Correlation of genetic distance with geographic distance along migratory routes

This news item from the University of Michigan speaks of a new study on human origins. See also, a related National Geographic story. I will blog more about it once it appears in the PNAS site.

Human genetic diversity supports 'Out of Africa' model

ANN ARBOR, Mich.—Small groups of settlers expanding outward from Africa are the most likely progenitors of the modern human population worldwide, according to a new study by researchers at the University of Michigan and Stanford University.

Researchers used computer modeling to examine a "serial founder effect" scenario as a possible explanation for the relationship between human populations' genetic and geographic distance from Africa, which are highly correlated, according to Noah Rosenberg, assistant research professor at the U-M Life Sciences Institute. He is co-author of the study, which appears in Proceedings of the National Academy of the Sciences.

Geographic distance very accurately predicted the genetic distance between populations. The more similar the genes, the shorter the geographic distance between any two people. The genetic diversity of populations was found to be greatest in Africa and least in the Americas.

The study assumes that there was a single ancestral group, and that subsets of the group expanded and colonized new locations outward from its initial starting point. A subset of settlers populated a new area, and as they did so, their genetic variation declined.

"No geographic location outside Africa accounts as well for the patterns of genetic diversity in the model," Rosenberg said. "If we assume that humans started at one location in Africa, there is an excellent fit of genetic variation to geographic distance. Every single point in Africa that was tested as a possible site of origin has more explanatory power than every site outside of Africa."

"We tried to explain this pattern of genetic and geographic distance using what might have happened in human history," said Rosenberg. "The original group populates a new area, but only some of the people from the original group move to the new area, so there is less genetic variation in the sub-group."

The "Out of Africa" model in paleoanthropology supposes that modern humans evolved within the African continent and migrated around the world. A counter-argument is the "multiregional" hypothesis, which holds that pre-humans left Africa and became modern humans at several locations around the globe. Rosenberg's latest data supports the out-of-Africa model.

A next step for researchers is to add genetic variants that predispose people to genetic diseases into the computer models.

"I'd like to link the geographic distribution of genetic diseases to human evolution," said Rosenberg. "This kind of work can help us figure out the extent to which the genetic factors responsible for globally distributed diseases are the same in all populations."