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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.

Mitochondrial Eve was soon joined by "Y chromosome Adam," an analogous father of us all, also from Africa. Increasingly refined DNA studies have confirmed this opening chapter of our story over and over: All the variously shaped and shaded people of Earth trace their ancestry to African hunter-gatherers.

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