October 14, 2008

Undiscovered Route Found of Early African Exodus

In a new study, researchers have found what they believe to be a new pathway that allowed modern humans to spread beyond their ancestral homeland about 120,000 years ago.

Rivers once flowed from the central Saharan watershed all the way to the Mediterranean, the team of researchers from the Universities of Bristol, Southampton, Oxford, Hull and Tripoli in Libya explain in the journal PNAS.

These rivers could have made up a "wet corridor" through Libya for ancient human migrations out of Africa into the rest of the world.

Researchers previously considered the Nile Valley to be the main route of human expansion into other continents.

The Sahara then covered most of North Africa, as it does now. So it would have presented a formidable obstacle for early modern humans wishing to cross from the south to the north of the continent.

Previous data show there was increased rainfall across the southern part of the Sahara between 130,000 and 170,000 years ago; in a gap between Ice Ages known as the last interglacial period.

Researchers sought to discover whether these wetter conditions had reached further north than previously considered.

Using geochemical tests, the scientists showed fossil river channels crossing the Sahara in Libya were active during the last interglacial. This would have created vital water courses across an otherwise arid region, the researchers reported.

Using geochemical tests, the scientists showed the channels were active during the last interglacial. This would have created vital water courses across an otherwise arid region, the researchers wrote in PNAS.

Researchers analyzed the forms, or isotopes, of different chemical elements in snail shells from two sites in the fossil river channels and from the shells of planktonic microfossils in the Mediterranean.

Their tests showed a distinct volcanic signature to these shells, which was quite different to rocks from surrounding sites.

"It's a possible route that the early modern humans could have taken," said lead author Anne Osborne, from the earth sciences group at Bristol.

"We now need to focus archaeological fieldwork around the large drainage channels an palaeo-lakes to test these ideas," said co-author Dr Nick Barton, from the University of Oxford.

Scientists may not know what routes were taken, but they do know that Homo sapiens had reached the Levant by around 100,000 years ago.

However, this appears to have been an early, failed foray outside Africa by modern humans. By 75,000 years ago, Neanderthals had replaced our species in the region.

Then, about 45,000 years ago, modern humans reoccupied the area.

Genetic evidence suggests that populations living outside Africa today are the descendents of a migration which originated in the east of the continent between 60-70,000 years ago.


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