Early civilizations emerged across Africa beginning around 4600 years ago. Nomadic groups migrated across the continent, spreading agriculture and ironworking. States developed in West Africa along trade routes for gold and salt, including the empires of Ghana and Mali. In East Africa, coastal city-states engaged in Indian Ocean trade, developing Swahili culture. Further south, stateless societies organized along family and clan lines, building monumental structures like Great Zimbabwe. Across the continent, African cultures developed rich artistic, musical, architectural and oral traditions.