Centuries BEFORE the coming of Islam and before the days of the Songhay, Mali and Ghana Empires (and thus before Timbuktu was built), there were some massive cities in the Niger Delta region which rivaled those of Mesopotamia

Historian Basil Davidson looks how kingdoms such as Ghana, Mali and Songhay provided a strong centralised governmental structure within which trade across West Africa thrived. In particular, he looks at the great city of Timbuktu and the influence of Islam.
http://www.lincoln.edu/history/his307/