The vast Sahara desert was not always a vast desolate ocean of sand. At the end of the last Ice Age, the Sahara Desert was just as dry and uninviting as it is today. But sandwiched between two periods of extreme dryness were a few millennia of plentiful rainfall and lush vegetation. During these few thousand years, prehistoric humans left the congested Nile Valley and established settlements around rain pools, green valleys, and rivers.
Some 12,000 years ago, the only place to live along the eastern Sahara Desert was the Nile Valley. But around 10,500 years ago, a sudden burst of monsoon rains over the vast desert transformed the region into habitable land. This opened the door for humans to move into the area, as evidenced by new radiocarbon dates of human and animal remains from more than 150 excavation sites.
Then climate changed again. Recent studies have suggested the Sahara gradually shifted from savannah 6,000 years ago, possibly as a result of a tiny shift in its axis, to its now vast oceans of sand just 2,700 years ago.
This raises the question: how could the Amazon rain forest have survived if the Saharan dust was absent between 10,500 and 6,000 years ago? Science claims that the Amazon rain forest is around 55 million years old.
 Claussen et al: Simulation of an abrupt change in Saharan vegetation in the mid-Holocene in Geophysical Research Letters - 1999