Over the millennia, a few steps a year and a few hops by boat added up. The wanderers had reached southeastern Australia by 45,000 years ago, when a man was buried at a site called Lake Mungo. Artifact-bearing soil layers beneath the burial could be as old as 50,000 years—the earliest evidence of modern humans far from Africa. No physical trace of these people has been found along the 8,000 miles from Africa to Australia—all may have vanished as the sea rose after the Ice Age. But a genetic trace endures. A few indigenous groups on the Andaman Islands near Myanmar, in Malaysia, and in Papua New Guinea—as well as almost all Australian Aborigines—carry signs of an ancient mitochondrial lineage, a trail of genetic bread crumbs dropped by the early migrants.
People in the rest of Asia and Europe share different but equally ancient mtDNA and Y-chromosome lineages, marking them as descendants of the other, stalled branch of the African exodus. At first, rough terrain and the Ice Age climate blocked further progress. Europe, moreover, was a stronghold of the Neandertals, descendants of a much earlier migration of pre-modern humans out of Africa.
Finally, perhaps 40,000 years ago, modern humans advanced into the Neandertals’ territory. Overlapping layers of Neandertal and early modern human artifacts at a cave in France suggest that the two kinds of humans could have met.
How these two peoples—the destined parvenu and the doomed caretaker of a continent—would have interacted is a potent mystery. Did they eye each other with wonder or in fear? Did they fight, socialize, or dismiss each other as alien beings?
All we know is that as modern humans and distinctly more sophisticated toolmaking spread into Europe, the once ubiquitous Neandertals were squeezed into ever shrinking pockets of habitation that eventually petered out completely. On current evidence, the two groups interbred rarely, if at all. Neither mtDNA from Neandertal fossils nor modern human DNA bears any trace of an ancient mingling of the bloodlines.
About 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. Following herds of game, skirting mountain ranges and deserts, they reached southern Siberia as early as 40,000 years ago. As populations diverged and became isolated, their genetic lineages likewise branched and rebranched. But the isolation was rarely if ever complete. "People have always met other people, found them attractive, and had children," says molecular anthropologist Theodore Schurr of the University of Pennsylvania.
Schurr’s specialty is the peopling of the Americas—one of the last and most contentious chapters in the human story. The subject seems to attract fantastic theories (Native Americans are the descendants of the ancient Israelites or the lost civilization of Atlantis) as well as ones tinged with a political agenda. The "Caucasoid" features of a 9,500-year-old skull from Washington State called Kennewick Man, for instance, have been hailed as proof that the first Americans came from northern Europe.
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.
Source: National Geography
Edited By: AHU – David A-O