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By Nidhi Subbaraman

The first humans to exit Africa 100,000 years ago crossed the Sahara to leave the continent. Hardly the desert it is now, that land was grassy and lush, greened by shifting monsoons and flowing rivers. It was those comfy conditions that likely drew northbound wanderers towards the first and most successful of human migrations out of the continent.

For three months of the year, three river systems spanned the North African desert and cut through dunes, turning their watersheds into swampy areas, a team of geographers report in a new study in PLOS ONE.

Using the computer modeling techniques used to forecast future climate patterns, they turned their gaze to the past, "hindcasting" for a precise, monthly read of rainfall and temperature in northern Africa 100,000 years ago. 

"The fact that people got out at this time and in quite an explosive way — that probably reflects an opportunity," Mike Rogerson, an author of the new study and a lecturer in Earth science and paleoclimate at the University of Hull in the U.K., told NBC News. 

Rainfall in the mountains would have taken three months to flow north towards the North African coast, evaporating along the way, and turning the surroundings into "great big swampy zones rather than a neat river system like the Nile," he said. The grass and shrubbery, and the grazing animals that fed on them, would have tided over the few months of dry spells, offering an exit route that was favorable to traveling humans all year round. 

The earliest human-made artifacts found in regions of the eastern Mediterranean date back 100,000 years, indicating that the first migration event took place then. (Homo sapiens wasn't the first to leave — our hominid cousins had left in prior millennia.) Dry river beds in North Africa, first spotted by aerial radar surveys, proposed a solution to the question: How would humans have survived the rough road through an arid desert? Answer: They didn't have to.

"It lined up very conveniently," Rogerson said of the landscape change. With the new study, "we've put a lot flesh on the skeleton" of a hypothesis. 

If the Sahara hadn't greened over, would humans have conquered the globe as successfully as they did? One way or another, "people would have got out," Rogerson said. 

Tom Coulthard, Jorge Ramirez, Nick Barton, Mike Rogerson and Tim Bruchare are authors of "Were Rivers Flowing across the Sahara During the Last Interglacial? Implications for Human Migration through Africa" published in the Sept. 11 issue of PLOS ONE.

Nidhi Subbaraman writes about science and technology. Follow her on Facebook, Twitter and Google+