The grasslands die around 3400 BCE and the survivors build Egypt

Around the thirty-fourth century BCE the climate of the eastern Sahara turned sharply drier and stayed that way. Steppe and semidesert grassland that had carried herds for millennia became the desert it is now, though patches held on well enough to support marginal livestock raising as late as about 2000 BCE. The pastoralists living out there had to go somewhere. They moved out of the emerging desert and settled among the communities already living along the Nile. That is the event that produced the Egypt of the textbooks, the thin green ribbon dependent on an annual flood. Before it, this was open country with mobile herders and a river running through it. After it, thousands of people and their cattle were compressed onto a floodplain a few kilometres wide. The competition for land and the density of population that followed drove polities to grow and swallow one another until the whole valley was unified around 3100 BCE.