<P> The history of West Africa began with the first human settlements around 4,000 BCE . It has been commonly divided into its prehistory, the Iron Age in Africa, the major polities flourishing, the colonial period, and finally the post-independence era, in which the current nations were formed . West Africa is west of an imagined north - south axis lying close to 10 ° east longitude, bordered by the Atlantic Ocean and Sahara Desert . </P> <P> Colonial boundaries are reflected in the modern boundaries between contemporary West African states, cutting across ethnic and cultural lines, often dividing single ethnic groups between two or more states . Early human settlers arrived in West Africa around 12,000 BCE . In the fifth millennium, as the ancestors of modern West Africans began entering the area, the development of sedentary farming began to take place in West Africa . The Iron industry, in both smelting and forging for tools and weapons, appeared in Sub-Saharan Africa by 1200 BCE, and by 400 BCE, contact had been made with the Mediterranean civilisations, and a regular trade included exporting gold, cotton, metal, and leather in exchange for copper, horses, salt, textiles, and beads . Culture developed further with the Nok culture (1000 BCE to 200 or 300 BCE), Serer people's ancient history, and construction of the Senegambian stone circles (between the third century B.C. and the sixteenth century C.E.). The Sahelian kingdoms were a series of kingdoms or empires that were built on the sahel, the area of grasslands south of the Sahara . They controlled the trade routes across the desert, and were also quite decentralised, with member cities having a great deal of autonomy . The Ghana Empire may have been established as early as the 4th century CE . It was succeeded by the Sosso in 1230, the Mali Empire in the 13th century CE, and later by the Songhai and Sokoto Caliphate . There were also a number of forest empires and states in this time period . </P> <P> Following the collapse of the Songhai Empire, a number of smaller states arose across West Africa, including the Bambara Empire of Ségou, the lesser Bambara kingdom of Kaarta, the Fula / Malinké kingdom of Khasso (in present - day Mali's Kayes Region), and the Kénédougou Empire of Sikasso . European traders first became a force in the region in the 15th century . The transatlantic African slave trade resumed, with the Portuguese taking hundreds of captives back to their country for use as slaves; however, it would not begin on a grand scale until Christopher Columbus's voyage to the Americas and the subsequent demand for cheap colonial labour . As the demand for slaves increased, some African rulers sought to supply the demand by constant war against their neighbours, resulting in fresh captives . European, American and Haitian governments passed legislation prohibiting the Atlantic slave trade in the 19th century, though the last country to abolish the institution was Brazil in 1888 . </P>

Which area of medieval africa was most associated with the production of salt