• Drought in Africa reduced the world population into small, isolated groups, study says
  • Separate study says number of humans may have fallen to 2,000
  • Analysis: Humans banded together again in Stone Age, increased in numbers

Migrations out of Africa appear to have begun about 60,000 years ago

WASHINGTON (AP) — Human beings may have had a brush with extinction 70,000 years ago, an extensive genetic study suggests.

-1.jpg
Spencer Wells
Geneticist Spencer Wells, says the study tells “truly an epic drama.”

The human population at that time was reduced to small isolated groups in Africa, apparently because of drought, according to an analysis released Thursday.

The report notes that a separate study by researchers at Stanford University estimated that the number of early humans may have shrunk as low as 2,000 before numbers began to expand again in the early Stone Age.

“This study illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species’ history,” said Spencer Wells, National Geographic Society explorer in residence.

“Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world. Truly an epic drama, written in our DNA.”

Wells is director of the Genographic Project, launched in 2005 to study anthropology using genetics. The report was published in the American Journal of Human Genetics.

Studies using mitochondrial DNA, which is passed down through mothers, have traced modern humans to a single “mitochondrial Eve,” who lived in Africa about 200,000 years ago.

Don’t Miss

Genographic Project

The migrations of humans out of Africa to populate the rest of the world appear to have begun about 60,000 years ago, but little has been known about humans between Eve and that dispersal.

The new study looks at the mitochondrial DNA of the Khoi and San people in South Africa, who appear to have diverged from other people between 90,000 and 150,000 years ago.

The researchers led by Doron Behar of Rambam Medical Center in Haifa, Israel, and Saharon Rosset of IBM T.J. Watson Research Center in Yorktown Heights, New York, and Tel Aviv University concluded that humans separated into small populations before the Stone Age, when they came back together and began to increase in numbers and spread to other areas.

Eastern Africa experienced a series of severe droughts between 135,000 and 90,000 years ago, and researchers said this climatological shift may have contributed to the population changes, dividing into small, isolated groups that developed independently.

Paleontologist Meave Leakey, a Genographic adviser, asked, “Who would have thought that as recently as 70,000 years ago, extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction?”

Today, more than 6.6 billion people inhabit the globe, according to the U.S. Census Bureau.

The research was funded by the National Geographic Society, IBM, the Waitt Family Foundation, the Seaver Family Foundation, Family Tree DNA and Arizona Research Labs.

Explorer-in-Residence/Emerging Explorer

-1.jpg
Photograph by Becky Hale

Current Projects: The Genographic Project

Spencer Wells is a leading population geneticist and director of the Genographic Project from National Geographic and IBM. His fascination with the past has led the scientist, author, and documentary filmmaker to the farthest reaches of the globe in search of human populations who hold the history of humankind in their DNA. By studying humankind’s family tree he hopes to close the gaps in our knowledge of human migration.

A National Geographic Explorer-in-Residence, Wells is spearheading the Genographic Project, calling it “a dream come true.” His hope is that the project, which builds on Wells’s earlier work (featured in his book and television program, The Journey of Man) and is being conducted in collaboration with other scientists around the world, will capture an invaluable genetic snapshot of humanity before modern-day influences erase it forever.

Wells’s own journey of discovery began as a child whose zeal for history and biology led him to the University of Texas, where he enrolled at age 16, majored in biology, and graduated Phi Beta Kappa three years later. He then pursued his Ph.D. at Harvard University under the tutelage of distinguished evolutionary geneticist Richard Lewontin. Beginning in 1994, Wells conducted postdoctoral training at Stanford University’s School of Medicine with famed geneticist Luca Cavalli-Sforza, considered the “father of anthropological genetics.” It was there that Wells became committed to studying genetic diversity in indigenous populations and unraveling age-old mysteries about early human migration.

Wells’s field studies began in earnest in 1996 with his survey of Central Asia. In 1998 Wells and his colleagues expanded their study to include some 25,000 miles (40,000 kilometers) of Asia and the former Soviet republics. His landmark research findings led to advances in the understanding of the male Y chromosome and its ability to trace ancestral human migration. Wells then returned to academia where, at Oxford University, he served as director of the Population Genetics Research Group of the Wellcome Trust Centre for Human Genetics at Oxford.

