NY Times , May 13, 2005
By NICHOLAS WADE
By studying the DNA of an ancient people in Malaysia, a team of geneticists says it has illuminated many aspects of how modern humans migrated from Africa.
The geneticists say there was only one migration of modern humans out of Africa; that it took a southern route to India, Southeast Asia and Australia; and that it consisted of a single band of hunter-gatherers, probably just a few hundred people strong.
Because these events occurred in the last Ice Age, when Europe was at first too cold for human habitation, the researchers say, it was populated only later, not directly from Africa but as an offshoot of the southern migration. The people of this offshoot would presumably have 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 solely through the female line. They are reported today in Science by a team of geneticists led by Dr. 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 woman, 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 occurred 65,000 years ago, pushed along the coasts of India and Southeast Asia and reached Australia by 50,000 years ago, the date of the earliest known archaeological site there.
The Malaysian people whom the geneticists studied are the Orang Asli. The term means "original men" in Malay.
They are probably descended from this first migration, because they have several ancient mitochondrial DNA lineages that are found nowhere else.
These lineages are 42,000 to 63,000 years old, the geneticists say. Subgroups of the Orang Asli, like the Semang, have probably been able to remain intact because they adapted to the harsh existence of living in forests, said Dr. Stephen Oppenheimer, the member of the geneticists' team who collected blood samples in Malaysia.
Some archaeologists theorize that Europe was colonized by a second migration that traveled north out of Africa. This fits with the earliest known modern human sites, dating from 45,000 years ago in the Levant and 40,000 years ago in Europe.
Dr. Macaulay's team says there could have been just 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 just one group succeeded in leaving Africa. One possibility is that because the migration occurred by continuous population expansion, leaving people in place at each site, the first emigrants may have blocked others from leaving. Another is that the terrain was so difficult for hunter-gatherers, who 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 about its a timing. Archaeologists tend to view the genetic data as providing invaluable information about the interrelationship between groups, 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, said Dr. Richard Klein, an archaeologist at Stanford. 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 that in the archaeological record in Africa, Dr. Klein said. Yet signs of modern human behavior do not appear in Africa until 50,000 years ago, the transition between the Middle and Later Stone Ages, 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, because the sea level has risen more than 200 feet since the last Ice Age. Dr. Klein expressed reservations about that argument, noting that people would not wait for the slowly rising sea levels to overwhelm them but would build new sites farther inland.
Dr. Macaulay said genetic dates had improved in recent years, now that it is affordable to decode the whole ring of mitochondrial DNA, and not just a small segment.
But he said he agreed "that archaeological dates are much firmer than the genetic ones" and that it was possible his 65,000-year date for the African exodus was too old.
Dr. Macaulay's team has been able to estimate the size of the population in Africa from which the founders descended. The calculation indicates a maximum of 550 women. 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.
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