A1Everybody loves a good story, and when it’s finished, this may be the greatest one ever told.

A2It begins in Africa with a group of people.

A3There are perhaps just a few hundred, surviving by hunting animals and gathering fruits, vegetables, and nuts.

A4It ends about 200,000 years later, with their seven billion descendants spread across the Earth.

B1In between is an exciting tale of survival, movement, isolation, and conquest, most of it occurring before recorded history.

B2Who were those first modern people in Africa?

B3What routes did they take when they left their home continent to expand into Europe and Asia?

B4When and how did humans reach the Americas?

B5For decades, the only proof was found in a small number of scattered bones and artifacts that our ancestors had left behind.

B6In the past 20 years, however, DNA technologies have allowed scientists to find a record of ancient human migrations in the DNA of living people.

C1“Every drop of human blood contains a history book written in the language of our genes,” says population geneticist¹ Spencer Wells.

C2The human genetic code, or genome, is 99.9 percent identical throughout the world.

C3The bulk of our DNA is the same.

C4However, the remainder is responsible for our individual differences—in eye color or disease risk, for example.

C5On very rare occasions, a small change—called a mutation—can occur.

C6This can then be passed down to all of that person’s descendants.

C7Generations later, finding that same mutation in two people’s DNA indicates that they share the same ancestor.

C8By comparing mutations in many different populations, scientists can trace their ancestral connections.

D1These ancient mutations are easiest to track in two places.

D2One is in DNA that is passed from mother to child (called mitochondrial DNA, or mtDNA).

D3The other is 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).

D4By comparing the mtDNA and Y chromosomes 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.

E1In the mid-1980s, a study compared mtDNA from people around the world.

E2It found that people of African descent had twice as many genetic differences from each other than did others.

E3Because mutations seem to occur at a steady rate over time, scientists concluded that modern humans must have lived in Africa at least twice as long as anywhere else.

E4They now calculate that all living humans maternally descend from a single woman who lived roughly 150,000 years ago in Africa, a “mitochondrial Eve.”

E5If geneticists are right, all of humanity is linked to Eve through an unbroken chain of mothers.

E6This Eve was soon joined by “Y-chromosome Adam,” the possible genetic father of us all, also from Africa.

E7DNA studies have confirmed that all the people on Earth can trace their ancestry to ancient Africans.

F1What seems certain is that at a remarkably recent date—probably between 50,000 and 70,000 years ago—one small group of people, the ancestors of modern humans outside of Africa, left Africa for western Asia.

F2They either migrated around the wider northern end of the Red Sea, or across its narrow southern opening.

G1Once in Asia, genetic evidence suggests, the population split.

G2One group stayed temporarily in the Middle East, while the other began a journey that would last tens of thousands of years.

G3Moving a little farther with each new generation, they followed the coast around the Arabian Peninsula, India, and Southeast Asia, all the way to Australia.

G4“The movement was probably imperceptible,” says Spencer Wells.

G5“It was less of a journey and probably more like walking a little farther down the beach to get away from the crowd.”

H1Archeological evidence of this 13,000-kilometer migration from Africa to Australia has almost completely vanished.

H2However, genetic traces of the group that made the trip do exist.

H3They have been found in the DNA of indigenous² peoples in Malaysia, in Papua New Guinea, and in the DNA of nearly all Australian aborigines.

H4Modern discoveries of 45,000-year-old bodies in Australia, buried at a site called Lake Mungo, provide physical evidence for the theories as well.

I1People in the rest of Asia and Europe share different but equally ancient mtDNA and Y-chromosome mutations.

I2These mutations show that most are descendants of the group that stayed in the Middle East for thousands of years before moving on.

I3Perhaps about 40,000 years ago, modern humans first advanced into Europe.

J1About the same time as modern humans pushed into Europe, some of the same group that had paused in the Middle East spread east into Central Asia.

J2They eventually reached as far as Siberia, the Korean peninsula, and Japan.

J3Here begins one of the last chapters in the human story—the peopling of the Americas.

J4Most scientists believe that today’s Native Americans descend from ancient Asians who crossed from Siberia to Alaska in the last ice age.

J5At that time, low sea levels would have exposed a land bridge between the continents.

J6Perhaps they—only a few hundred people—were traveling along the coast, moving from one piece of land to the next, between a freezing ocean and a wall of ice.

J7“A coastal route would have been the easiest way in,” says Wells.

J8“But it still would have been a hell of a trip.”

J9Once across, they followed the immense herds³ of animals into the mainland.

J10They spread to the tip of South America in as little as a thousand years.

K1Genetic researchers can only tell us the basic outlines of a story of human migration that is more complex than any ever written.

K2Many details of the movements of our ancestors and their countless individual lives can only be imagined.

K3But thanks to genetic researchers—themselves descendants of mtDNA Eve and Y-chromosome Adam—we have begun to unlock important secrets about the origins and movements of our ancient ancestors.

Scientists have long believed that modern humans originated in Africa, because that’s where they’ve found the oldest bones. Geneticists have come to the same conclusion based on analysis of human DNA. However, there is less consensus about the routes our ancestors took. For example, genetic data suggests that Europe might have been settled by an inland migration from India, rather than directly from the Middle East. “I think the broad human prehistoric framework is in place,” says geneticist Peter Forster of the McDonald Institute for Archaeological Research, “and we are now fitting in the details.”