The first kingdom on this stretch of the Nile was a Nubian one

Political scale grew across the middle and northern reaches of the Middle Nile in the second half of the fourth millennium BCE, and it grew on both sides of what later became the Egyptian frontier. The largest polity between roughly 3500 and 3200 BCE may have had its centre in Nilo-Saharan-speaking country immediately south of Upper Egypt. Its capital and its royal burials were at Qustul in Lower Nubia, partway between the first and second cataracts, and its authority seems to have reached north past the first cataract into southern Upper Egypt. Take that seriously and the standard story of Egyptian state formation shifts. The first state of real size in the valley that became Egypt was ruled from Nubia by Nilo-Saharan-speaking kings. The unification under the First Dynasty around 3100 BCE then looks like the end of a process that had begun centuries earlier, upriver, in a place the textbooks decline to name.

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.