Most of the Sahara was a land of lakes and rivers full of fish. There were meadows, forests, and fertile valleys offering sustenance for a large human and animal population. It staggers the mind to discover that fish harpoons have been found in many areas of the Sahara. In remote regions, rock carvings and paintings exist by the tens of thousands. Some of the cattle in these paintings wore discs between the horns as in Egyptian paintings.
Some rock drawings show Phoenician work or influence.
Pottery, tools, and polished stone weapons have been found in abundance. The desert did not form gradually over long eons of time. Other kinds of evidence show the same kind of very rapid growth or development. When you take a tour through a cave, you have apparent evidence right before your eyes of stalagmites and stalactites that took many millions of years to form. Little specifically seems to be known about the growth rate, except that it is a vaguely slow process.