Answer :
Since the land started to change into an inhabitable desert the old inhabitants need to migrate to a place they could survive, Since their was no water, food or vegetation.
When the Sahara started to decline the people living there encountered the ferocious heat during the day and freezing temperatures during the night. They wanted to move east or south to look for remaining plains or forests to live in before the Sahara became a desert it was filled with vegetation and life. When the people moved to their new homes it most likely made them more cautious and careful with their new resources. Over the course of 5000 years, the amount of rain falling over the Sahara drastically decreased. As most of the Sahara is towards the North and mostly inland, it means that less water falls there. The Sahara is huge, covering ~9,000,000 square kilometers. It is called Sahara, because that is translated into desert. The Sahara is mainly stone plateaus, sand seas, and mostly sand dunes. Wind and little rain shapes most of the desert features. Because of this, the area has little suitable places to farm, has little vegetation, and the little vegetation it has consists of shrubs and little grasslands. The place is extremely hot during the days and has a dramatic contrast during the night. Because of all this, many people that saw that the desert is a harsh area to live in, try to move closer to plains and forests that are still inside Africa. This led to a spread of people from Northern Africa to other parts of the land.