It is believed that human story begins in Africa and ends about 200,000 years later with their seven billion descendants (后代) spread across the Earth, living in peace or at war, their faces lit by campfires and computer screens.
In between is an exciting tale of survival, movement, isolation, and conquest, most of it occurring before recorded history. Who were those first modern people in Africa? What routes did they take when they left their home continent 60,000 years ago to expand into Europe and Asia? When and how did humans reach the Americas? For decades, the only proof was found in a small number of scattered bones and artifacts that our ancestors left behind on their journeys. In the past 20 years, however, DNA technologies have allowed scientists to find 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. The human genetic code, or genome, is 99.9 percent identical throughout the world. But while the bulk of our DNA is the same, what's left is responsible for our individual differences — in eye color or disease risk, for example. On very rare occasions, a small change, called a mutation, can occur, which is then passed down to all of that person's descendants. Generations later, finding that same mutation in two people's DNA indicates that they share the same ancestor. By comparing mutations in many different populations, scientists can trace their ancestral connections.
These ancient mutations are easiest to track in two places: in DNA that is passed from mother to child (called mitochondrial DNA, or mtDNA for short), and in DNA that travels from father to son (known as the Y chromosome, the part of DNA that determines a child will be a boy). By comparing the mtDNA and Y chromosomes (染色体) of people from various populations, geneticists can get a rough idea of where and when those groups separated in the great migrations around the planet.
