April 24, 2008

Humans Narrowly Escaped Extinction 70,000 Years Ago

An extensive genetic study released Thursday said that humans may have narrowly escaped extinction brought on by a drought some 70,000 years ago, after which the entire population was reduced to small isolated groups in Africa. 

The report notes that a separate Stanford University study estimated the number may have dwindled to as low as 2,000 before expanding 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, according to an Associated Press report.

"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."

Wells serves as director of the Genographic Project, which studies anthropology using genetics.   The initiative was launched three years ago.

Previous studies using mitochondrial DNA passed down through mothers had traced modern humans to a single "mitochondrial Eve" who inhabited Africa 200,000 years ago.

But the migrations of humans out of Africa to the rest of the world appear to have started about 60,000 years ago, with little understood about the time between Eve and that dispersal.

The new study examined mitochondrial DNA of South Africa's Khoi and San people, which seem to have diverged from other humans between 90,000 and 150,000 years ago.

The researchers  concluded that humans had separated into small populations prior to the Stone Age, when they came back together, grew in number and began migrating to other areas of the world.

Between 135,000 and 90,000 years ago, areas of Eastern Africa experienced a series of severe droughts.  The researchers believe this shift in climate may have played a role in the populations dividing into small, isolated groups that developed independently of one another.

"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," Paleontologist Meave Leakey, a Genographic adviser told the Associated Press.

More than 6.6 billion people inhabit the globe today, according to the U.S. Census Bureau.

The research was 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.  It was funded by the National Geographic Society, IBM, the Waitt Family Foundation, the Seaver Family Foundation, Family Tree DNA and Arizona Research Labs.

The study was published in the April 24 issue of American Journal of Human Genetics.


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