African south of the Sahara lived largely in nomadic, hunter-gatherer groups up until 200 BC. As a result, African populations were very sparse. There are several speculations as to why sub-Saharan Africans remained in hunting-gathering groups, but they are all guess-work. Perhaps the most reasonable explanations involve the abundance of resources and the protection that their isolation gave them from invasion and migration pressures.
Still, early sub-Saharan Africans developed metallurgy at a very early stage, possibly even before other peoples. Around 1400 BC, East Africans began producing steel in carbon furnaces (steel was invented in the west in the eighteenth century). The Iron Age itself came very early to Africa, probably around the sixth century BC, in Ethiopia, the Great Lakes region, Tanzania, and Nigeria. Iron technology, however, only spread slowly across Africa; it wasn't until the first century AD that the smelting of iron began to rapidly diffuse throughout the continent.
The instrument of that spread was the Bantu migrations. Bantu is a family of languages that are closely related and represent the largest linguistic family of African languages. Bantu speaking people migrated out of north-central Africa in the last century BC and these migrations continued all throughout the first millenium AD. They migrated south into the rain forest regions around the Congo and they migrated east into the East African highlands. Wherever they migrated, they imposed their language, which mixed with and replaced indiegenous languages. How they managed to impose their language on such a wide range of people across such a huge swathe of territory is anyone's guess. Further migrations in the first millenium then displaced the earlier Bantu immigrants, who pushed farther east and south. These Bantu immigrants would eventually found the civilization of the Mwenumatapa, or "Great Zimbabwe" civilization. Not only did the Bantu spread iron-smelting techniques across Africa, they also were responsible for diffusing agriculture, particularly agriculture of high-yield crops such as yams, bananas, and plantains. The spread of agriculture led to the explosive growth of village life all throughout Africa.
Urban settlement began at a very early date in Africa. The earliest urban settlement were stone-walled towns in southern Mauritania that date back to sometime in the second millemium BC. An explosion of urban settlement in the Sahel region immediately south of the Sahara began between 600 and 200 BC. The Sahel is a hot, dry savannah that can support human agriculture and settlement. The first urban settlements were Sahelian: Jenne, Gao, and Kumbi (later Kumbi Saleh, the capital of the kingdom of Ghana). All of these urban centers grew up in oasis and river regions which could support such large populations.