Living in the Past

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As scientists clinked champagne glasses earlier this year over the formal publication of the human genome, there was a lot of speculation about how much this rough index to the secrets of human life would affect humankind's future, opening up new vistas for medical therapeutics even as it poses wrenching ethical dilemmas. But in order to look forward, we have to look back. The human genetic code is foremost a record of our quirky and adventurous past, scarred and spurred by accidental developments. All the evidence of our humble evolutionary beginnings is still there. Much of our genome is disturbingly reminiscent of that of the fruit fly, and 99% of it all is shared with chimpanzees.

Somewhere amid what it says about our ancient past and possible future, our DNA conveys information about our more recent past — and perhaps the means to extend history into the realm of prehistory. And so it is that in the last decade, and at an increasing pace over the past few years, genetics has rejuvenated and somewhat confused the far older and dustier field of prehistoric archaeology. What had been a largely humanistic quest for first principles has been bolstered by complex statistical analyses of genetic evidence, allowing new voices to emerge from the long silence that makes up almost all of the history of Homo sapiens sapiens. In Europe alone, which manifests the least human genetic variation of any continent, genetic research is changing long-held notions of ethnic identity and origin. Among the most startling findings: some 80% of the gene pool of modern Europeans stems from ancestors who came to the Continent more than 11,000 years ago. The vast majority of Europeans, be they Italians or Swedes, whether they pride themselves on their aristocratic or their peasant origins, can trace their ancestry to just seven female lineages and as few as 10 male ones. Most of them came to Europe as Paleolithic hunter-gatherers, and far from being wiped out by the superior technologies later brought from the Middle East by Neolithic farmers, they might well have lived side by side with the newcomers for millennia.

Alongside the classical archaeologists unburying, dating and correlating physical artifacts, a growing number of molecular anthropologists — or archaeogeneticists — are working to reconstruct the genetic heritage of modern Europeans. The data from the digs and the labs don't always sit easily with one another, and when increasingly bold ideas about linguistic ties are thrown into the mix, the discussion can get heated indeed. The tension among those disciplines is a creative one, though. "We have all different kinds of data, and a synthesis has just begun to emerge," says Colin Renfrew, director of the McDonald Institute for Archaeological Research at the University of Cambridge, whose own work has been spurring on that synthesis for decades. "But there's no doubt that in the end there was one past, where events happened in a place and at a time."

For the last few years, some of the most revealing remnants of that single past have been found not on archaeological sites, but in modern genes. Last year two separate groups of scholars presented evidence that modern Europeans are mostly descendants of people who came to the Continent in the Upper Paleolithic era, more than 11,000 years ago.

A group of researchers led by Ornella Semino of the University of Pavia and including Stanford University's Luca Cavalli-Sforza, one of the pioneers of genetic anthropology, analyzed data from the Y chromosomes of 1,007 men throughout Europe and the Middle East. Much of the gender-determining Y chromosome is passed on from father to son without recombining, but it is earmarked by certain telltale mutations. Semino's team found that just 10 mutations, each of which represents a lineage stretching back into prehistoric times, account for 95% of all the samples taken.

By studying the way these mutations are distributed among present-day European men, and then comparing them with what archaeological sources have revealed about the Continent's settlement by humans, they concluded that 80% of these chromosomes trace back to the most ancient migrations into Europe from Central Asia and the Middle East. These chromosomes had been carried by the people who survived Europe's last ice age in three refuges — one in present day Ukraine, another in the northern Balkans and a third in the Iberian Peninsula — before about 13,000 years ago, when they repopulated the vast regions of a Continent that teemed with large game as the ice sheets receded.

Those data meshed neatly with an analysis conducted by scholars at the University of Oxford of mitochondrial DNA, which is passed on exclusively from mother to child. Using more than 4,000 samples from current residents of Europe and the Middle East, Martin Richards (now of the University of Huddersfield) and his colleagues parsed out mutations that they believe originated in Europe. Using known mutation rates for a particular part of the mitochondrial DNA, they were able to distinguish the weight of subsequent migrations in the current genetic makeup of Europeans. Their strikingly similar conclusion: at least 80% of the lineages of present-day Europeans stretch back to the Paleolithic era.

Those studies radically challenge previous theories of the ancestry of today's Europeans. The development of agriculture in the Middle East about 10,000 years ago is thought to have set off a veritable population explosion: archaeological sites have shown that the revolutionary innovation spread into the Eastern Mediterranean and the Balkans, then westward into Europe at a rate of about one kilometer a year.

The theory, fleshed out in detail by Cavalli-Sforza, was that a gradual but inexorable migration into Europe brought agriculture with it. Fortified by sheep, goats, barley and domesticated wheat, these Neolithic peoples were widely thought to have displaced the hunters and gatherers who had inhabited Europe — in much sparser numbers — until then.