When you just look at the modern map it's absurd to think about the number of people that live in Africa north of Sahara. It looks like a wasteland that stretches all the way to the coast.
Also, with this timeline, it seems plausible that the earliest Egyptian empires may have been around just early enough to actually experience this desertification. No doubt countless tribal peoples had (or have) oral histories describing how their homeland was eaten away by desert.