Agriculture in Africa began in the heart of the Sahara desert, 4 which by 5200 BC was much more humid and densely populated than today. Several native species were domesticated, the most important being pearl millet, sorghum and cowpea, which spread through West Africa and the Sahel. The Sahara at this time was like the Sahel is at present. Its wide open fields allow an easy cultivation, but they make impossible the intensive cultivation of the soil, since it is poor and the rain limited. Local crops were also not ideal and produced fewer calories than those in other regions. These factors limited surpluses and maintained sparse and dispersed populations.
Many empires grew around coastal areas or large rivers that serve as part of important trade routes. The kingdoms of Mali and the Songhai Empire grew along the Niger River between 1200 and 1590. Berber traders from the Sahel, a region south of the Sahara desert, traded copper, horses, weapons and clothing brought from North Africa on the camel trains.6 Trade with the Berbers, and other groups, led to the growth of the Ghanaian Empire, which exchanged gold, kola nuts, and slaves. Western Africans had a demand for salt, which is collected in the desert oases, and used to preserve food, as well as for seasoning, and due to the exponential growth of the population they had to increase the agricultural production they needed for their survival .
