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Human exodus from Africa helped by wetter weather

03.09.2007.
Source: Roxanne Khamsi, New Scientist

Our ancestors may have succeeded in migrating out of Africa thanks to a shift towards wetter weather that made it easier to find food while on the move.

Sediment samples recently taken from the bottom of Lake Malawi in Africa reveal that the continent experienced a series of long-lasting droughts that ended 70,000 years ago around the same time that Homo sapiens is thought to have started exiting the continent for Europe and Asia.

Genetic studies have provided evidence that early modern humans migrated outwards from Africa at this point in the past, but until now scientists have had few clues about what enable them to do so.

To better reconstruct a picture of the past Christopher Scholz at Syracuse University in New York State, US, and his colleagues analysed sediment samples from Lake Malawi in south-eastern Africa.

The team drilled deep into the mud at the bottom and obtained several sediment cores, including one 400-metre-long core measuring 3 inches in diameter. So far, his team has succeeded in analysing the top 80 metres of this sediment core, giving them information about climate patterns up to 145,000 years ago.

Stabilising climate

The analysis revealed that between 135,000 to 75,000 years ago the region experienced several periods of severe drought, causing water levels of the giant lake to shrink down to just 100 metres.

Scholz says that the climate in the region appears to have stabilised around 70,000 years ago with the lake around 700 m deep, with more consistent rainfall than before. He attributes this stabilisation to changes in the Earth's orbit around the Sun that astronomers have suggested occurred around that time.

The new finding overturns previous assumptions that the most severe droughts in tropical areas of Africa occurred around 18,000 years ago as a result of atmospheric changes related to the growth of glaciers elsewhere on the globe.

"People suspected drought in the past, but they had the timing and the causative mechanism all wrong," Scholz explains.

Conflicting evidence

He imagines that the shift towards wetter climes in Africa created more lush landscapes that made it easier for early modern humans to find food and traverse tough terrain as they migrated towards Europe and Asia.

The sediment samples from Lake Malawi give us a more detailed view of prehistoric ecological conditions, says Kaye Reed at Arizona State University, Tempe, Arizona. She cautions, however, that one should not automatically generalise about the climate across the whole continent based on a few sediment samples: "We know today that there are several different types of habitats within Africa caused by different patterns of climate."

On the other hand, another new study which examined cave formations in the Negev Desert in Israel found evidence that a period of drought began in that region starting 110,000 years ago, suggesting that the climate changes seen by Scholz's group might have applied across the continent and beyond (Geology (DOI: 10.1130/G23794A.11).

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0703874104)