BioEd Online

The first modern humans to emigrate from Africa may have done so by sticking to the coast.

Analysis of surviving aboriginal populations in Southeast Asia suggest that they arose from a single wave of migrants who left the Horn of Africa more than 65,000 years ago. By following the coasts, say the authors of the new analyses, early humans may have been able to colonize the globe with remarkable speed - reaching far-flung lands such as Australia within just a few thousand years.

Most experts agree that modern humans arose in Africa before spreading throughout the world. But while archaeological evidence suggests that humans moved north into Egypt and the Middle East, climate records show that this region was an inhospitable desert until 50,000 years ago, making this an unlikely choice of route.

Journeying east around the coastlines of Somalia and eventually India would have been one alternative, says Vincent Macaulay of the University of Glasgow, UK. "It wouldn't have been difficult to live on the coast," he says. "In fact, it would have been quite appealing."

Living alone

Macaulay and his colleagues tested this idea by studying DNA from the Orang Asli people in Malaysia, who are not thought to have interbred with other groups. "We're interested in working out who the first people to move out of Africa were, so the obvious people to sample are indigenous ones," he explains.

The researchers collected cheek-swab samples from 260 Orang Asli tribespeople, and analysed their mitochondrial DNA, which is passed down unaltered from mother to child. They then compared this with mitochondrial DNA from other populations - the difference between them reflects the time since the groups diverged.

Another research group, led by Lalji Singh of the Centre for Cellular and Molecular Biology in Hyderabad, India, carried out similar tests on indigenous people living on the Andaman Islands in the Indian Ocean.

By comparing these with data on mitochondrial DNA from other populations in the region, the researchers conclude that the two populations are descended from a single group of people, containing perhaps 600 females of reproductive age, who lived in India around 65,000 years ago. The studies are reported in this week's issue of Science¹,².

The authors suggest that if the Andaman and Orang Asli people travelled by a circuitous inland route, they could not have stayed so closely related to the original Indian population.

Quick movers

If humans tended to migrate along coasts, it might explain how they moved so quickly, Macaulay suggests. The earliest human remains found in Australasia are some 60,000 years old, which means the early pioneers must have averaged several kilometres a year.

Being confined to coastlines, where there is limited space to expand, might have meant that settlers used up their resources more quickly and were then forced to move on, Macaulay says. What's more, if sea levels have risen since then, many archaeological remains may now be under the sea floor, which could why most remains have been found inland.

The fact that these tribes are related to a single small population suggests that humans emigrated from Africa only once, adds Philip Endicott, who studies human migrations at the University of Oxford, UK. But he cautions that mitochondrial DNA does not provide very accurate timing, because it is smaller than the overall human genome and so is more susceptible to mutational quirks.

"I'm not yet convinced that there was one small, rapid exit - I'm not sure that's the whole story," adds Chris Stringer, an anthropologist at the Natural History Museum in London. "But the coastal route is plausible."

Macaulay and his team now plan to sample more populations from India and Arabia, and to look at other genetic markers besides mitochondrial DNA, to add more weight to the theory.