About 70,000 years ago mankind came close to being wiped out by a 'volcanic winter'. Our science editor Dr David Whitehouse reports.

A new hypothesis about recent human evolution suggests that humans came close to extinction because of a 'volcanic winter' that occurred 71,000 years ago.

Some scientists estimate that there may have been as few as 15,000 humans alive at one time.

The 'volcanic winter' lasted about six years. It was followed by 1,000 years

of the coldest Ice Age on record. It brought widespread famine and death to human populations around the world. It also affected subsequent human evolution.

This was because of the 'bottleneck' effect. The rapid decrease, in our ancestors' populations, in turn, brought about the rapid 'differentiation' - or genetic divergence - of the surviving populations.

The idea is being advocated by Professor Stanley Ambrose of the University of Illinois. He believes that the eruption of Mount Toba in Sumatra caused the bottleneck.

"Modern human races may have diverged abruptly, only 70,000 years ago," he writes in the Journal of Human Evolution.

Geneticists have thought for some time that humans passed through a recent evolutionary bottleneck but they had little idea what may have caused it.

Scientists believe that an eruption of Toba caused a volcanic winter that lasted six years and significantly altered global climate for the next 1,000 years.

During those six years there was substantial lowering of global temperatures, drought and famine. No more than 15,000 people survived.

When better conditions returned the human population was able togrow once more and develop the genetic diversity we see today.

"When our African recent ancestors passed through the prism of Toba's volcanic winter, a rainbow of differences appeared," Professors Ambrose adds.

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