"Who would have thought that as recently as 70,000 years ago, extremes of climate had reduced our population to such small numbers that we were on the very edge of extinction," added Meave Leakey, Professor of Paleontology at Stony Brook University.

An international team studied the maternally-transmitted mitochondrial DNA from subjects in southern and eastern Africa.

Paleontology has revealed that our species originated in Africa approximately 200,000 years ago.

Previous studies using mitochondrial DNA have traced modern humans to a single 'Eve' alive at that time.

This latest study shows that that humans separated into small populations prior to the Stone Age.

One reason for this could be a series of severe droughts that struck Eastern Africa around 100,000 years ago.

When conditions improved, the isolated populations began to come together and eventually left Africa to colonise the world.

"It was only around 40,000 years ago that they became part of a single pan-African population, reunited after as much as 100,000 years apart," said Doron Behar, of the Rambam Medical Center at Haifa.

From that tiny population of 2,000 individuals, the human race has now grown to around 6.6 billion, according to the latest estimates.

"This study illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species' history," said Spencer Wells, National Geographic Society explorer in residence.

"Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world."

"Truly an epic drama, written in our DNA."