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Did buried rivers in the Sahara help humans migrate out of Africa?

Did prehistoric rivers across the Sahara support human migration routes? This figure simulates the probability of surface water during the last interglacial period. (Coulthard et al/PLOS ONE)


Descriptors not usually associated with the Sahara desert: wet, humid, laced by green corridors, critical for supporting human life.

But a British-led research team announced in a study published yesterday that they believe three rivers used to flow across the Sahara, and that those rivers -- now buried --provided a likely route for the migration of homo sapiens out of Africa.

"It's exciting to think that 100,000 years ago there were three huge rivers forcing their way across 1,000 kilometres of the Sahara desert to the Mediterranean -- and that our ancestors could have walked alongside them," said lead author Tom Coulthard in a statement. Coulthard is a professor of physical geography at the University of Hull.

Scientists are still debating crucial points about human dispersal from Africa and into the Levant and Mediterannean. Did everyone migrate all at once, together along a single route? Did everyone migrate all at once but along several routes? Or were there many migrations at many different times? And what were those routes?

By analyzing stalagmites, among other things, previous research showed that humidity spiked higher in the Sahara region around 100,000 years ago. That coincides with some of the earliest evidence of homo sapiens in the Sahara. And fossil evidence shows there was water in the Sahara at some point.

But if waterways existed, did they occur at the right time to be helpful for human migration -- the so-called "green corridors" hypothesis?

Archaeology has not yet been able to definitively answer those questions.

So Coulthard and his colleagues used a different method -- hydrological earth modelling -- to try to see what the region would have looked like in the last interglacial period, which lasted from roughly 130,000 to 110,000 years ago. Homo sapiens originated in Africa 200,000 years ago and were present in the Near East by 125,000 years ago.

They used rainfall data taken from simutions where the African monsoon was positioned much further north. They used previous analysis that showed the existence of watersheds that would have drained from the Saharan mountains to the Mediterannean. They also incorporated satellite imagery that showed traces of river beds associated with these watersheds but now buried by dunes. They adjusted for where coastlines would have been: sea levels were up to 20 metres higher in some places at that point.

The researchers ran several simulations that accounted for different levels of evaporation, but stayed within the most conservative of outcomes.

They discovered that both perennial and patchily-flowing rivers would have run throughout the Sahara. Some peter out in the middle of the dessert, while others combine to form a solid river system. And the simulations also showed huge lagoons and wetlands in what is now Lybia, some over 70,000 kilometres square.

Coulthard's group is confident enough in their data to say the question now should not be whether rivers existed, but what exactly they looked like.

The viability of these rivers gives a big boost to the theory that homo sapiens would have used them to travel into the Levant and the Mediterannean.

Now it's up to the archaeologists to show whether early human skeletons or artefacts are buried along these routes.

Kate Allen is the Star's science and technology reporter. Find her on Twitter at @katecallen.






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