Following a stint as head of research for a Massachusetts-based biotechnology company, Wells made the decision in 2001 to focus on communicating scientific discovery through books and documentary films. From that was born The Journey of Man: A Genetic Odyssey, an award-winning book and documentary that aired on PBS in the U.S. and National Geographic Channel internationally. Written and presented by Wells, the film chronicled his globe-circling, DNA-gathering expeditions in 2001-02 and laid the groundwork for the Genographic Project.

Since the Genographic Project began, Wells’s work has taken him to over three dozen countries, including Chad, Tajikistan, Morocco, Papua New Guinea, and French Polynesia, and he recently published his second book, Deep Ancestry: Inside the Genographic Project. He lives with his wife, a documentary filmmaker, in Washington, D.C.

Spencer Wells is risking life and limb to collect DNA from the most isolated, remote peoples on the planet. Five years, 100,000 samples, and 40 million dollars later, he’ll have a new road map to human history.
By Michael Shnayerson

-1.jpg
WALKING THROUGH TIME: Spencer Wells, accompanied by a party of Bushmen, treks across a dry watering hole in northern Namibia.

Spencer Wells knows exactly where he wants to go next: the Tibesti mountains. He wants to fly to Libya, now that it’s open to Westerners again, then hail a camel caravan across the Libyan desert to Chad, where the seven inactive volcanoes of the Tibesti rise 11,000 feet (3,353 meters) from the central Sahara: a private world of crags and chasms seldom seen by more than a handful of outsiders. Like any intrepid traveler, he’s unfazed by the prospect of deadly North African windstorms and burning desert heat. The land mines near the border of Libya and Chad do pose a problem, but local guides can thread a path. As for the fierce and isolated Tubu, who’ve ruled the Tibesti long enough for Herodotus to have named them the Troglodytes, they’re the real payoff. Wells wants to learn their oral history, how their ancestors exacted tribute from traders passing to and from the Middle East, and what this crossroads reveals about one of Earth’s earliest cultures on the continent where all human life began. Then he wants to stick a cheek swab into each of their mouths to collect a generous gob of DNA-rich saliva.

Wells, 36, is a population geneticist using science in global pursuit of the greatest story not yet told: the story of how humankind traveled from its origins in Africa to populate the planet. The most telling clues lie with isolated, indigenous tribes like the Tubu, for their DNA remains, in a sense, the purest. Their unique genetic markers, characteristic mutations in a defined sequence of DNA, are like flags waving from the place their ancestors have inhabited for thousands of years—the starting point for ancient migrations. Any venturesome Tubu who crossed the Sahara to see the outlying world, and propagated in the process, passed on one or another of those genetic markers to his or her offspring. Any traveler who came through the Tibesti and intermarried did the same. Wells might take a cheek swab from an investment banker in Boston and find that same genetic marker: proof that one of those Tubu created a family line that leads, in some circuitous way, over continents and generations, from the Tibesti to an oak-paneled office in Back Bay. It’s in the hope of tracing myriad journeys such as this that Wells, a newly named National Geographic Explorer-in-Residence, is undertaking one of the most ambitious and expensive research adventures in the National Geographic Society’s 117-year history: the grandly named Genographic Project.

-2.jpg

At a cost of 40 million dollars over five years, the brunt of it borne by National Geographic, IBM, and the Waitt Family Foundation, the Genographic Project under Wells’s direction is establishing 11 DNA-sampling centers around the world, with the goal of collecting 100,000 cheek swabs or blood samples from mostly indigenous peoples like the Tubu. A sense of urgency infuses the project: Year by year, at an ever quickening rate, the outside world is crowding in on, and at the same time absorbing, indigenous peoples. A Tubu who moves to Paris will still have the genetic markers that distinguish him as a Tubu, but the geographical context for his markers will be gone. As for the Tubu who remain in the Tibesti mountains, they may marry more with outsiders as modern technology makes contact more likely. Generation by generation, tracing the last routes of historical migration of such isolated people grows that much harder. Wells wants to map as many routes as he can while their geographical origins are relatively intact.

Wells has a nifty and novel idea to help fund and publicize the project. “We’re taking this directly to the people,” he declares. “Because in addition to doing this work with indigenous populations, we’re going to be offering for sale to the public, in the developed world mostly, the opportunity to do this cheek-swab test to see how they fit into the family tree.” For about a hundred dollars, a contributor gets his or her very own cheek-swab kit along with a map of migratory routes, as Wells has charted them thus far, that is like an explorer’s parchment map of the New World. “Because these participation kits are totally anonymous, there’s no way anyone can find out anything about your history except you,” Wells says. “Once the results are ready, you can access the Web site (www.nationalgeographic.com/genographic

) for extensive details about genetics, archaeology, history, and the context for genetic variation. Your sample, if you choose, can be put into our database, so that it adds to this increasing data set about genetic variation all over the world. But when you purchase your cheek-swab kit, you’re also funding research, and part of the money will be channeled back to taking samples from the 100,000 people.”

