he pharaohs of ancient Egypt owed their existence to prehistoric climate change in the eastern Sahara, according to an exhaustive study of archaeological data that bolsters this theory.
Starting at about 8500 B.C., researchers say, broad swaths of what are now Egypt, Chad, Libya, and Sudan experienced a "sudden onset of humid conditions."
. . .
But around 5300 B.C. this climate-driven environmental abundance started to decline, and most humans began leaving the increasingly arid region.
"Around 5,500 to 6,000 years ago the Egyptian Sahara became so dry that nobody could survive there," said Stefan Kröpelin, a geoarchaeologist at the University of Cologne in Germany and study co-author.
This has been kicking around for some time, largely due to many similarities between early Valley artifacts and those in the Sahara (see here for a couple references). I think it was either Fekri Hassan or Karl Butzer (probably the latter) who really pushed this idea of Western desert proto-agriculturalists moving into the Valley during a time of increased aridity, adopting some newly introduced domesticates from SW Asia and eventually producing the Neolithic/Predynastic. See also work by Angela Close and Fred Wendorf as well.