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05/13/2005:

"Geneticists Link Modern Humans to Single Band Out of Africa"

A team of geneticists believe they have shed light on many aspects of how modern humans emigrated from Africa by analyzing the DNA of the Orang Asli, the original inhabitants of Malaysia. Because the Orang Asli appear to be directly descended from the first emigrants from Africa, they have provided valuable new clues about that momentous event in early human history.

The geneticists conclude that there was only one migration of modern humans out of Africa - that it took a southern route to India, Southeast Asia and Australasia, and consisted of a single band of hunter-gatherers, probably just a few hundred people strong.

A further inference is that because these events took place during the last Ice Age, Europe was at first too cold for human habitation and was populated only later - not directly from Africa but as an offshoot of the southern migration which trekked back through the lands that are now India and Iran to reach the Near East and Europe.

The findings depend on analysis of mitochondrial DNA, a type of genetic material inherited only through the female line. They are reported in today's issue of Science by a team of geneticists led by Vincent Macaulay of the University of Glasgow.

Everyone in the world can be placed on a single family tree, in terms of their mitochondrial DNA, because everyone has inherited that piece of DNA from a single female, the mitochondrial Eve, who lived some 200,000 years ago. There were, of course, many other women in that ancient population, but over the generations one mitochondrial DNA replaced all the others through the process known as genetic drift. With the help of mutations that have built up on the one surviving copy, geneticists can arrange people in lineages and estimate the time of origin of each lineage.

With this approach, Dr. Macaulay's team calculates that the emigration from Africa took place about 65,000 years ago, pushed along the coastlines of India and Southeast Asia, and reached Australia by 50,000 years ago, the date of the earliest known archaeological site.

The Orang Asli - meaning "original men" in Malay - are probably one of the surviving populations descended from this first migration, since they have several ancient mitochondrial DNA lineages that are found nowhere else. These lineages are between 42,000 and 63,000 years old, the geneticists say.

Groups of Orang Asli like the Semang have probably been able to remain intact because they are adapted to the harsh life of living in forests, said Dr. Stephen Oppenheimer, the member of the geneticists' team who collected blood samples in Malaysia.

Some archaeologists believe that Europe was colonized by a second migration, which traveled north out of Africa. This fits with the earliest known modern human sites - which date to 45,000 years ago in the Levant and 40,000 years ago in Europe.

But Dr. Macaulay's team says there could only have been one migration, not two, because the mitochondrial lineages of everyone outside Africa converge at the same time to the same common ancestors. Therefore, people from the southern migration, probably in India, must have struck inland to reach the Levant, and later Europe, the geneticists say.

Dr. Macaulay said it was not clear why only one group had succeeded in leaving Africa. One possibility is that since the migration occurred by one population budding into another, leaving people in place at each site, the first emigrants may have blocked others from leaving.

Another possibility is that the terrain was so difficult for hunter-gatherers, who must carry all their belongings with them, that only one group succeeded in the exodus.

Although there is general, but not complete, agreement that modern humans emigrated from Africa in recent times, there is still a difference between geneticists and archaeologists as to the timing of this event. Archaeologists tend to view the genetic data as providing invaluable information about the interrelationship between groups of people, but they place less confidence in the dates derived from genetic family trees.

There is no evidence of modern humans outside Africa earlier than 50,000 years ago, says Dr. Richard Klein, an archaeologist at Stanford University. Also, if something happened 65,000 years ago to allow people to leave Africa, as Dr. Macaulay's team suggests, there should surely be some record of this event in the archaeological record within Africa, Dr. Klein said. Yet signs of modern human behavior do not appear in Africa until the transition between the Middle and Later Stone Age, 50,000 years ago, he said.

"If they want to push such an idea, find me a 65,000-year-old site with evidence of human occupation outside of Africa," Dr. Klein said.

Geneticists counter that many of the coastline sites occupied by the first emigrants would now lie under water, since sea level has risen more than 200 feet since the last Ice Age. Dr. Klein expressed reservations about this argument, noting that rather than waiting for the rising sea levels to overwhelm them, people would build new sites further inland.

Dr. Macaulay said that genetic dates have improved in recent years now that it is affordable to decode the whole ring of mitochondrial DNA, not just a small segment as before. But he said he agreed "that archaeological dates are much firmer than the genetic ones" and that it is possible his 65,000-year date for the African exodus is too old.

Dr. Macaulay's team has been able to estimate the size of the population in Africa from which the founders are descended. The calculation indicates a maximum of 550 women, but the true size may have been considerably less. This points to a single group of hunter-gatherers, perhaps a couple of hundred strong, as the ancestors of all humans outside of Africa, Dr. Macaulay said.
Full: nytimes.com