For a man just weeks from the public announcement of this global gambit, Wells is forgivably a bit tense on an early spring day in his fourth-floor office at National Geographic’s Washington, D.C., headquarters. Ruddy and fit—a whole lot more fit than your typical laboratory-bound scientist—he radiates a steely cool, like a field marshal on the eve of battle. The inevitable layman’s queries about genetics elicit crisp details about mitochondrial DNA and Y chromosomes in gleaming, perfectly formed paragraphs. Most of us talk in analog; Wells is digital. Only a framed picture on the shelf suggests that not all goes according to plan. Wells’s two young daughters are with their mother in Geneva; a divorce is under way. For ten years of fieldwork around the world, Wells is paying a human toll.

To Wells, the Genographic Project is a perfect double helix of history and science, the origins of which trace, for him, to a university science lab in Lubbock, Texas, which became a second home when he was all of nine years old. That was when Wells’s mother, a professor at Texas Tech University’s medical school, took time off to earn a Ph.D. in biology and let her son hang out while she performed her experiments. The same year, 1979, Wells was also strongly influenced by English polymath James Burke’s ten-part television series Connections, with its anecdotal braiding of science and history. After earning a B.S. in biology at 19, Phi Beta Kappa, from the University of Texas at Austin, Wells took his own Ph.D. at Harvard University in evolutionary genetics—the historical side of the science, as he says—and studied fruit flies with world-renowned population geneticist Richard Lewontin.

Fruit flies had served as an ideal test case since the early 20th century, when Thomas Hunt Morgan used them in his Fly Room at Columbia University to show how chromosomes worked, to prove that chromosomes were made up of genes, and to show how genes were passed down. But at the end of the day, Wells says, “I’m not that interested in fruit fly history. But I am interested in human population history. I wanted to apply some of those methods to human history.”

“What I appreciate about Spencer is that he is not the kind of scientist who is only interested in his favorite molecule or DNA mutation,” says fellow geneticist and Genographic Project coordinator Lluis Quintana-Murci of the Pasteur Institute in Paris. “Many scientists tend to be closed off in their little rooms. He always had much broader interests—human biology and history. And he uses genetics as a tool to unravel the past.”

For that, Wells went to Stanford University to work with the “grand old man” of human population genetics, Luca Cavalli-Sforza, who is now a chairman of the Genographic Advisory Board. He wanted to learn what the bold new field of genome sequencing—identifying every gene in a living thing and mapping its relation to every other gene—could do for tracing human history.

For years, scientists had studied blood for genealogical clues. Blood characteristics suggested the different cultures of a family tree. But blood was little help for telling when a Middle Eastern people, say, might have migrated to western Europe and intermarried. What the scientists needed, Wells says, was a clock.

That clock came in the form of DNA. The seemingly endless ribbons of DNA found in human hair, saliva, blood—any cell in the body—are made up of three billion individual units, known as A, C, G, and T. Subtle variations in that sequence are what genetically distinguish one person from another. As incredibly able as the human system is at replicating each unique sequence from one generation to the next, a small number of variations, or mutations, do occasionally crop up. By analyzing specific regions of the DNA, comparing the results to known reference sequences, and identifying differences that are anthropologically significant, geneticists are able to track mutations.

“They’re like spelling mistakes,” Wells says. “Imagine you’re copying a very long document, and occasionally you’ll put an A where there should be a C. And that mistake has been translated down through the generations, and more mistakes have accumulated. So the longer the lineage has been in existence, the more mistakes the sequence is going to have. And if you know the rate at which those mistakes occur, you can actually estimate how long this individual has been evolving since that origin, how long his DNA has been accumulating changes.”

