're all interconnected.
JEFFREY BROWN: That, of course, blows up concepts of pure races and national identity, ideas of genetics misused by the Nazis and many others into our own time. Differences? Yes. But more connections and mixing than we'd known. It's hardly the end of the story. Reich says the ancient DNA revolution is just beginning. And there's plenty the DNA doesn't tell us, just why people migrated at a given time, for example, what, in fact, where they thinking? New secrets, he says, will continue to be unlocked.
DAVID REICH: It's gonna really profoundly change the way we do archaeology, history, linguistics, sociology, even demography, and even sort of economic history, because we will be able to learn things about, for example, how population sizes have changed over time.
JEFFREY BROWN: It's all there in the ancient bones. For the "PBS NewsHour," I'm Jeffrey Brown at the Harvard Medical School in Cambridge, Massachusetts.
