Why Did Humans Migrate Out Of Africa?

Transcript

GUY RAZ, HOST:

It's the TED Radio Hour from NPR. I'm Guy Raz. And our show today - how it all began, ideas about our origins. So if you'd been hanging out in, say, the town of Whitehorse, which is in Canada's Yukon Territory, this past week, this is what you'd have heard on the radio.

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UNIDENTIFIED ANNOUNCER: On 96.1 The Rush.

MARJ: Hi, I'm Marj. And this is Marj's Market. Tired of paying rent? There are alternatives.

RAZ: And around the same time, around 6,000 miles to the Southeast, you would've heard this in Peru.

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UNIDENTIFIED MAN #1: (Spanish spoken).

UNIDENTIFIED MAN #2: (Spanish spoken).

UNIDENTIFIED MAN #1: (Spanish spoken).

RAZ: About 8,000 miles Northeast, this was on the air in St. Petersburg, Russia.

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UNIDENTIFIED MAN #3: (Russian spoken).

RAZ: And more than 3,000 miles South, this in Kenya.

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UNIDENTIFIED MAN #4: (Foreign language spoken).

RAZ: So earlier in the show, David Christian explained that it was language that gave us an edge as a species. More than 6,000 languages are spoken around the world, so how did that happen? How did we come to look and sound so different? That's what Spencer Wells has been trying to figure out.

SPENCER WELLS: I mean, I spend my life traveling visiting places like Chad and Tajikistan and Papua New Guinea and Palau.

RAZ: Spencer Wells, you might remember, is the geneticist who analyzed my DNA earlier in the show. He's also an explorer for National Geographic. And he told us that one of the things he really likes to do when he travels is to look at faces.

WELLS: You know, you see people who seem to be so different from each other. And kind of the underlying theme of our work is, well, how different are they really?

RAZ: Turns out, not much. And while Louise Leakey and her family proved that through prehistoric bones and fossils, Spencer Wells looks for the evidence of our common origins in living, breathing human beings. He's using the tools of molecular genetics to figure out when human populations began to migrate from Africa and spread across the globe.

More on that in a minute. But first, let's go back to the beginning. So we are about 200,000 years old, right?

WELLS: Approximately. Based on fossil records, yeah.

RAZ: Like the way the human - the way humans are.

WELLS: Things that we would recognize as being like us if they were sitting here in the studio.

RAZ: 200,000 years ago, where are we living and what do we sort of look like?

WELLS: We were living as a very small group of hunter-gatherers out in savannas of likely Eastern Africa, so present-day Ethiopia, Kenya, Tanzania. And it was in that kind of crucible of East African hunter-gatherer society that all of the modern human characteristics arose.

RAZ: So 200,000 years ago, we're all dark. Our skin is darker, right?

WELLS: Yeah, we're a species of hairless primate. And we evolved in the tropics. And there was no SPF 50 200,000 years ago.

RAZ: Yeah.

WELLS: So we had to have some sort of natural sunscreen, and that was melanin.

RAZ: It's amazing to think about the age of our universe. 200,000 years is nothing. That's like a blimp in time. It's like it's not even a second on the 24-hour clock.

WELLS: Exactly.

RAZ: And yet we have...

WELLS: It's a few thousand human generations.

RAZ: Yeah. So we're talking about a huge change in the way we look in such a short period of time.

WELLS: Absolutely. So why?

RAZ: Yeah, why?

WELLS: That's one of the big unsolved mysteries. We know that there's been adaptation. So we started off dark living in the tropics in Africa. And my ancestors, for instance, who came from northern Europe...

RAZ: You're a very white guy, you know.

WELLS: My buddy, Skip Gates, who does all of the PBS shows on African-American lives and so on, is filming with me once. And he says, Spencer, you know it takes the whitest man in the world to tell us we all came from black Africans.

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RAZ: Spencer Wells picks up the story from the TED stage.

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WELLS: And how recently do we share this ancestry? Was it millions of years ago? - which we might suspect by looking at all this incredible variation around the world. No. The DNA tells a story that's very clear. Within the last 200,000 years, we all share an ancestor - a single person, mitochondrial Eve; you might have heard about her - in Africa. An African woman who gave rise to all the mitochondrial diversity in the world today.

But what's even more amazing is that if you look at the Y-chromosome side, the male side of the story, the Y-chromosome Adam only lived around 60,000 years ago. That's only about 2,000 human generations - the blink of an eye in an evolutionary sense. That tells us we were all still living in Africa at that time. This was an African man who gave rise to all the Y-chromosome diversity around the world. It's only within the last 60,000 years that we have started to generate this incredible diversity we see around the world. Such an amazing story. We're all effectively part of an extended African family.

