Humans on earth

Human beings may have had a brush with extinction 70,000 years ago, an extensive genetic study suggests.
The human population at that time was reduced to small isolated groups in Africa, apparently because of drought, 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 expand again in the early 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. Truly an epic drama, written in our DNA." The report was published in the American Journal of Human Genetics.

Previous studies using mitochondrial DNA -- which is passed down through mothers -- have traced modern humans to a single "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 Eve and that dispersal.

The new study looks at the mitochondrial DNA of the Khoi and San people in South Africa, who appear to have diverged from other people between 90,000 and 150,000 years ago.

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, New York, and Tel Aviv University concluded that humans separated into small populations before 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 researchers said this climatological shift may have contributed to the population changes, dividing into small, isolated groups that developed independently.

Paleontologist Meave Leakey, a Genographic adviser, said: "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.

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