Human beings may have had a brush with extinction about 70,000 years ago, an extensive genetic study suggests.
The human population at that time had been gradually reduced to small isolated groups across eastern and southern Africa, apparently because of massive droughts lasting tens of thousands of years, according to an analysis released Thursday.
The report notes that a separate study by researchers at Stanford University estimated the number of early humans may have shrunk as low as 2,000 before numbers began to rapidly expand again in the period known as the Late Stone Age.
"This study illustrates the extraordinary power of genetics to reveal insights into some of the key events in our species' history," Spencer Wells, National Geographic Society explorer in residence, said in a statement.
"Tiny bands of early humans, forced apart by harsh environmental conditions, coming back from the brink to reunite and populate the world," he added. "Truly an epic drama, written in our DNA."
Wells is director of the Genographic Project, launched in 2005 to study anthropology using genetics. The report was published in the American Journal of Human Genetics.
Previous studies using mitochondrial DNA — which is passed down through mothers — have shown that all modern humans share one female ancestor (out of thousands), the "mitochondrial Eve," who lived in Africa about 200,000 years ago.
The migrations of humans out of Africa to populate the rest of the world appear to have begun about 60,000 years ago, but little has been known about humans between the time of mitochondrial Eve and that dispersal.
The new study looks at the mitochondrial DNA of the Khoi and San people in South Africa, formerly known as Hottentots and Bushmen, who appear to have diverged from other people between 90,000 and 150,000 years ago.
The implication is that the Khoi and San were not the only divergent group at the time, but that other groups all reconnected to form a pan-African population about 40,000 years later.
[That would imply that the reconnection took place after at least one divergent group had already left the continent to become the ancestors of Asians, Europeans, Australians, Polynesians and Native Americans.
Genetic studies lend credence to that last scenario, as non-Africans have significantly less genetic diversity than Africans.
Sister species of humans already existed in Eurasia, such as Homo neanderthalensis in Europe and the Middle East. The assumption is that modern humans drove them to extinction as they expanded across the continent.
As for the Khoi and San, they remained isolated in southern Africa until relatively recently, when Bantu-speaking herders and farmers gradually moved south in the first millennium A.D, followed by a much greater and swifter wave after 1400 A.D.]
The researchers, led by Doron Behar of Rambam Medical Center in Haifa, Israel and Saharon Rosset of IBM T.J. Watson Research Center in Yorktown Heights, N.Y., and Tel Aviv University, concluded that humans separated into small populations prior to the Stone Age, when they came back together and began to increase in numbers and spread to other areas.
Eastern Africa experienced a series of severe droughts between 135,000 and 90,000 years ago, and the researchers said this climatological shift may have contributed to the population changes, dividing into small, isolated groups which developed independently.
Paleontologist Meave Leakey, a Genographic adviser, commented: "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?"
Today more than 6.6 billion people inhabit the globe, according to the U.S. Census Bureau.
The research was funded by the National Geographic Society, IBM, the Waitt Family Foundation, the Seaver Family Foundation, Family Tree DNA and Arizona Research Labs.