Sedentary farming began in, or around the fifth millennium B. C, as well as the domestication of cattle. Though there is some uncertainty, some archaeologists believe that iron metallurgy was developed independently in sub-Saharan Africa (possibly in West Africa). Archaeological sites containing iron smelting furnaces and slag have been excavated at sites in the Nsukka region of southeast Nigeria in what is now Igboland: dating to 2000 BC at the site of Lejja (Eze-Uzomaka 2009) and to 750 BC and at the site of Opi (Holl 2009). Smelting furnaces appear in the Nok culture of central Nigeria by about 550 BC and possibly a few centuries earlier. Ironworking technology allowed an expansion of agricultural productivity, and the first city-states later formed. Northern tribes developed walled settlements and non-walled settlements that numbered at 400. In the forest region, Iron Age cultures began to flourish, and an inter-region trade began to appear. The desertification of the Sahara and the climatic change of the coast caused trade with upper Mediterranean peoples to be seen.