Initially Cavalli-Sforza’s team focused on something called mitochondrial DNA—a good choice because it appears frequently in the cell, so it’s easy to find and track. “It’s effectively the remnant of a bacteria that became engulfed by the cell about a billion years ago,” Wells explains. “And all higher organisms have these structures in their cells.” As Cavalli-Sforza’s group knew, mitochondrial DNA is inherited strictly maternally. A mother passes it to her son, but the son can’t pass it on. Only from mother to daughter does it keep descending, generation to generation. In most African tribes, women did the traveling, mostly to find mates, while the men stayed put. So the story that mitochondrial DNA told was only half of the story. Cavalli-Sforza’s geneticists needed a piece of male DNA they could study to prove their theory of African origins and migration. That was when the team’s researchers began studying variations in the Y chromosome, which is passed down from fathers to sons and had, up until then, been a maddeningly inscrutable bit of DNA.

By studying how mutations had accumulated in both mitochondrial DNA and Y chromosomes and determining the rate at which those mutations occur—like counting tree rings—the geneticists made a dramatic conclusion: The populations with the greatest number of mutations were in sub-Saharan Africa. They had the oldest living lineages, which meant they were, beyond the shadow of a genealogical doubt, directly related to the earliest of our traceable ancestors. Their DNA marked the spot where humankind began.

Archaeology had suggested this, of course. But what the geneticists saw from their DNA sequencing of current-day Africans is that their ancestors appeared to have lived as the only humans on Earth as recently as 60,000 years ago. That was when they started migrating, taking their genetic mutations with them, and passing them down. Why did they leave when they did? Because 10,000 years before that, one of the Pleistocene epoch’s worst cold snaps nearly drove humankind to extinction, and these were the survivors, whose better brains made them more adaptable.

If Steven Spielberg were making the movie, a fur-garbed posse of these plucky migrants—the first adventure travelers!—would set off together at dawn across the savanna, covering miles each day in search of fresh prey. In fact, the “Great Leap Forward,” as anthropologist Jared Diamond puts it (in a facetious borrow from Mao Zedong), probably occurred less than 10,000 years ago. The intellectual leap, Diamond and Wells contend, came first: A few children born with higher intelligence and better communication skills, capable of fashioning better tools, passed their superior genes to offspring who came to dominate their clans.

Smarter humans learned to hunt certain species. As those species migrated a few miles every year or so, or died out on the clan’s home turf, the hunters pursued them. Drought accelerated these trends by causing animals to disperse more widely. Eventually, a few migrants reached the Indian Ocean and adopted a fishing life. Little by little, they migrated north up Africa’s east coast. Some traversed the Red Sea—perhaps on simple log rafts. Others headed farther north to the Mediterranean. By 45,000 years ago, as garbage dumps from the time attest, hunters with relatively sophisticated tools were ensconced on the Mediterranean’s shores, engaged in art and other shows of complex culture, and curious about the unknown lands that lay before them in every direction.

Wells wanted DNA samples to fill in this tantalizingly sketchy picture of humankind’s origins. Where had the first of those migrating Africans gone? What routes had they taken, in just tens of thousands of years, to populate the rest of the world?

-1.jpg
Following different routes out of Africa, successive waves of early humans migrated into new territories, eventually populating the entire globe save Antarctica. This map shows this complex web of migrations in their broadest strokes. Maps by Joyce Pendola.

Somewhere between 80,000 and 50,000 years ago, Africa saved Homo sapiens from extinction. Charting the DNA shared by more than six billion people, a population geneticist—and director of the Genographic Project—suggests what humanity “owes” its first home.

by Spencer Wells

For more about the Genographic Project, visit nationalgeographic.com.

-2.jpgGuest editor Bono as a toddler, circa 1961, with maps showing the migrations of his matrilineal (top) and patrilineal ancestors (middle), based on analysis of his DNA. His father’s ancestors were among the first modern humans to enter Europe. Courtesy of the Hewson family.

Do you think you know who you are? Maybe Irish, Italian, Jewish, Chinese, or one of the dozens of other hyphenated Americans that make up the United States melting pot? Think deeper—beyond the past few hundred years. Back beyond genealogy, where everyone loses track of his or her ancestry—back in that dark, mysterious realm we call prehistory. What if I told you every single person in America—every single person on earth—is African? With a small scrape of cells from the inside of anyone’s cheek, the science of genetics can even prove it.

Here’s how it works. The human genome, the blueprint that describes how to make another version of you, is huge. It’s composed of billions of sub-units called nucleotides, repeated in a long, linear code that contains all of your biological information. Skin color, hair type, the way you metabolize milk: it’s all in there. You got your DNA from your parents, who got it from theirs, and so on, for millions of generations to the very beginning of life on earth. If you go far enough back, your genome connects you with bacteria, butterflies, and barracuda—the great chain of being linked together through DNA.

