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exercise for the other main clusters of European lineages. They found that just eleven clusters, containing some 40 individual lineages, accounted for three quarters of the present day European population. The most ancient, the U5 cluster, had a time-in-Europe date of 50,000 years, give or take 5,000 years each way. This fits well with the archaeological date of 45,000 years for a site in Bulgaria that marks the earliest known presence of modern humans in Europe and hence denotes the start of the Upper Paleolithic age. And it suggests that the Aurignacians, the first people to enter Europe, belonged to the U5 mitochondrial lineage.

Richards could not tell how many people had entered Europe during this first entry of modern humans. But taking the number of lineages in the U5 cluster as a fraction of the whole, he calculated that about 7% of today's Europeans are descended from these first arrivals.

All but one of the other clusters arrived at various times in between, from 35,000 to 15,000 years ago. Altogether some 87% of Europeans are descended from people who arrived before the end of the Pleistocene ice age. Only 13% are descended from ancestors who came to Europe around 10,000 years ago, mostly in the form of the J cluster of mitochondrial lineages. These arrivistes would presumably represent the immigrants from the Near East who brought knowledge of agriculture into Europe and were the harbingers of the Neolithic age.

A major factor that shaped the present population of Europe were the glaciers of the Last Glacial Maximum which drove people back into the southern refuges of the Iberian peninsula. Europe was then repopulated by people spreading northeastward from these refuges as the glaciers retreated after 15,000 years ago. 126 The lineage clusters V and H, which had entered Europe earlier, were prominent in the reexpansion. Some 45 to 50% of most European populations belong to this cluster, and 60% of Basques do, as might be expected if the Basque region of southwestern France and northeastern Spain was the source of the recolonization.

The Richards team's reconstruction of the population history of Europe brought to light an unexpected fact: that most Europeans are descended from the first settlers who arrived during the Upper Paleolithic era. Only a minority arrived during the Neolithic age. This is the reverse of expectation; archaeologists had assumed that the people who introduced farming to Europe in the Neolithic age overwhelmed the earlier inhabitants with their larger populations. The findings suggest that the people of the Upper Paleolithic did not die out; they switched from foraging to settlement and adopted the new farming techniques. Since men and women migrate together, studies of Y chromosome lineages should corroborate the conclusions drawn by the Richards group from mitochondrial DNA. To a large extent they do. A recent analysis by Ornella Semino of the University of Pavia and Peter Underhill of Stanford University has established that 95% of European men belong to just 10 lineages of the Y 127

chromosome tree. The researchers cannot find a Y chromosome lineage that matches up specifically with U5, the mitochondrial DNA cluster that signals the first arrivals in Europe 45,000 years ago. But they see evidence for lineages of men, all carrying a mutation known as M173, who arrived between 40,000 and 35,000 years ago and whom they consider the likely bearers of the Aurignacian culture.

A second migration of Y chromosome owners arrived in Europe from the Near East some 20,000 to 25,000 years ago. The mutation that defines these lineages is known as M170. These men seem to have been the bearers of the Gravettian culture that succeeded the Aurignacian, Semino and her colleagues say. The work of the Richards and Semino teams lays the basis for what many hope will be a grand synthesis between genetics and archaeology. If the geneticists can firm up the dates of entry into Europe of the various mitochondrial DNA and Y chromosome lineages, archaeologists may be able to tie these population movements into the sequence of culturally distinct occupations they have defined for the Upper Paleolithic period. And if historical linguists should succeed in reconstructing a family tree of human languages, as discussed in chapter 10, it may even be possible to say what language was spoken by the people of these ancient lineages. Such a link can already be suggested in at least one instance: if people of the mitochondrial lineage J were indeed those who arrived 10,000 years ago bringing the agricultural techniques of the Neolithic, then they may have spoken the Indo-European tongue from which so many of today's European languages are descended.

The Upper Paleolithic in Eastern Asia

The population history of East Asia cannot yet be written in the same detail as that of Europe. Although the two halves of the Eurasian continent developed separately, there have clearly been links between them. One lies with the men who brought the Aurignacian culture to Europe. Their branch of the Y chromosome genealogy, defined by mutation M173, is a brother to the M3 lineage that is found in some Siberian populations and many American Indians; the two lineages presumably originated from the same source, perhaps in India. Upper Paleolithic sites similar to those in Europe and dating from 40,000 to 25,000 years ago are found across Siberia and around Lake Baikal. The Siberians probably lived in much the same way as their European cousins, hunting the large herds of hoofed species that grazed the Eurasian steppe land. Like the Europeans, their millennia of foraging life were disrupted by the rigors of the Last Glacial Maximum.

Siberia may be something of a backwater in the contemporary world, but in the days of the Last Glacial Maximum its inhabitants accomplished two historic achievements. One was the domestication of the dog, the first species to be drawn into human service. The second, of lesser immediate importance, was the discovery and inhabitation of North and South America. Dogs have lost their working status in most modern societies. But they spread like wildfire in the prehistoric world. They could be trained to help hunt other animals. They made good bed warmers during cold Siberian nights. They would have been a self-transporting source of meat in case of emergency. But probably none of these is the reason that dogs spread so quickly from one end of Eurasia to another.

In antithesis to the Sherlock Holmes tale that hinges on the dog that didn't bark in the night, a crucial problem of dog origins is why they do. Wolves almost never bark. Barking was probably a character that was selected by the dog's first domesticators. That suggests they weren't much interested in using dogs for hunting, where a bark is no asset. But if the first use of dogs was in sentry duty, to warn of strangers, intruders, and attackers creeping in for a dawn raid, then a fierce and furious bark would have made a dog an invaluable defense system.

Dogs may thus have played an important role in early human history, especially if they helped make possible the transition from foraging to settled societies. People who settled down in one place would have been under constant risk of attack. It is perhaps significant that the first settlements occurred at the same time as dogs were domesticated.

Dogs are wolves that have been genetically adapted to live with people. In biological relationships between two species, it is common for each to evolve in response to the other. Have people adapted so as to live with dogs? Communities that learned to make use of dogs as sentries may have gained a substantial advantage, especially in conditions of constant warfare, over those whose members did not learn how to establish rapport with dogs. Another way in which dogs may have altered early human societies is by

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