14 May 2005
When humans left Africa, they traveled south
A team of geneticists believe they have illumined 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 that it 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 from an offshoot of the southern migration that trekked back, through the territory that is now taken up by India and Iran, to reach the Near East and Europe.
The team of geneticists led by Vincent Macaulay, of the University of Glasgow, calculates that the emigration from Africa took place around 65,000 years ago, pushed along the coastlines of India and Southeast Asia, and reached Australia by 50,000 years ago. The Orang Asli - meaning "original men" in Malay - are probably one of the relict populations descended from this first migration since they have several ancient mitochondrial DNA lineages between 42,000 and 63,000 years old, the geneticists say.
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 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 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.
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 is that the terrain was so difficult for hunter-gatherers, who must carry all their belongings with them, that only group succeeded in the exodus.
There is no evidence of modern humans outside Africa earlier than 50,000 years ago, says Richard Klein, an archaeologist at Stanford University. Also, if something happened 65,000 years to allow people to leave Africa, as Macaulay's team suggests, there should be a record of it in the archaeological record within Africa, Klein said. Yet signs of modern human behavior do not appear in Africa until the transition between the Mesolithic and Neolithic 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," he said.
Source: International Herald Tribune (13 May 2005)