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Today, all non-Africans are known to have descended from a small group of people that ventured into Eurasia around 50,000 years ago. However, fossil evidence shows that there were numerous failed dispersals before this time that left no detectable traces in living people.

In a paper in Nature, new evidence explains for the first time why those earlier migrations didn't succeed. A consortium of scientists led by Prof. Eleanor Scerri of the Max Planck Institute of Geoanthropology in Germany, and Prof. Andrea Manica of the University of Cambridge has found that before expanding into Eurasia 50,000 years ago, humans began to exploit different habitat types in Africa in ways not seen before.

"We assembled a dataset of archaeological sites and environmental information covering the last 120 thousand years in Africa. We used methods developed in ecology to understand changes in human environmental niches, the habitats humans can use and thrive in, during this time," says Dr. Emily Hallett of Loyola University Chicago, co-lead author of the study.

"Our results showed that the human niche began to expand significantly from 70,000 years ago, and that this expansion was driven by humans increasing their use of diverse habitat types, from forests to arid deserts," adds Dr. Michela Leonardi of London's Natural History Museum, the study's other lead author.

"This is a key result," explains Professor Manica. "Previous dispersals seem to have happened during particularly favorable windows of increased rainfall in the Saharo-Arabian desert belt, thus creating 'green corridors' for people to move into Eurasia. However, around 70,000–50,000 years ago, the easiest route out of Africa would have been more challenging than during previous periods, and yet this expansion was sizable and ultimately successful."

Many explanations for the uniquely successful dispersal out of Africa have been made, from technological innovations to immunities granted by admixture with Eurasian hominins. However, no technological innovations have been apparent, and previous admixture events do not appear to have saved older human dispersals out of Africa.

Here the researchers show that humans greatly increased the breadth of habitats they were able to exploit within Africa before the expansion out of the continent. This increase in the human niche may have been a result of positive feedback of greater contact and cultural exchange, allowing larger ranges and the breakdown of geographic barriers.

"Unlike previous humans dispersing out of Africa, those human groups moving into Eurasia after approximately 60–50,000 years ago were equipped with a distinctive ecological flexibility as a result of coping with climatically challenging habitats," says Prof. Scerri. "This likely provided a key mechanism for the adaptive success of our species beyond their African homeland."