By James Shreeve
Genetic trails left by our ancestors are leading scientists back across time in an epic discovery of human migration.
Get a taste of what awaits you in print in the March 2006 issue of National Geographic's article "The Greatest Journey" from this compelling excerpt:
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.
And, also from the Research Division of National Geographic:
Scientists who trace modern human origins often talk about a special kind of DNA, mitochondrial DNA (mtDNA). But do you know what mitochondrial DNA is? And do you know what mitochondria do? Mitochondria are tiny structures found within the cell that resemble grains of rice. They produce the energy that cells need to function, and they have their own small complement of DNA, containing just 37 genes in humans. Compare this to the human nuclear genome (the entire set of genetic instructions found within the nuclei of all the cells), which contains some 30,000 genes. But mitochondrial DNA is especially powerful for studying the ancestry of modern humans. While the nuclear genome is reshuffled with each new generation as the father's and mother's DNA recombine, mitochondrial DNA is passed directly from a mother to her children. As a result it preserves patterns of ancient markers that help scientists map the path modern humans have taken around the world.
Get the whole story in the pages of National Geographic magazine.