The history of East Africa has been divided into its prehistory, the major polities flourishing, the colonial period, and the post-colonial period, in which the current nations were formed. East Africa is the eastern region of Africa, bordered by North Africa, Central Africa, Southern Africa, the Indian Ocean, and the Sahara Desert. Colonial boundaries are reflected in the modern boundaries between contemporary East African states, cutting across ethnic and cultural lines, often dividing single ethnic groups between two or more states.