The Sahara would seem to be an effective barrier for migration of anything other than birds. And so many scientists have assumed that early humans made their trek out of Africa---on their way to spread over the rest of the world---through the lush Nile River valley. However, there is little evidence that early humans actually took that route.
But there have been tantalizing clues that the Sahara has not always been the huge desert obstacle it is today. There are Nile crocodiles, fish and molluscs living in isolated Saharan oases, for example. How did they get there?
In a new study in PNAS, scientists from the United Kingdom say that the Sahara has gone through humid periods during which there have been lakes, rivers and inland deltas, all linked together and channeling water and creatures across the land. This vast waterway would have allowed the dispersal of animals, and with the animals, humans followed.
The last time this linked waterway was filled was in the early Holocene, around 10,000 years ago. The researchers examined the distributions of stone points and various Nilo-Saharan languages and found that the movement of humans during this last humid period was influenced by the movement of aquatic species; the human hunters were following their prey into what had been desert.
Whether humans were able to pass through the Sahara even earlier isn't clear---there just isn't enough data to show that there were earlier periods where the whole regions was wet---but there is evidence that a "green Sahara" may have existed around 100,000 to 125,000 years ago, around the time when modern humans were migrating out of the continent.