<P> The drying of the Sahara created a formidable barrier between the northern and southern portions of the continent . Two important exceptions were Nubian Sudan, which was linked to Egypt by the Nile and Ethiopia, which could trade with the northern regions over the Red Sea . Powerful states grew up in these regions such as Kush in Nubia (modern day Northern Sudan and Southern Egypt) and Aksum in Ethiopia . Especially from Nubia, ideas and technologies from the Middle East and Europe reached the rest of Africa . </P> <P> Historians believe that iron working developed independently in Africa . Unlike other continents Africa did not have a period of copper and bronze working before their Iron Age . Copper is quite rare in Africa while iron is quite common . In Nubia and Ethiopia, iron, trade, and agricultural surpluses lead to the establishment of cities and civilizations . </P> <P> Ordinarily, in the sparsely populated areas, this same period saw the expansion of the Bantu speaking peoples . The Bantu expansion began in Southern Cameroon around 4000 years ago . Bantu languages are spoken there today and there is archaeological evidence for incoming Neolithic farmers in Northern Gabon c. 3800 BC . It is known that Bantu expansion was extremely rapid and massive, but its exact engine remains controversial . This period predated iron, which appears in the archaeological record by 2500 BC . </P> <P> One of the early expansions of Bantu was the migration of the Bubi to Fernando Po (Bioko). They were still using stone technology at first . The difficulties of cutting down the equatorial forest for farming have led to the suggestion that the primary expansion was along river valleys, a hypothesis supported by studies of fish names . Another factor may have been the arrival of southeast - Asian food crops, notably the AAB plantain, the cocoyam and the water - yam . Linguistic reconstructions suggest that the only livestock possessed by the proto - Bantu was the goat . Over the centuries the entire southern half of Africa was covered with the group, excluding only the Kalahari desert . Their expansion only ended relatively recently . In the year 1000, Arab traders described that the Bantu had not reached as far as Mozambique, and European settlers observed the Bantu expansion into South Africa under the Zulu and others . </P>

Discuss the economic changes in africa during the last 100years with particular reference to nigeria