Monday, Apr. 30, 2001

Living in the Past

By JAMES GRAFF

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. The emerging genetic picture tells a different story, one that identifies the earliest Europeans as scraggly, persistent foragers who had hunkered down during the glacial age. "This research changes the whole debate about Europe, shifts it back in time from the Neolithic era of farming to the Paleolithic era of hunter-gatherers," says Bryan Sykes, professor of human genetics at the University of Oxford and a pioneer of mitochondrial DNA analysis. "There's now a much clearer sense that the genes we carry lived through the Ice Age, that our ancestors were hunting bison and reindeer with essentially the same genetic makeup we have today."

Like any other scientific innovation, tracing founder lineages through the Y chromosome of men or the mitochondrial DNA of women is open to many questions. Some scientists think, for instance, that even the relatively high number of samples — whether a lock of hair or a dollop of saliva — used in the most recent studies is still too small to give a full picture of genetic variation. Others harbor doubts that the rate of mutation of mitochondrial DNA is constant enough to support conclusions about chronological dating. "Physical anthropology remains the gold standard for dating," says geneticist Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, a member of the team at the University of California, Berkeley that in 1987 identified "Mitochondrial Eve" as the 140,000- to 280,000-year-old ancestor of all living humans. "But we're getting better at genetic dating all the time."

Carbon dating has been recalibrated in recent decades to give archaeologists strong certainties about the ages of key artifacts for Europe's prehistory, from the drawings in France's Chauvet Cave (32,000 years old), to possibly Neanderthal milk teeth found in Cavallo, Italy (31,000 years old), to the Minoan civilization on Crete (3,700 years old). That means there is always the prospect of a physical discovery — an agricultural site that doesn't fit in time or space, say — that can blow existing theories out of the water.

So, though anthropologists of even the most humanist bent can't afford to ignore the precipitous pace of discovery among geneticists, neither can any search for an integrated picture of the past rely on molecular anthropology alone. "Genetics tells us about the travels of human genes — the boy-meets-girl of the story," says Marek Zvelebil, an archaeologist at the University of Sheffield. "But gene exchange is different from language or cultural exchange. Who are we in the long term? There are at least three identities — genetic, linguistic and cultural — and we're all a mix of these."

Take for instance the key question of how agriculture spread in Europe. The genetic evidence of recent years certainly challenges the notion that agriculturalists from the Middle East wiped out previous immigrants to Europe, but it doesn't answer the question of how farming did spread. Zvelebil's reading of archaeological sites suggests that in many parts of Eastern and Northern Europe, there were porous frontiers between foragers and farmers that could last thousands of years.

In the Eastern Baltic, for instance, foragers traded seal fat, amber, slate and flint for the farmers' pottery and grain. In coastal regions where oysters or other shellfish were plentiful, foragers felt no particular compulsion to take up the tasks of horticulture. Where farming did spread, he says, it was often through a process of gradual adoption by hunter-gatherers rather than continual migration of farmers. "Gene flow just doesn't correspond to the cultural patterns," he says.

Nor, most scholars now agree, do they correspond very well to linguistics. Sykes has pointed out that the Basques, who speak a non-Indo-European language amid a sea of Indo-European ones, lack the genetic markers that would indicate they have been in Europe longer than their French and Spanish neighbors (though there are markers — such as a much higher frequency of RH-negative blood types — that point to their distinctiveness). And most speakers of Hungarian, a Finno-Ugric language surrounded by Indo-European tongues, don't appear genetically much different from their Slavic neighbors.

So what? Well, it complicates matters, suggesting that gradual cultural exchange has played a quiet but constant role in human history — and that invasions aren't necessarily all they have been cracked up to be. Thirty years ago the dominant theory was that the precursor of the Indo-European languages came to Europe on the tongues of warrior horsemen from the Pontic steppes of present-day Ukraine, and that the broad dispersal of those languages across the Continent was a tribute to their martial success. Then in 1987 Renfrew made a powerful case that it was the Neolithic farmers who brought the language with them from the Middle East, and that along with their barley and wheat they sowed the overwhelming dominance of their tongue throughout Europe. But as the genetic evidence now suggests, neither warriors nor farmers were able to keep their language to themselves. The Indo-European language family — from Lithuanian and Catalan to Swedish and English — spread far more successfully across Europe than the genes of its original progenitors did.

Linguists have their own ideas about how change occurs; they have managed through a rough philological equivalent of genetic research to work back from modern languages to common roots, thus reconstructing Proto-Indo-European, a purely theoretical tongue. But as Renfrew points out, if the difficulties of dating genetic change are vexing, the ones for dating linguistic change are even harder: though linguists can chart the rate of change from, say, late Latin to early Spanish, they can't prove the same rate applies for other languages before the advent of writing.

Renfrew sees evidence that linguists — like their colleagues in other disciplines when they look at prehistoric developments — are beginning to think outside the box and relate language to "tangible material processes" like floods, the spread of agriculture and demographic developments. Currently the prehistory of language is, as Renfrew puts it, "at the edge of knowability," but that could change in a matter of decades if the feverish pace of cross-fertilization of molecular anthropology and archaeology continues.

And there is no reason why it will not. "Thirty or 40 years ago the story of Europe was basically one of watching the covered wagons roll west, full of pottery, wheat and barley, pushing aside the hunter-gatherers," says Clive Gamble, an archaeologist at the University of Southampton. Further back, archaeology was harnessed to political ends, subsumed in Nazi Germany to the dogma of Aryan man, and in most other places in Europe to a kind of manifest destiny.

The new research is almost certain — like the genome itself — to suggest a more nuanced and complicated idea of what it means to be a human being. We are all more similar than racists or nationalists like to think: the genetic variance throughout the 6 billion humans on earth amounts to less than that in a single troop of chimpanzees. But those genes have afforded us an ability to adapt from foraging for hazelnuts to searching the Web in the evolutionary blink of an eye. What happens in the next blink is anybody's guess.