14 April 2009

Ancient African Genetic History

DNA based stories that tell the story of human evolution in broad strokes are fascinating, even if they need to be taken with a grain of salt, and buffered by recognition of closely we are all related.

A 2008 study directed by Quintana-Murci, based on an analysis of one mitochondrial DNA region, suggested that maternal ancestors of African pygmies [who mostly live hunter-gatherer lifestyles in forests] and [Bantu] farmers diverged no more than 70,000 years ago. The new study focused on 23 nuclear DNA regions and one stretch of mitochondrial DNA.

“We can now provide a more general view of the genetic history of these African populations,” Quintana-Murci says.

Quintana-Murci’s team further concludes that two groups of modern-day pygmies, one located in west-central Africa and the other in east-central Africa, separated from an ancestral population roughly 20,000 years ago. That makes sense in light of earlier evidence that global cooling around that time led to a shrinking of central African forests into separate eastern and western zones, where different groups of pygmies would have settled, according to the researchers.

Still, pygmies’ retreat into different forested regions did not totally prevent gene flow between western and eastern groups, the new study suggests. In addition, the study finds that Bakola pygmies in western Africa and Twa pygmies in eastern Africa share a substantial amount of DNA with neighboring farmers, suggesting breeding with the taller populations.

Pygmies’ DNA also contains signatures of steep population declines that occurred as recently as 250 years ago in eastern Africa and 2,500 years ago in western Africa. Agricultural expansion into forests or major epidemics could have drastically cut pygmies’ numbers, Quintana-Murci speculates. . . . the team’s calculations for the timing of population divisions may be off by a few thousand or even tens of thousands of years . . . If pygmies indeed split into eastern and western groups around 20,000 years ago, it’s also unclear when they began to interbreed with nearby farmers[.] . . .

Quintana-Murci and his colleagues’ study investigated present-day genetic variation in 236 Africans recruited from five agricultural and seven pygmy populations. Participants came either from eastern or western Africa.

Modern humans are believed to have left Africa for Eurasia via the Middle East sometime around 100,000 years ago. The Bantu-Pygmy split would have taken place tens of thousands of years after the Out of Africa migration, close in time to the first arrivals of modern humans in Australia, but long before the Neolithic revolution in which settled agriculture was established. The second split, of the two main Pygmy populations could have coincided with the early Neolithic revolution.

Sub-Saharan African genetic diversity is greater than the diversity of the entire non-African population of the world, so these kinds of studies, limited as they are by small sample size, provide important insights into the evolutionary roots of modern humans. This information is also ephemeral, as technology breaks the links between geography and ancestry.

1 comment:

Andrew Oh-Willeke said...

See also here, a paper published in the Journal Nature describing cranial variation studies confirming the genetic out of Africa scenario.