RAZ: And what happens? When do they - how do they start to move out? And where do they go?

WELLS: Well, so the evidence is that there might've been a little brief foray into the Middle East, in the Arabian Peninsula, as early as 120,000 years ago. But they didn't go very far beyond that. But the big blast came out around 60,000 years ago. So that's 2,000 human generations.

RAZ: So the question of course is what happened? Why didn't humans start to leave Africa earlier than that?

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WELLS: Well, that's a big question - these why questions, particularly in genetics and the study of history. When all else fails, talk about the weather. What was going on to the world's weather around 60,000 years ago? Well, we were going into the worst part of the last ice age. The Northern Hemisphere had massive, growing ice sheets. New York City, Chicago, Seattle - all under a sheet of ice. Most of Britain, all of Scandinavia - covered by ice several kilometers thick. Now Africa is most tropical continent. We weren't covered in ice in Africa, rather Africa was drying out at that time. The reason for that is that ice actually sucks moisture out of the atmosphere. If you think about Antarctica, it's technically a desert. And Africa was turning to desert. The Sahara was much bigger then than it is now. And the human habitat was reduced to just a few small pockets compared to what we have today. The evidence from genetic data is that the human population around this time - roughly 70,000 years ago -crashed to fewer than 2,000 individuals. We nearly went extinct. We were hanging on by our fingernails.

RAZ: We were almost completely wiped out as a species. And if that wasn't bad enough...

WELLS: We had the eruption of a mega-volcano, the largest volcanic eruption in the last 20 to 30 million years - Mount Toba in Sumatra, which today is Lake Toba. And it blew its top, and it spewed all of this ash into the atmosphere and sulfur dioxide into the atmosphere. And this had the effect of creating a global nuclear winter, in effect. And temperatures dropped by 15 or 20 degrees Celsius, on average.

RAZ: Wow. It just blocked the sun.

WELLS: Yeah. It blocked out the sun, basically. And so the animals and the plants become sparser, and so the human population becomes sparser. We can also look at the genetic variation that we see today. And humans have remarkably little genetic variation for a species of large ape. We're highly inbred. And it's because the population size around the time of that volcanic eruption dropped down to maybe as few as 2,000.

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WELLS: But then, 50, 60, 70,000 years ago, somewhere in that region, all hell breaks loose. Art makes its appearance. The stone tools become much more finely crafted. The evidence is that human began to specialize in particular prey species at particular times of the year. The population size started to expand. Probably, according to what many linguists believe, fully modern language - syntactic language, subject, verb, object that we use to convey complex ideas, like I'm doing now - appeared around that time. We became much more social. The social networks expanded. This change in behavior allowed us to survive these worsening conditions in Africa. And they allowed us to start to expand around the world.

RAZ: What do we imagine that that migration was like?

WELLS: Well, I think it was really just a question of people moving a little bit further in search of better food supplies or water supplies. There might have some wanderlust involved.

RAZ: So it wasn't like, hey, let's go, you know, 10,000 miles? It was a very slow migration over thousands and thousands...

WELLS: Well, every long journey starts with a step. And in this case, people had no idea what was out there. I mean, there was nobody tweeting from Siberia saying, hey, guys, come on up here. There's lots of, you know, reindeer up here on the tundra. They didn't know what they were going to encounter. I mean, it's really amazing if you think about. You're setting off on the biggest journey in the history of your species. You have no idea where you're going, but you are smart enough to be able to figure out solutions to all the problems that are going to be thrown in your way.

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WELLS: The reason you're alive today is because of those changes in our brains that took place in Africa around 60, 70,000 years ago, allowing us not only to survive in Africa but to expand out of Africa. An early coastal migration along the south coast of Asia, leaving Africa around 60,000 years ago, reaching Australia very rapidly by 50,000 years ago. Slightly later migration up into the Middle East. These would have been savanna hunters. Entering Europe around 35,000 years ago. And finally, a small group migrating up through the worst weather imaginable - Siberia, inside the Arctic Circle during the last Ice Age - temperatures of -70, -80, even -100, perhaps - migrating into the Americas, ultimately reaching that final frontier. An amazing story. And it happened first in Africa. The changes that allowed us to do that. The evolution of this highly adaptable brain that we all carry around with us, allowing us to create novel cultures. Allowing us to develop the diversity that we see on a whirlwind trip like the one I've just been on. Thank you very much.

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RAZ: Spencer Wells is a geneticist and the director of The Genographic Project at National Geographic. You can check out his entire TED Talk at ted.npr.org. Transcript provided by NPR, Copyright NPR.