The report by Stefan Kropelin with the Institute of Prehistoric Archeology at the University of Cologne in Germany contradicts previous research suggesting the change from trees and lush grasslands to desolate desert occurred within a few centuries, The New York Times said Friday.

Kropelin's team analyzed layers of sediment in a core drilled from the bottom of Lake Yoa in northeastern Chad. He said the changing types of pollen that drifted to the bottom of the lake tell a story about the surrounding terrain.

The report was published in the journal Science.