What event that began 3000 years ago is linked to the spread of cultures in africa

By about 3000 B.C., much of the area was desert. The
droughts that have recently affected Africa indicate that the desiccation, or
drying up, of the Sahara is continuing and the desert is growing.

As the Sahara became less habitable, the populations moved north toward
the Mediterranean coast and south into the area of the dry sahel, or fringe,
and, especially, onto the grassy savannas suitable for agriculture and
grazing. Savannas stretch across Africa from the mouth of the Senegal River on
the west coast to Lake Chad and the Upper Nile valley. This broad region, the
Sudan, became a center of cultural development. The movement of peoples into
the Sudan and toward the Nile valley and the Mediterranean set the stage for
major developments in the subsequent history of Africa.

Agriculture, Iron, And The Bantu Peoples

Agriculture may have developed independently in Africa, but many scholars
believe that the spread of agriculture and iron throughout Africa linked that
continent to the major centers of civilization in the Near East and
Mediterranean world. The drying up of the Sahara had pushed many peoples to
the south into sub-Saharan Africa. These were the ancestors of the Negro
peoples. They settled at first in scattered hunting-and-gathering bands,
although in some places near lakes and rivers people who fished, with a more
secure food supply, lived in larger population concentrations. Agriculture
seems to have reached these people from the Near East, since the first
domesticated crops were millets and sorghums whose origins are not African but
West Asian. The route of agricultural distribution may have gone through Egypt
or Ethiopia, which long had contacts across the Red Sea with the Arabian
peninsula. There is evidence of agriculture prior to 3000 B.C.

Once the idea of planting diffused, Africans began to develop their own
crops, such as certain varieties of rice, and they demonstrated a continued
receptiveness to new imports. The proposed areas of the domestication of
African crops lie in a band that extends from Ethiopia across the southern
Sudan to West Africa. Subsequently, other crops, such as bananas, were
introduced from Southeast Asia, and in the 16th century A.D. American crops,
such as maize and manioc, spread widely throughout Africa.