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November 14, 2000
 

Scientists Rough Out Humanity's 50,000-Year-Old Story

By NICHOLAS WADE

From what had seemed like irreversible oblivion, archaeologists and population geneticists believe they are on the verge of retrieving a record of human history stretching back almost 50,000 years.

The record, built on a synthesis of archaeological and genetic data, would be a bare bones kind of history without individual names or deeds. But it could create a chronicle of events, however sketchy, between the dawn of the human species at least 50,000 years ago and the beginning of recorded history in 3,500 B.C. The events would be the dated migrations of people from one region to another, linked with the archaeological cultures and perhaps with development of the world's major language groups.

The new element in this synthesis is the increasing power of geneticists to look back in time and trace the history of past populations from analysis of the DNA of people alive today.

"It is astonishing how much archaeology is beginning to learn from genetics," Dr. Colin Renfrew, a leading archaeologist at the University of Cambridge in England, said at a conference on human origins held last month at the Cold Spring Harbor Laboratory on Long Island.

In one of the most detailed genetic reconstructions of population history so far, Dr. Martin Richards of the University of Huddersfield in England and many colleagues have traced the remarkably ancient ancestry of the present-day population of Europe.

Some 6 percent of Europeans are descended from the continent's first founders, who entered Europe from the Near East in the Upper Paleolithic era 45,000 years ago, Dr. Richards calculates. The descendants of these earliest arrivals are still more numerous in certain regions of Europe that may have provided them with refuge from subsequent waves of immigration. One is the mountainous Basque country, where people still speak a language completely different from all other European languages. Another is in the European extreme of Scandinavia. Another 80 percent arrived 30,000 to 20,000 years ago, before the peak of the last glaciation, and 10 percent came in the Neolithic 10,000 years ago, when the ice age ended and agriculture was first introduced to Europe from the Near East.

It used to be thought that the most important human dispersals occurred in the Neolithic, prompted by the population increases made possible by the invention of agriculture. But it now seems that the world filled up early and the first inhabitants were quite resistant to displacement by later arrivals.

Dr. Richards's estimates, reported in the current issue of The American Journal of Human Genetics, are based on analysis of mitochondrial DNA, a genetic element that occurs in both men and women but that is transmitted only through the mother; thus, they reflect only the movement of women.

The movement of men can be followed through analysis of the Y chromosome, but the Y chromosome is harder to work with and data are only just now becoming available. In an article in the current issue of Science, Dr. Peter A. Underhill of Stanford University and colleagues reported the first analysis of the European population in terms of the Y chromosome. Although this agrees with the mitochondrial DNA findings in major outline, suggesting that Europe was populated mostly in the Paleolithic period with additions in the Neolithic, there are some points of difference.

The earliest migration into Europe according to mitochondrial DNA took place from the Near East 45,000 years ago, but Dr. Underhill and his colleagues said they could see no corresponding migration in the Y chromosome data.

They have found a very ancient Y chromosome mutation that occurs in Siberia as well as Europe. They boldly link this mutation with the bearers of the Aurignacian culture who entered Europe 40,000 years ago. The culture appears in Siberia at about the same time, as if these early people had spread both east and west.

Dr. Underhill and his colleagues associate another mutation, which is common in India, Pakistan and Central Asia as well as Europe, with the people of the Kurgan culture who, according to one theory, expanded from southern Ukraine and spread the Indo-European languages.

Dr. Underhill's report tries to make the grand synthesis between archaeological and genetic data, but it will probably be some time before the specialists in each area agree on how the two types of data should be associated.

"It is very exciting that the geneticists now have internal dating procedures, but really I think the dates are very loose indeed," Dr. Renfrew said in an interview.

Geneticists believe that the world outside Africa was populated by the migration of a very small number of people who left east Africa about 50,000 years ago. These modern humans, with their more advanced and inventive culture, are thought to have displaced the archaic hominids like the Neanderthals, which had emigrated from Africa many thousands of years earlier.

These Paleolithic populations created sophisticated stone tools and left evidence of their advanced culture in the cave paintings of southern France, dating to at least 30,000 years ago. Although anatomically modern humans first appear in Africa about 150,000 years ago, their archaeological remains show little sign of modern human behavior.

Dr. Richard Klein, an archaeologist at Stanford University, has suggested that some genetic change, perhaps as profound as the invention of language, occurred in Africa around 50,000 years ago, and that it was these behaviorally modern humans who both spread within Africa and populated the rest of the globe.

This thesis was challenged at the Cold Spring Harbor conference by two archaeologists, Dr. Sally McBrearty of the University of Connecticut and Dr. Alison Brooks of George Washington University. They argued that each of the components said to characterize the Paleolithic revolution in human behavior, like stone blades, long distance trade and art, can be found in Africa at earlier dates.

"So all the behaviors of the Upper Paleolithic have an African pedigree," Dr. McBrearty said. The behaviors were gradually assembled as a package and exported, "which is why it appears suddenly in Europe 40,000 years ago," she said.

Dr. Klein said in an interview that he doubted some of the early dates proposed by Dr. McBrearty and Dr. Brooks, and that even if the dates were correct, modern behaviors conferred such an advantage that they should appear in a broad pattern, not just at the handful of places cited by his critics. To understand what happened in the past, it is necessary to look for patterns and ignore the "noise," he said.

