Ancient green corridors helped our early ancestors out of Africa
14 October 2008, by Tom Marshall
Ancient people moving northwards out of Africa had more routes open to them than previously thought, according to research published today.
A photograph from NASA's space shuttle Endeavour of the Libyan desert showing ancient rivers (1994).
Archaeologists have tended to believe human migration out of Africa and into the Near East took place along the Nile valley. But new research into the ancient climate suggests people could have travelled along other watercourses that crossed the modern-day Sahara desert.
Today this is one of the world's least-hospitable landscapes. Scientists have long been puzzled by discoveries of bones of water-loving animals like hippos and crocodiles in locations like modern-day Chad. This research suggests an answer: around 120,000 years ago the region was much wetter, with numerous rivers and oases.
A generalised map of the Sahara shows the location of the sample sites and the fossilised river courses. Click for bigger version.
'What's exciting here is that we now know from satellite imagery that there are a number of major dry drainage channels across the Sahara that could have been used as migration routes, perhaps from Chad to southern Libya and beyond,' says Dr Nick Barton, an archaeologist at Oxford University. 'This challenges the assumption that the Nile valley was the only corridor to the north that was open to early humans.'
The greener Sahara was caused by a major increase in the African monsoon between around 130,000 and 117,000 years ago, itself a consequence of cyclical variations in the Earth's orbit around the sun.
Called an 'insolation maximum' by climate scientists, this caused a bigger temperature differential between land and sea. The resulting pressure gradient greatly intensified the region's annual monsoons - called a 'monsoon maximum' - and this in turn meant the monsoon could penetrate far enough north to cross the mid-Saharan watershed, which lies at about 21-23° north. Beyond this point, any water falling as rain flowed north, creating rivers stretching across the Sahara to the Mediterranean.
'This challenges the assumption that the Nile valley was the only corridor to the north that was open to early humans.'
Professor Eelco Rohling of the University of Southampton's School of Ocean and Earth Science, who is based at the National Oceanography Centre, Southampton (NOCS), says the research will help refine future climate models. This is because monsoon maxima provide a difficult test case for models as they test whether they still work under very different climate conditions.
'This study proves that past 'greening of the Sahara' was more extensive than climate models suggest,' he says. 'The underlying processes remain to be fully explained, and the evidence forms a critical test for the next generation of regional climate models of the African monsoon.'
Which way? Discussing the route with Libyan guides.
The research, published today in Proceedings of the National Academy of Sciences, was carried out by PhD student Anne Osborne, supervised by Dr Derek Vance of Bristol University and co-supervised by Rohling. They were joined by collaborators from the universities of Oxford, Hull and Tripoli.
Osborne, funded by the Natural Environment Research Council, says, 'Space-born radar images showed fossil river channels crossing the Sahara in Libya, flowing north from the central Saharan watershed all the way to the Mediterranean.'
Earlier work had used satellite imagery and extensive field work to map the features of the ancient landscape, showing long-dry riverbeds running across Libya.
Osborne's research builds on this by analysing levels of neodymium isotopes to trace the path taken by ancient monsoon floodwaters. This works because the comparatively young rocks of the Earth's crust in the volcanic mountains of the mid-Sahara have a very different balance of neodymium isotopes from that of the much older material of the North African lowlands.
Water in the Libyan Sahara.
Osborne found that erosion in the volcanic mountains of the central Saharan watershed left rivers with a unique neodymium signature. She was able to follow this signature all the way to the Mediterranean by analysing snail shells from dried riverbeds. Because these creatures spent their whole lives in these rivers, their shells took on the neodymium signature of the waters flowing past them.
The researchers combined this with analysis of oxygen and neodymium isotopes in the microfossils in seabed cores from the Mediterranean. These isotopes indicate a greater contribution of freshwater around the Gulf of Sirte, north of modern-day Libya, than around the mouth of the Nile.
The combined analyses revealed that a large quantity of freshwater, originating from a region of young crust, poured into the Mediterranean at the time. This signal was more dilute near the mouth of the Nile than near Libya. This suggests that the bulk of the fresh water with a young crust signature did not come via the Nile.
Camping in the Sahara, en route to the southern sample site.
Barton connects the results to the distinctive artefacts that archaeologists call Arterian. These are found across much of Northern Africa, but are rarely discovered in the Nile valley, and include sea shells that have been drilled and mounted on string, in what are likely to have been early necklaces or bracelets.
'We have good reason to think these artefacts were left by behaviourally modern humans across much of North Africa. We suggest - if these populations were coming out of Eastern Africa - that they migrated via a series of routes across the Sahara and maybe also along a more southerly route via the Sahel,' he argues.
The researchers at Bristol hope to continue the research with further fieldwork in Libya to carry out more extensive sampling. They also want more detailed satellite imagery to give them an idea where to look. The ultimate goal is to map the paths of these ancient watercourses in much greater detail and look for further evidence of prehistoric human migrations.
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