Wet spells

While about 40 per cent of hydrocarbons in today's dust come from water-dependent plants, this rose to 60 per cent, first between 120,000 and 110,000 ago and again from 50,000 to 45,000 years ago. So the region seemed to be in the grip of unusually wet spells at the time.

That may have been enough to allow sub-Saharan Stone Age Homo sapiens to migrate north: the first fossils of modern humans outside Africa date from 93,000 year ago in Israel. And both genetic analysis and archaeology show that humans didn't spread extensively beyond Africa until 50,000 years ago, suggesting a second migration at the time of the second wet spell.

Fossil record

Ian Tattersall of the American Museum of Natural History in New York is impressed by the findings. "They tie in approximately with the information we have from the fossil record."

Castañeda’s team is not the first to suggest that wet spells may have come in handy. Last year, Anne Osborne of the University of Bristol, UK, suggested that the first migrants may have used a now-buried network of river channels in the Libyan Sahara, which dates roughly 120,000 years.