Climate change is thought to explain mankind’s exodus from Africa to colonise other parts of the world.

A switch from drought to wetter conditions led to a population expansion and the spread of early humans to other continents about 70,000 years ago.

The prolonged period of drought some 90,000-135,000 years ago had created such stresses on Homo sapiens that the population had crashed, researchers believe. This could help to explain why mankind is thought to be descended from a relative handful of people in Africa.

“We’ve got an explanation for why that might have occurred,” said Professor Andrew Cohen, of the University of Arizona.

“Tropical Africa was extraordinarily dry about 100,000 years ago. Archaeological evidence shows relatively few signs of human occupation during the mega-drought period.”

The idea was consistent with previous studies, which have suggested an earlier exodus about 125,000 years was “ultimately unsuccessful”. Researchers concluded that there was an ecological reason for the human exodus after sediment and fossil samples revealed that Africa had been racked by a series of prolonged droughts, which were “more extreme and widespread” than any other dry periods identified in tropical Africa and would have devastated plant and animal life.

Sediment from core samples of Lake Malawi, one of the world’s deepest lakes, revealed that water levels dropped by at least 1,986ft (600m).

Lake Malawi is 2,316ft deep today but during the mega-drought, which researchers said was the most likely explanation for the drop in water levels, it fell to just 410ft deep.

During this period the land around the lake turned into semi-desert, an arid scrubland habitat with limited resources for primates - in stark contrast to the lush vegetation today.

In their report, published in the journal Proceedings of the National Academy of Sciences, the scientists said: “The end of arid conditions in tropical Africa closely coincides with the onset of aridity elsewhere on the continent.

“Thus, a likely period for human population expansion out of equatorial Africa would have been during the climatic ‘crossover’ time, between 90 and 70 thousand years ago.”

The interval was long enough, they said, to tally with other evidence suggesting that Homo sapiens reached Australia about 50,000 years ago.

The core samples provided a record of the quantities and types of plankton, invertebrates and pollen that dropped to the bottom of the lake. Species found during the mega-drought period lived only in shallow, algae-rich waters indicating turbid waters. Lake Malawi, in the Great Rift Valley, would have been “algae-filled and pea-soup green”. From 90,000 to 70,000 years ago, the water levels in the lake rose to current levels.

Scientists involved in the study said the discovery of mega-droughts suggested that cichlid fish, for which the lake is famous, evolved much slower than previously believed. It had been thought that the fish had evolved 15,000-25,000 years ago but the core samples suggested that it could have originated from as long ago as 90,000 years.

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