Between 1000 and 500 B.C.E., a classical period took shape in several centers of Asia, northern Africa, and southern Europe. These centers lasted until roughly the 5th century C.E. The classical civilizations built on the achievements of earlier river valley civilizations. Each major classical civilization overlapped one or more of the earlier centers geographically. The pace of change stepped up. Innovations were dramatic. Empires developed as a political form, and great thinkers and religious leaders drew traditional cultural elements together in striking new statements that were the bases for whole new cultures.