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New Model Refines View of Human Population Relationships

NEW YORK (GenomeWeb) – A pair of investigators from Harvard Medical School and the Broad Institute has taken a fresh look at the human population dispersal patterns after ancient humans made their way out of Africa, using a statistical model that argues against widespread migration via the so-called "southern route."

The team considered allele frequency and admixture patterns in array-based genotyping data for modern humans from 10 populations around the world and used available Denisovan and Neanderthal sequences to estimate gene flow from these archaic hominin groups. The resulting model, described yesterday in Molecular Biology and Evolution, suggests that modern human populations split into an eastern and a western group some 45,000 years ago, not long after the main out-of-Africa migration.

Such patterns argue against a pronounced southern migration of modern humans into present-day Australia and Southeast Asia, the team noted, though the data available so far do not conflict with the possibility that a subset of ancient modern humans made their way to the region along this route.

"We don't see evidence of ancestry from an early southern dispersal in present-day populations, although we can't rule out a small proportion," lead author Mark Lipson, a genetics researcher at Harvard Medical School, said in a statement.

"There is some older archaeological evidence from Asia," Lipson added, "and while our results suggest that the earliest inhabitants probably would not have been closely related to Asian and Australian populations today, it would be extremely interesting to see DNA from those sites."

Several past studies have sought clues to historical human migrations using ancient remains from modern humans and archaic hominins, along with genetic variant and pathogen patterns in present-day populations, explained Lipson and co-author David Reich, a genetic researcher with Harvard Medical School and the Broad Institute.

But there is ongoing debate over the ancestry of individuals in Australia, New Guinea, and parts of Southeast Asia, they noted, with some groups proposing a divergence for these populations during an earlier out-of-Africa migration that preceded a split between the eastern and western Eurasian ancestry groups.

With Affymetrix array-based genotyping profiles or genome sequencing for individuals from a dozen human or archaic human populations and from chimpanzees, the duo developed a statistical model to tease apart ancestry and admixture patterns in these populations.

With this approach, the researchers saw signs of nine main admixture events, including half a dozen involving Neanderthals or Denisovans. But the data did not support the notion that Australasian populations have ancestry from modern humans who left Africa long before the main out-of-Africa migration.

Instead, their initial model and subsequent analyses suggested that present-day populations primarily carry ancestry from eastern and western groups that began diverging 45,000 or more years ago, before mixing with one another and with archaic hominins during subsequent migration events.

Although details of these admixture events may shift somewhat as new data become available, Lipson called the current model "a detailed synthesis of existing data and a good basis for further work."

"While our results still represent a simplified picture, they provide a useful summary of deep Eurasian population history that can serve as a null model for future studies and a baseline for further discoveries," he and Reich wrote.