Did ancient river channels guide humans out of Africa?
By Ewen Callaway The first humans to leave Africa didn’t have to struggle over baking sand dunes to find a way out – instead they might have followed a now-buried network of ancient rivers, researchers say. Chemical analysis of snail fossils suggests that monsoon-fed canals criss-crossed what is now the Sahara desert as modern humans first trekked out of Africa. Now only visible with satellite radar (see an image), the channels flowed intermittently from present-day Libya and Chad to the Mediterranean Sea, says Anne Osborne, a geochemist at the University of Bristol, UK, who led the new study. Up to five kilometres wide, the channels would have provided a lush route from East Africa – where modern humans first evolved – to the Middle East, a likely second stop on Homo sapiens‘ world tour. Archaeological, genetic and palaeontological evidence have pointed to the Nile River Valley and Red Sea as other potential alleys for human migration out of Africa. To make a case for the channels, Osborne’s team excavated snail fossils buried by half a metre of sand from a channel in Libya and compared their chemical makeup to rocks from volcanoes hundreds of kilometres away. By measuring the decay of a radioactive metal locked into the shells and rocks, Osborne’s team showed that the buried snails must have incorporated water that flowed from the volcanoes. Other climate records point to a sometimes-green Sahara around this time, and Osborne thinks that seasonal monsoons could have supported a patchwork of life-saving oases across the desert. Chris Stringer, a palaeoanthropologist at London’s Natural History Museum, says Osborne’s team makes a good climatological case for the importance of the Saharan channels in human migrations. North African human bones and artefacts closely match those in the Middle East, but, he says, a greener Sahara could have connected already existing populations in both spots to achieve the same effect. Better proof could come with archaeological finds documenting a human migration across the Sahara, he says. Yet it’s a task that few researchers have taken on so far. “It’s up to the archaeologists now to go and have a search,” Osborne says. Journal reference: PNAS (DOI: