Humans came 'close to extinction'

Volcanic ash may have caused a worldwide winter

By Dr David Whitehouse BBC News Online science editor

A new hypothesis about recent human evolution suggests that we came very 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 a so-called 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 to grow 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," Professor Ambrose said.