Six thousand years ago, the massive arid region dominating northern Africa was quite green, a patchwork of trees and savannas as well as many sparkling lakes.

The region, larger than Australia, also was inhabited, according to the European-US-Canadian team of scientists behind a study in Science dated May 9.

Most of the physical elements that could tell the tale of the Sahara's geographic evolution have been lost. The scientists studied layers of sediment in one of the largest remaining Sahara lakes, Yoa, in a remote spot in northern Chad, which took them back through six millennia of climate history.

They looked at sediments, did soil tests and reviewed biological indicators such as plant and tree pollen and spores that were present before the desert encroached. They also studied the remains of aquatic microorganisms.

Their findings contradicted previous modeling that indicated a rapid collapse of vegetation in the region in a sudden end to the African Humid Period, about 5,500 years ago, said Stefan Kropelin, a geologist at the Prehistoric Archaeology Institute of the University of Cologne who took part in the new study.

In 2000, a study by Peter de Menocal of Columbia University of sediments in the west of Mauritania found a sudden increase in wind-carried dust blown off the Sahara region, suggesting swift climate change.

But data from Lake Yoa shows the opposite, and the transition to desert took its time, said Kropelin. He said he believed de Menocal's data were not wrong but misinterpreted.

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