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After Near Extinction, Humans Split Into Isolated Bands

Amitabh Avasthi
for National Geographic News
April 24, 2008
 
After nearly going extinct 150,000 years ago, humankind split into small groups—living in isolation for nearly a hundred thousand years before "reuniting" and migrating out of Africa, a new gene study says.

At one point our species may have been down to as few as 2,000 individuals, probably due to climate change—a longstanding theory bolstered by the new findings.

The research fills a gap in our understanding of what was happening in Africa before humans first left the continent.

"The assumption has always been that the original population [in sub-Saharan Africa] was very small but probably a single population," said Spencer Wells, head of the Genographic Project, which oversaw the study.

"Turns out, that is not the case."

(The National Geographic Society owns National Geographic News and funds the Genographic Project.)

The study appears in the current issue of the American Journal of Human Genetics.

Separate Ways

Around 200,000 years ago, modern humans emerged as a distinct species. All people alive today can trace their ancestry back to these humans, according to previous studies.

By the time the first great migrations out of Africa began, around 60,000 years ago, humanity had split into distinct populations with unique genetic lineages.

So what happened between 200,000 years ago and 60,000 years ago?

To find out, Wells and his colleagues analyzed 624 complete genomes of mitochondrial DNA—which is passed down from mothers—from various indigenous populations across sub-Saharan Africa. A genome is a person's complete set of DNA (quick overview of human genetics).

The researchers tracked mutations in genetic samples. Samples from Khoisan hunter-gatherers in South Africa were particularly revealing—perhaps not surprising, as the Khoisan have some of the oldest paternal and maternal lineages among modern humans.

The team found that a population that had probably originated in eastern Africa split about 150,000 years ago. One group went south, the other northeast.

Humankind "remained apart for nearly a hundred thousand years and then about 40,000 years ago, [then] reunited to become part of a single pan-African population," said Doron Behar, a Genographic associate researcher based at Rambam Medical Center in Haifa, Israel.

We Almost Didn't Make It

While it is not fully clear why the populations split in the first place, climate change may have played a role, the researchers say. (Related: "Did Climate Change Trigger Human Evolution?" [February 2, 2006].)

"There seems to have been some major climatic events that probably contributed to the separation," said Wells, pointing to evidence that Lake Malawi, in what is now Mozambique, went through a series of severe droughts during that time.

"The population size was driven down to probably as low as 2,000 individuals, perhaps—just a few hundred individuals in each population," Wells added.

"We were on the brink of extinction."

Once the rough climatic conditions let up, the populations apparently expanded and ultimately moved out of Africa—perhaps helped by the new tools and technologies of the late Stone Age.

Often Overlooked

Colin Groves is an anthropologist at the Australian National University in Canberra.

He says the findings "remind us that, before the spread of Homo sapiens out of Africa, things were nonetheless going on within Africa, involving population splits.

"Human diversity within Africa is fascinating, and with the previous focus on non-African peoples, it has tended to be overlooked or forgotten," he added.

But archaeologist Peter Forster says, "The conclusions do not strike me as being particularly new.

"We published along these lines since 1997. But it's good to know that independent analyses come to similar conclusions."

Like the Genographic scientists, Forster, of Britain's Anglia Ruskin University, has analyzed mitochondrial DNA to study human migrations out of Africa.

Medical Potential?

Though the new findings are not intended for medical use, an outside expert suggested that they may have medical potential.

"This [study] will be used as a reference for future genetic research on African populations, including disease studies," said Antonio Torroni, a geneticist at the University of Pavia in Italy, who was not involved in the new research.

"It is essential not only for reconstructing the history of specific African populations, but it is also an essential requirement for reliable association studies between various mitochondrial DNA [lineages] and complex disorders linked to aging, athletic performances, diabetes, Parkinson's disease, and Alzheimer's disease."
 

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