NEW YORK (GenomeWeb News) – Agriculture may have reached southern Africa as early as 2,000 or so years ago, arriving with a migration of pastoralists from in and around present-day Tanzania, according to new research.
In a paper appearing online this week in the Proceedings of the National Academy of Sciences, an international team of researchers analyzed Y-chromosome data for hundreds of individuals from more than a dozen populations in eastern and southern Africa. Their research suggests the movement of individuals from eastern to southern-central Africa about 2,000 years ago — hundreds of years before Bantu-speaking individuals from western Africa introduced their own agricultural methods to the region.
“There’s a long history in Africa that includes people moving from one area to another,” lead author Brenna Henn, an anthropology doctoral student at Stanford University, told GenomeWeb Daily News. Now, she added, there’s evidence for an extra, previously unidentified migration. That, in turn, adds credence to the notion that the spread of agriculture was paired with the physical movement of populations within Africa.
Agriculture originated in the Middle/Near East more than eight thousand years ago, spreading from there to Europe and Africa, and beyond. But there is still debate about how it was passed from one region or population to the next.
Some believe agriculture was communicated via cultural diffusion, with neighboring groups passing along sheep, cow, pottery, and so on without migrating. Others suggest there was demic diffusion, with agricultural people physically moving from one region to another. The former model predicts relatively little gene flow. The latter, on the other hand, would have left tell-tale genetic signatures that could be used to trace the migration.
To test the demic diffusion theory in eastern and southern Africa, Henn and her colleagues analyzed the Y-chromosome DNA of 454 individuals from 13 populations in Tanzania and in the Namibia-Angola-Botswana border region.
The researchers, who have been studying click-speaking populations in these areas for several years, identified a new Y-chromosome polymorphism called E3b1f-M293 that they thought might provide new insights into potential relationships between pastoral, agricultural, and hunter-gatherer populations in these areas.
Using this SNP, along with microsatellite data, the team defined an M293 haplogroup that seems to have originated in eastern Africa — in or around Tanzania — some nine to 11 thousand years ago. The precise origin of M293 cannot be determined without obtaining and testing additional samples from populations in Kenya, Sudan, and Ethiopia, they noted.
When they looked at the distribution of M293, the researchers found evidence of a historical human migration — consistent with the notion that there was physical movement of pastoralists from eastern to southern Africa. In particular, based on the data available to date, M293 seems to occur most often in northern Tanzania and in click-speaking populations in Namibia and Angola.
“Given the directionality and data…we conclude that eastern African individuals contributed M293 to southern African populations within the last few thousand years,” Henn and her co-workers wrote. “The scale of this migration may have been small, minimally four male individuals.”
This migration appears to be completely distinct from the migration of Bantu-speaking people from western Africa to southern Africa about 1,500 years ago, Henn said. During that migration, Bantu-speaking people from western Africa apparently introduced new agricultural methods, including iron tools, to southern Africa.
But the latest evidence suggests that at least some populations in southern Africa may have already been exposed to pastoralism by the time this group arrived. “If indeed M293 is indicative of the spread of pastoralism,” the authors noted, “then the Y-chromosome does not support the model of Bantu-speaking agropastoralists initially introducing sheep to southern Africa.”
Consistent with the notion that the two migrations were distinct, the researchers found that Bantu-speaking populations had different M293 profiles that arose more recently than those present in eastern and southern Africa.
Archeological evidence also supports the Tanzania to southern Africa migration theory, the researchers noted. For instance, some pottery found in northern Namibia, northern Botswana, and Zambia resembles the style of that discovered on the Kenya/Tanzania border.
So far, the researchers don’t have enough information to determine the precise migration route, owing to a lack of sampling from Zambia, Zimbabwe, DR Congo, and Botswana, Henn said. And, the researchers noted, “without samples from the other regions of Namibia, Botswana, and South Africa we cannot address the question of how pastoralism spread after it reached south-central Africa.”
In the future, the team plans to explore these and other questions. They have already collected additional samples from individuals in South Africa who are descended from pastoralists and hunter-gatherer populations, Henn said. And, she added, efforts by other groups to sequence ancient human and animal DNA will likely provide even more historic insights into the migration of humans and agricultural animals within Africa.