In An African Classical Age (1998) he argues for a conception of the period from 1000 BC to 400 AD in East Africa as a "classical age" during which a variety of major technologies and social structures first took shape. His Civilizations of Africa: A History to 1800 (2002), brings together the whole of African history from the close of the last ice age down to the end of the eighteenth century. With the archaeologist Merrick Posnansky, he also edited The Archaeological and Linguistic Reconstruction of African History (1982), at that time a state-of-the-field survey of the correlation of linguistic and archaeological findings in the different major regions of the continent.