The Sahara Desert has long been considered a forbidding and impenetrable barrier to human migration from southern Africa, but new evidence suggests there were three brief intervals of time when it transformed into a vast green savannah which provided the ideal conditions for the early movement of ancient humans.

According to scientists at McMaster and the University of New Mexico, one of those intervals coincides precisely with the age of the oldest known remains of modern humans—dating back roughly 315,000 years ago—which were discovered recently by another team of researchers in Morocco.

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