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Ancient humans may have trekked out of Africa to escape arid climes. This is the result suggested by a record of climate in East Africa spanning the past 200,000 years.

"It raises the possibility that drought, rather than rainy conditions, prompted early humans to migrate," says lead author Jessica Tierney at the University of Arizona.

Modern humans are widely agreed to have evolved in Africa and are thought to have migrated out 65,000 to 55,000 years ago. They may have left via East Africa and headed to Arabia, although this isn't settled.

Previous studies showed that many parts of Africa, like the Sahara, have had many wet and dry periods. A marine core, collected from the Gulf of Aden off Africa's east coast in 1965, has sediments dating to 200,000 years ago. Analysing this allowed Tierney and her colleagues to construct an extended timeline of climate shifts in north-east Africa.

They focused on a chemical called alkenone that is made by marine algae, whose composition changes as the temperature of the sea surface alters. In this way, they determined temperatures every 1600 years, going back 200,000 years.

The team also created a rainfall record by analysing the wax of leaves blown out to sea and buried in the sediment. Plants make subtly different leaf wax depending on rainfall.

Changing climes

Between 130,000 and 80,000 years ago, the team found that north-east Africa was warm and wet, in line with previous evidence. At this time, humans were expanding throughout Africa, leading some to suggest that the green Sahara made it easier for people to move out into Europe and Asia.

But between 75,000 and 55,000 years ago, the climate turned dry and cold - and genetic studies suggest this is when the major out-of-Africa migration happened. So instead of a comfortable climate making it easy to leave Africa, Tierney says, maybe the difficult climate made the move necessary.

The study successfully ties together all the disparate lines of evidence about past Saharan climates, says Nicholas Drake of King's College London.

However, the interpretation is less clear because the date of the out-of-Africa migration is highly contentious. Two recent studies found evidence that modern humans were in Sumatra at least 63,000 years ago and in Australia by 65,000 years ago, which implies an earlier exit. Other studies hint at multiple waves of dispersal dating back 130,000 years.

"I don't think we can really say that dispersal coincided with North Africa becoming drier and colder," says archaeologist Huw Groucutt at the University of Oxford.

Journal reference: Geology, DOI: 10.1130/G39457.1