<P> In the Aïr Mountains, present - day Niger, copper was smelted independently of developments in the Nile valley between 3000 and 2500 BC . The process used was not well developed, indicating that it was not brought from outside the region; it became more mature by about 1500 BC . </P> <P> A major migration of Sahel cattle farmers took place in the third millennium BCE, and the pastoralists encountered the developed hunter - gatherers of the Guinea region . Flint was considerably more available there and made the use of microliths in hunting far easier . The migration of the Sahel farmers was probably caused by the final desiccation of the Sahara desert in this millennium, which contributed greatly to West Africa's isolation from cultural and technological phenomena in Europe and the Mediterranean Coast of Africa . </P> <P> In the western Sahel, the rise of settled communities was largely the result of domestication of millet and sorghum . Archaeology points to sizeable urban populations in West Africa beginning in the 2nd millennium BCE . Symbiotic trade relations developed before the trans - Saharan trade, in response to the opportunities afforded by north - south diversity in ecosystems across deserts, grasslands, and forests . The agriculturists received salt from the desert nomads . The desert nomads acquired meat and other foods from pastoralists and farmers of the grasslands and from fishermen on the Niger River . The forest dwellers provided furs and meat . </P> <P> Tichit (Dhar Tichitt) and Oualata were prominent among the early urban centres, dated to 2000 BCE, in present - day Mauritania . About 500 stone settlements littered the region in the former savannah of the Sahara . Its inhabitants fished and grew millet . It has been found that the Soninke of the Mandé peoples were responsible for constructing such settlements . Around 300 BCE, the region became more desiccated and the settlements began to decline, most likely relocating to Koumbi Saleh . From the type of architecture and pottery, it is believed that Tichit was related to the subsequent Ghana Empire . Old Jenne (Djenne) began to be settled around 300 BCE, producing iron and with sizeable population, evidenced in crowded cemeteries . Living structures were made of sun - dried mud . By 250 BCE, Jenne was a large, thriving market town . </P>

What were the two most important commodities traded in early west african civilizations