Clearer Picture On African Human Expansion Revealed
October 23, 2012

Clearer Picture On African Human Expansion Revealed

Brett Smith for - Your Universe Online

One of the biggest sticking points in the scientific debate over the story of human evolution and eventual diaspora from Africa has been how it is told from two opposite sides of the scientific spectrum–the clinically-based genetics community and the field-based anthropological community.

The debate has caused some sniping over the years due to conflicting theories. But now, a group of geneticists from Stanford University set out to reconcile some of those differences through a comprehensive literature review published in the latest edition of the Proceedings of the National Academy of Sciences (PNAS).

Through that review–the Stanford team was able to create a new framework that can be referenced by future studies in both the genetics and anthropology fields.

"People are doing amazing genome sequencing, but they don't always understand human demographic history" said co-author Brenna Henn, a postdoctoral fellow in genetics at the Stanford School of Medicine. "We wanted to write this as a primer on pre-human history for people who are not anthropologists."

The geneticists were able to show how the outward expansions from Africa that occurred between 45,000 and 60,000 years ago can be correlated to historical changes that can be drawn from genetic evidence.

"The basic notion is that all of these disciplines have to be considered simultaneously when thinking about movements of ancient populations," said lead author Marcus Feldman, a professor of biology at Stanford. "What we're proposing is a story that has potential to explain any of the fossil record that subsequently becomes available, and to be able to tell what was the size of the population in that place at that time."

In the review, the geneticists noted that human expansion into every inhabitable zone on Earth has been accompanied by a loss of genetic diversity that they attribute to what they call a “serial founder effect.”

This effect was the result of the migration of small groups that established unique settlements with a limited gene pool. It can be seen in “the genetics of human parasites, morphology, and linguistics,” according to the review.

"If you know something about the demographic history of populations, you may be able to learn something about the reasons why a group today has a certain genetic abnormality — either good or bad," Feldman said in a news release. "That's one of the reasons why in our work we focus on the importance of migration and history of mixing in human populations.”

“It helps you assess the kinds of things you might be looking for in a first clinical assessment,” he added. “It doesn't have the immediacy of prescribing chemotherapy — it's a more general look at what's the status of human variability in DNA, and how might that inform a clinician."

According to the review, this history gave rise to the two types of genetic variation in humans: genomes from African subcultures, which retain an exceptional number of unique variants, and populations living outside of Africa, which contain less unique genetic variants. The identification of these two categories is relevant for mapping genotypes to phenotypes and for inferring the power of natural selection in human history, the geneticists wrote.