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Genetics Suggest Population Expansion in Africa Began in Stone Age

NEW YORK (GenomeWeb News) – Modern human populations started expanding some 40,000 years ago, according to a paper appearing appeared online today in PLoS ONE.

Researchers from the University of Arizona and the University of California at San Francisco used multi-locus sequence analysis to assess genetic signatures found in nearly 200 individuals from seven populations around the world. Their results suggest human population expansions in Africa started about 40,000 years ago during the Stone Age — a more recent expansion time than that predicted from previous studies.

"[B]oth hunter-gathers (San and Biaka) and food-producers (Mandenka and Yorubans) best fit models with population growth beginning in the Late Pleistocene," senior author Michael Hammer, a genetics researcher at the University of Arizona, and his co-authors wrote. "These dates are concurrent with the appearance of the Late Stone Age in Africa, supporting the hypothesis that population growth played a significant role in the evolution of Late Pleistocene human cultures."

Previous studies based on mitochondrial DNA, Y-chromosome data, or autosomal microsatellites provided a broad range of estimates about when modern human population expansion began, dating as far back as about a few hundred thousand years ago. But such estimates often conflict with one another and are based on one or a few sequences that may be under selective pressure, the researchers explained.

In an effort to generate more reliable data for teasing apart human population history, Hammer and his team used Sanger sequencing to re-sequence roughly 6,000 bases of nuclear DNA from each of about 20 autosomal non-coding regions for 184 individuals.

These regions were selected because they were sites with lots of crossing over events but were also far from protein-coding genes and not likely to be under selection. By looking at all of the areas together, Hammer told GenomeWeb Daily News, it's possible to overcome the noise detected at any single region.

The individuals tested belonged to seven different populations: San, Biaka, Mandenka, Yoruban, French Basque, Han Chinese, and Melanesian.

When the team analyzed their data using multi-locus analysis, they found evidence suggesting that both hunter-gatherer populations (such as the San from Namibia and the Biaka from the Central African Republic) and food-producer populations (such as the Mandenka from Senegal and Yorubans from Nigeria) began expanding roughly 40,000 years ago during the Late Pleistocene period.

That predates the advent of farming in Africa, Hammer noted, and is consistent with archeological evidence suggesting there was a burst of populations interacting and sharing tools and cultural innovations at that time.

Overall, the team concluded that human populations in Africa began a ten-fold expansion some 36,000 years ago. Their data hint that expansion may have been a tad earlier and faster in the hunter-gatherer population — about a 13-fold expansion starting about 41,000 years ago — than in the food-producing populations, which expanded approximately seven-fold starting some 31,000 years ago.

In the future, the team plans to do additional studies looking at more populations from different parts of the world. And, Hammer said, they also hope to employ next-generation sequencing technology to look at even more regions in the genome.

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