Video from 2017.
"As little as 6,000 years ago, the vast Sahara Desert was covered in grassland that received plenty of rainfall, but shifts in the world's weather patterns abruptly transformed the vegetated region into some of the driest land on Earth. Now a researcher is trying to uncover the clues responsible for this enormous climate transformation -- and the findings could lead to better rainfall predictions worldwide." Source: Science Daily (2016).
Notes: The Sahara was probably turned into a desert by a change in the position of the Hadley Cell, "an atmospheric circulation pattern in the tropics that produces winds called the tropical easterlies and the trade winds," according to Windows To The Universe.
"The Hadley circulation is a tropical atmospheric circulation that rises near the equator. It is linked to the subtropical trade winds, tropical rainbelts, and affects the position of severe storms, hurricanes, and the jet stream. Where it descends in the subtropics, it can create desert-like conditions." (Science Daily)
This theory fits in with the general picture we have of how Egypt was first inhabited. Six-thousand years ago -- roughly 4,000 BC -- the northern African grasslands were populated by small bands of Stone Age hunter-gatherers (the Neolithic period in predynastic Egypt is thought to have started around 6,000 BC). Then, when the trade winds and rain belts shifted, transforming the Sahara into a desert, these prehistoric peoples migrated to the Nile region where the Archaic or Early Predynastic Period started after the unification of Upper and Lower Egypt around 3000 BC.