Monday, October 14, 2013


Ancient rivers, their remains now lying beneath the Sahara desert, once formed green corridors at the surface which our ancestors followed on their great trek out of Africa. A climate model has given us an image of what the landscape would have looked like around 100,000 years ago, suggesting that early humans went west out of sub-Saharan Africa and followed a vast and fertile river system to the Mediterranean.

For decades, there has been speculation that three now-dry North African rivers once served as green pathways for our early ancestors. The waterways would have supported lush flora and fauna to supply early humans with food as they trekked across the continent to the Mediterranean and on to Eurasia. "But no one has been able to work out how much water was in these rivers, when and where exactly they flowed, and how far they reached across the desert," says hydrologist Tom Coulthard of the University of Hull, UK.

Coulthard and his colleagues modeled the climate of the last interglacial period to see how the monsoon rains would have run down the trans-Saharan mountains' north face and flowed across the landscape. Even after accounting for ground absorption and evaporation, the model indicated that there was enough water to have carved green corridors through the desert.

The most promising of the three reconstructed rivers, says Coulthard's colleague Michael Rogerson, is the Irharhar, which flowed 800 kilometers due north to humid regions along the Algeria-Tunisia border. Intriguingly, it is the westernmost of the three systems, yet we know that humans eventually walked east out of Africa. Rogerson suggests that
after reaching the end of the Irharhar, early humans may have taken advantage of marine resources and walked eastward along the North African coast, traversing the Nile delta before migrating into the Middle East.

"As they walked down the river, early humans would have had resources they could use immediately at the Irharhar's far end," he says. Any humans who followed the other two rivers would have been left stranded in inhospitable surroundings. "The other two rivers deliver you to the central parts of Libya, which we think were quite arid then."

Archaeological evidence is more concentrated to the west, says Rogerson, tallying with the idea that most humans would have followed the Irharhar. Classic stone tools like spearheads have been found in the surrounding area,raising the possibility that early humans settled along the river's banks.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0074834


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