In fact most scientists agree that today's Native Americans descend from ancient Asians who crossed from Siberia to Alaska in the last ice age, when low sea level would have exposed a land bridge between the continents. But there's plenty of debate about when they came and where they originated in Asia.
For decades the first Americans were thought to have arrived around 13,000 years ago as the Ice Age eased, opening a path through the ice covering Canada. But a few archaeologists claimed to have evidence for an earlier arrival, and two early sites withstood repeated criticism: the Meadow-croft Shelter in Pennsylvania, now believed to be about 16,000 years old, and Monte Verde in southern Chile, more than 14,000 years old.
The DNA of living Native Americans can help settle some of the disputes. Most carry markers that link them unequivocally to Asia. The same markers cluster in people who today inhabit the Altay region of southern Siberia, suggesting it was the starting point for a journey across the land bridge. So far, the genetic evidence doesn't show whether North and South America were populated in a single, early migration or two or three distinct waves, and it suggests only a rough range of dates, between 20,000 and 15,000 years ago. Even the youngest of those dates is older than the opening of an inland route through the Canadian ice. So how did the first Americans get here? They probably traveled along the coast: perhaps a few hundred people hopping from one pocket of land and sustenance to the next, between a frigid ocean and a looming wall of ice. "A coastal route would have been the easiest way in," says Wells. "But it still would have been a hell of a trip." Beyond the glaciers lay immense herds of bison, mammoths, and other animals on a continent innocent of other intelligent predators. Pushed by population growth or pulled by the lure of game, people spread to the tip of South America in as little as a thousand years.
The genes of today's Native Americans are helping to bring their ancestors' saga to life. But much of the story can only be imagined, says Jody Hey, a population geneticist at Rutgers University. "You can't tell it with the richness of what must have happened." With the settling of the Americas, modern humans had conquered most of the planet. When European explorers set sail 700 years ago, the lands they "discovered" were already full of people. The encounters were often wary or violent, but they were the reunions of a close-knit family.
Perhaps the most wonderful of the stories hidden in our genes is that, when unraveled, the tangled knot of our global genetic diversity today leads us all back to a recent yesterday, together in Africa.