I've noted before how science is often about finding missing pieces of the jigsaw and fitting them in place. This paper by Tom J. Coulthard, Jorge A. Ramirez, Nick Barton, Mike Rogerson, Tim Brücher, reported in New Scientist by Alyssa A. Botelho illustrates that as well as anything by adding a little bit more information to the account of human migration out of Africa.
The assumed routes Homo sapiens took out of Africa usually include migration north down the Nile which crosses the Sahara as a narrow fertile band in an otherwise arid desert, and by coastal spread from the Horn of Africa across the Red Sea and along the edge of the Arabian Peninsula. However, that didn't tie in with the archaeological evidence of stone tools in the western Sahara and Mediterranean coastal region which indicate human habitation much further west than the traditional routes suggest. For this reason, others had proposed a once-fertile Sahara with rivers running north to the Mediterranean. The problem was in working out how much water would have been in these rivers (and so whether they could have supported a human population) or where they were located.
Human migration north through Africa is contentious. This paper uses a novel palaeohydrological and hydraulic modelling approach to test the hypothesis that under wetter climates c.100,000 years ago major river systems ran north across the Sahara to the Mediterranean, creating viable migration routes. We confirm that three of these now buried palaeo river systems could have been active at the key time of human migration across the Sahara. Unexpectedly, it is the most western of these three rivers, the Irharhar river, that represents the most likely route for human migration. The Irharhar river flows directly south to north, uniquely linking the mountain areas experiencing monsoon climates at these times to temperate Mediterranean environments where food and resources would have been abundant. The findings have major implications for our understanding of how humans migrated north through Africa, for the first time providing a quantitative perspective on the probabilities that these routes were viable for human habitation at these times.
So it looks likely that the most westerly river provided the route across the Sahara, bringing our ancestor up to the Atlas Mountains and eventually to the shores of the Mediterranean in the area of the present-day Tunisia-Algeria border - further west than we normally assume but consistent with the stone tool evidence. The suggestion is that we then spread eastward along the coast to the Nile Delta and then into the Middle East and eventually into Europe and Asia, where we found our cousins, the Neanderthals and Denisovans who had been living there for some 200,000 years - the descendants of an earlier migration out of Africa by our immediate ancestors, H. heidelbergensis.
And so another piece of the jigsaw has been fitted into the fascinating human story of the last 100,000 years or so.