Why did farming spread to north africa

Africa is a continent of great size, almost 12 million square miles or
about three times the size of the United States. Most of it lies in the
tropics and, although we often think of Africa in terms of its rain forests,
less than ten percent of the continent is covered by tropical forests, and
those are mostly in West Africa. Much of the African surface is covered by
savannas, or open grasslands, and by arid plains and deserts. In geological
terms, the continent is really formed by a series of high plateaus broken in
the east by the Great Rift valley and the mountains that surround it. Large
rivers - the Congo, the Nile, the Zambezi, and the Niger - begin in the
interior of the continent and flow to the sea over great falls and cataracts
that mark the passage from the plateau to the coast. These falls have
historically made movement from the coast to the interior difficult, but the
great river systems have also provided the interior of Africa with routes of
communication.