What about humanity, though? What about creatures you would recognize as being like you if they were peering over your shoulder right now? It turns out that every person alive today can trace his or her ancestry back to Africa. Everyone’s DNA tells a story of a journey from an African homeland to wherever you live. You may be from Cambodia or County Cork, but you are carrying a map inside your genome that describes the wanderings of your ancestors as they moved from the savannas of Africa to wherever your family came from most recently. This is thanks to genetic markers—tiny changes that arise rarely and spontaneously as our DNA is copied and passed down through the generations—which serve to unite people on ever older branches of the human family tree. If you share a marker with someone, you share an ancestor with him or her at some point in the past: the person whose DNA first had the marker that defines your shared lineage. These markers can be traced to relatively specific times and places as humans moved across the globe. The farther back in time and the closer to Africa we get, the more markers we all share.

What set these migrations in motion? Climate change—today’s big threat—seems to have had a long history of tormenting our species. Around 70,000 years ago it was getting very nippy in the northern part of the globe, with ice sheets bearing down on Seattle and New York; this was the last Ice Age. At that time, though, our species, Homo sapiens, was still limited to Africa; we were very much homebodies. But the encroaching Ice Age, perhaps coupled with the eruption of a super-volcano named Toba, in Sumatra, dried out the tropics and nearly decimated the early human population. While Homo sapiens can be traced to around 200,000 years ago in the fossil record, it is remarkably difficult to find an archaeological record of our species between 80,000 and 50,000 years ago, and genetic data suggest that the population eventually dwindled to as few as 2,000 individuals. Yes, 2,000—fewer than fit into many symphony halls. We were on the brink of extinction.

And then something happened. It began slowly, with only a few hints of the explosion to come: The first stirrings were art—tangible evidence of advanced, abstract thought—and a significant improvement in the types of tools humans made. Then, around 50,000 years ago, all hell broke loose. The human population began to expand, first in Africa, then leaving the homeland to spread into Eurasia. Within a couple of thousand years we had reached Australia, walking along the coast of South Asia. A slightly later wave of expansion into the Middle East, around 45,000 years ago, was aided by a brief damp period in the Sahara. Within 15,000 years of the exodus from Africa our species had entered Europe, defeating the Neanderthals in the process. (Neanderthals are distant cousins, not ancestors; our evolutionary lineages have been separate for more than 500,000 years.) We had also populated Asia, learning to live in frigid temperatures not unlike those on the Moon, and around 15,000 years ago we walked across a short-lived, icy land bridge to enter the Americas—the first hominids ever to set foot on the continents of the Western Hemisphere. Along the way we kept adapting to new climates, in some cases lost our dark tropical skin pigmentation, developed different languages, and generated the complex tapestry of human diversity we see around the world today, from Africa to Iceland to Tierra del Fuego. But the thing that set it all in motion, the thing that saved us from extinction, happened first in Africa. Some anthropologists call it the Great Leap Forward, and it marked the true origin of our species—the time when we started to behave like humans.

Africa gave us the tool we needed, in the form of a powerful, abstract mind, to take on the world (and eventually to decode the markers in our DNA that make it possible to track our amazing journeys). Perhaps just a few small genetic mutations that appeared around 50,000 years ago gave humans the amazing minds we use to make sense of the confusing and challenging world around us. Using our incredible capacity to put abstract musing into practice, we have managed to populate every continent on earth, in the process increasing the size of our population from a paltry few thousand to more than six billion. Now, 50 millennia after that first spark, times have changed. A huge number of things have contributed to Africa’s relative decline on the world stage, perhaps most important geography. As Jared Diamond describes in his masterly book Guns, Germs, and Steel, Eurasia, with its East-West axis, allowed the rapid latitudinal diffusion of ideas and tools that would give its populations a huge advantage after the initial leap out of Africa. Couple that with the results of colonial exploitation over the past five centuries, and Africa, despite many strengths and resources, is once again in need, as it was 70,000 years ago. This time, though, things are different.

The world population that was spawned in Africa now has the power to save it. We are all alive today because of what happened to a small group of hungry Africans around 50,000 years ago. As their good sons and daughters, those of us who left, whether long ago or more recently, surely have a moral imperative to use our gifts to support our cousins who stayed. It’s the least we can do for the continent that saved us all thousands of years ago.

For more about the Genographic Project, visit nationalgeographic.com.

Dr. Spencer Wells is explorer-in-residence at the National Geographic Society and the director of the Genographic Project.