The synthesis of archaeology with population genetics may provide a basis into which a third discipline can join, that of historical linguistics. Most linguists insist that languages change so rapidly that their roots cannot reliably be traced further back than 5,000 years. Only a few, like Dr. Joseph Greenberg of Stanford, believe that some elements of language remain constant, enough to reconstruct all the world's languages into just 14 superfamilies of a much great antiquity.

The signature of these ancient superfamilies can be seen in the geographic distribution of languages, Dr. Renfrew said. In some areas of the world, like the Caucasus, New Guinea and South America, there are many language families packed into a small area, which he called a mosaic zone. In other areas, a single language family covers a broad area or spread zone. The Indo-European languages, which stretch from Europe to India, are one such example. Another is Afro-Asiatic, the superfamily that includes the languages of Ethiopia and Somalia and Semitic languages like Arabic and Hebrew.

The spread zones, Dr. Renfrew said, are mostly the result of recent dispersals caused by agricultural inventions. The mosaic zones "may be those of the first humans to occupy those areas, at least in Australia and America," he said.

The language spoken by the ancestral human population may never be known, though Dr. Greenberg has tried to reconstruct a few words of it. But some linguists who study the click languages of southern Africa feel they are very ancient. This belief is supported by genetic evidence showing that the Khoisan peoples, the principal speakers of click languages, belong to the most ancient of all the human lineages, based on mitochondrial DNA.

Dr. Anthony Traill, a click language expert at the University of the Witwatersrand in Johannesburg, said that linguistically the languages fell into three separate groups whose relationship, aside from the clicks, was hard to establish. The clicks must be ancient, he said, because "the chances of clicks being invented after being lost is zero." The only use of clicks outside of Africa is in an Australian aboriginal initiation languages in which the clicks are used as meaningless sounds.

"The idea that clicks were lost from all languages other than Khoisan," Dr. Traill said, "is stimulating, but I don't know what to make of it."

Of the three disciplines that bear on human origins historical linguistics, population genetics and archaeology only archaeology has a rock-solid method of dating, based on radiocarbon and other kinds of radioactive decay.

But geneticists are now improving their dating methods, even though the dates are still very approximate, to the point that they can begin to correlate their findings with the archaeologists'. The geneticists' first foray into human prehistory was the famous "mitochondrial Eve" article of 1987 by the late Allan Wilson, showing that when people around the world were placed on a family tree constructed from their mitochondrial DNA, the tree was rooted in African populations, in an individual who lived about 200,000 years ago.

Though the methodology of the paper was imperfect, its result was unchanged after the method had been corrected, and geneticists have developed a growing confidence in mitochondrial DNA dates. The mitochondrial DNA trees trace back to a single individual, not because there was only one Eve the ancestral human population is thought to have contained about 10,000 people but because the lineages of all the other Eves have gone extinct. The process is easy to visualize by thinking of an island population with 10 surnames. In each generation, some men will have no children or only daughters and their surnames will disappear until only one is left; the Y chromosome and mitochondrial DNA follow the same pattern.

The first major branch points in the mitochondrial Eve tree have been called the daughters of Eve and they fall in a geographic pattern with some daughters of Eve being characteristic of Africa, some of Asia and the Americas and some of Europe and the Near East.

Dr. Richards and his colleagues have analyzed the ancestry of the present European population by looking within the major daughter of Eve branches for subbranches that occur both in Europe and the Near East, from western Iran through Turkey and Arabia to Egypt, because the Near East is the probable source of most of the ancestral populations that entered Europe.

The subbranches from each region were then dated by counting the number of mutations that had occurred in the mitochondrial DNA sequence from the beginning of the subbranch until today. If the subbranch was older in the Near East than Europe, it indicated a migration into Europe. By this method Dr. Richards's team was able to date the migrations into Europe. They also picked up a sizable back-migration from Europe to the Near East.

The geneticists working on the Y chromosome may eventually be able to date migrations with similar precision. The major class of mutation on the Y is so rare that the ticks of the mutation clock are too many thousands of years apart to be reliably averaged. But a second kind of mutation occurs more rapidly and the combination of the two may make a reasonable clock.

Analysis of the Y chromosome has already yielded interesting results. Dr. Ariella Oppenheim of the Hebrew University in Jerusalem said she had found considerable similarity between Jews and Israeli and Palestinian Arabs, as if the Y chromosomes of both groups had been drawn from a common population that began to expand 7,800 years ago.

In the middle ages, the Vikings settled in Greenland but contact with their colonies was lost at the beginning of the 15th century. In 1720, by which time the Danes had long become Protestants, there arose considerable concern that the missing colonists, if they still existed, would be Roman Catholics and in need of conversion. An expedition was sent to Greenland but found only ruined houses and Eskimos. Did the Vikings perish or intermarry? An analysis of Greenlanders' mitochondrial DNA shows only genetic signatures typical of the New World, and it indicates their unalloyed descent from Eskimos of Alaska. "It looks bad for the Vikings," said Dr. Peter Forster of the University of Cambridge, a co- author of the study.

Dr. Douglas Wallace of Emory University, who pioneered the use of mitochondrial DNA to analyze human origins, said of the emerging type of analysis: "The Y chromosome has a great future. But it is a very new technology."