By Roland Pease

BBC science unit, San Francisco



The sediments are an archive of past climate conditions

The evidence comes from sediments drilled up from the beds of Lake Malawi and Tanganyika in East Africa, and from Lake Bosumtwi in Ghana.

It shows equatorial Africa experienced a prolonged period of drought.

It is possible, scientists say, this was the reason some of the first humans left Africa to populate the globe.

Certainly, those who remained on the continent at that time would have had to be extremely resilient to make it through such hard times.

"This was a profound impact on the landscape," said Christopher Scholz, from Syracuse University, US.

"So it must have had a major impact, not just on humans but on all species in equatorial Africa at this time."

Tight group

Dr Scholz presented data from the drilling project here at the Fall Meeting of the American Geophysical Union.

The cores reveal that prior to 75,000 years ago, Lake Malawi, which is currently an inland sea some 550km long and 700m deep, was reduced to a couple of pools no more than 10km across and 200m deep.

Only a prolonged continent-wide drought could have had this effect. What makes the timing so fascinating is that it ties in with the "Eve hypothesis" of human evolution.

Genetic studies suggest modern human society is descended from a group of around 10,000 individuals who lived in East Africa at the time of this crisis.

Immediately after its end, human populations started to expand rapidly - and many of our ancestors began moving out of Africa and into the Middle East, Asia and Europe.

Driving force

Scientists are increasingly convinced that tragedies in the deep past have shaped human evolution.

The intriguing thought is that we owe our existence to a small band of survivors who clung on to life during a crisis of epic proportions or who simply decided they had to move to find water.

Viewed from space: Lake Bosumtwi is in an old impact crater

"And, also, there may be a connection with the exodus of early modern humans out of Africa and this climatic release.

"There's been recognition that speciation of hominids is controlled by environmental factors - whether that's long-term changes in aridfication in Africa or perhaps the dramatic increase in variability in environmental conditions, such as in precipitation, temperature, and so forth."