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Human Journey
MARCH 2006
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By James Shreeve
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Genetic trails left by our ancestors are leading scientists back across time in an epic discovery of human migration.
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Everybody loves a good story, and when it's finished, this will be the greatest one ever told. It begins in Africa with a group of hunter-gatherers, perhaps just a few hundred strong. It ends some 200,000 years later with their six and a half billion descendants spread across the Earth, living in peace or at war, believing in a thousand different deities or none at all, their faces aglow in the light of campfires and computer screens.
In between is a sprawling saga of survival, movement, isolation, and conquest, most of it unfolding in the silence of prehistory. Who were those first modern people in Africa? What compelled a band of their descendants to leave their home continent as little as 50,000 years ago and expand into Eurasia? What routes did they take? Did they interbreed with earlier members of the human family along the way? When and how did humans first reach the Americas?
In sum: Where do we all come from? How did we get to where we are today? For decades the only clues were the sparsely scattered bones and artifacts our ancestors left behind on their journeys. In the past 20 years, however, scientists have found a record of ancient human migrations in the DNA of living people. "Every drop of human blood contains a history book written in the language of our genes," says population geneticist Spencer Wells, a National Geographic explorer-in-residence.
The human genetic code, or genome, is 99.9 percent identical throughout the world. What's left is the DNA responsible for our individual differences—in eye color or disease risk, for example—as well as some that serves no apparent function at all. Once in an evolutionary blue moon, a random, harmless mutation can occur in one of these functionless stretches, which is then passed down to all of that person's descendants. Generations later, finding that same mutation, or marker, in two people's DNA indicates that they share the same ancestor. By comparing markers in many different populations, scientists can trace their ancestral connections.
In most of the genome, these minute changes are obscured by the genetic reshuffling that takes place each time a mother and father's DNA combine to make a child. Luckily a couple of regions preserve the telltale variations. One, called mitochondrial DNA (mtDNA), is passed down intact from mother to child. Similarly, most of the Y chromosome, which determines maleness, travels intact from father to son.
The accumulated mutations in your mtDNA and (for males) your Y chromosome are only two threads in a vast tapestry of people who have contributed to your genome. But by comparing the mtDNA and Y chromosomes of people from various populations, geneticists can get a rough idea of where and when those groups parted ways in the great migrations around the planet.
In the mid-1980s the late Allan Wilson and colleagues at the University of California, Berkeley, used mtDNA to pinpoint humanity's ancestral home. They compared mtDNA from women around the world and found that women of African descent showed twice as much diversity as their sisters. Since the telltale mutations seem to occur at a steady rate, modern humans must have lived in Africa twice as long as anywhere else. Scientists now calculate that all living humans are related to a single woman who lived roughly 150,000 years ago in Africa, a "mitochondrial Eve." She was not the only woman alive at the time, but if geneticists are right, all of humanity is linked to Eve through an unbroken chain of mothers.
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