March 10, 2015
Ancient African herders used ‘no fly zones’ to migrate south
Experts have long believed that it was one little but dangerous insect, the tsetse fly, that halted the southward migration of ancient herders thousands of years ago. But new research published Monday in the Proceedings of the National Academies of Science now suggests otherwise.As Dr. Fiona Marshall, a professor of anthropology at Washington University in St. Louis, and her colleagues explain in the new study, herders migrated from eastern to southern Africa about 2,000 years ago, but only in small numbers. The tsetse fly, a disease-carrying insect known for spreading sleeping sickness, was long believed to have impeded their migration.
“Archaeologists have argued that the presence of tsetse flies around Lake Victoria, Kenya, created a barrier that prevented migration and forced subsistence diversification,” the authors wrote. “This study, using stable isotope analysis of animal teeth, reveals the existence of ancient grassy environments east of Lake Victoria, rather than tsetse-rich bushy environments.”
Overturning previous assumptions
By analyzing animal remains from a nearly 2,000-year-old settlement located near Gogo Falls in the modern woodlands of southern Kenya, the Marshall and her colleagues report that they have discovered evidence that contradicts previous assumptions about the conditions of the era.
“This study overturns previous assumptions about environmental constraints on livestock management in a key area for southward movement of early herders,” Marshall, a co-author of the new study, explained in a statement Monday. “It reveals that the vegetation east of Lake Victoria was then much different than it is today and that ancient grassy environments may have provided an important corridor for herders moving into southern Africa.”
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The existing theory appeared to make sense, as it was quite possible that the dense wooded bushlands of south-central Africa that are home to the tsetse fly may have represented a serious disease-related threat to the early herders. Their presence could have acted as a barrier to keep the people from moving their sheep, cattle and goat herds too far south.
However, the new research indicates that both domestic and wild herbivores living in the area at the time of the Gogo Falls settlement had diets that consisted primarily of grassland vegetation – a discovery that makes it unlikely that the bush-loving tsetse flies would have been present in the same area, and thus overturns long-held theories that the disease-carrying insects were at fault.
Can I get your isotopic autograph?
Marshall, along with colleagues from the University of Utah and Ross University in Florida, conducted stable isotope analyses on samples of tooth enamel recovered from ancient animal remains during a 1983 expedition at the Gogo Falls site.
Since both grassland and woodland vegetation have unique isotopic signatures in the teeth of animals that consume them, Marshall and her colleagues were able to confirm that the area was surrounded by extensive grasslands at the time that the Gogo Falls settlement was established.
By establishing that the tsetse flies probably were not a major issue at this time, the authors of the PNAS paper suggest that the use of both domestic and wild food sources was by choice, and was not born out of necessity. Previously, the diverse diet of the settlement was attributed to a decline in livestock production related to the presence of tsetse flies, Marshall explained.
“Instead of this ecological explanation, our isotopic findings support the notion that herders may simply have interacted with hunter-gatherer groups already living in these areas, adapting to their foraging styles,” she said. “This suggests that social factors may have played a greater role than previously thought in subsistence diversity during the spread of pastoralism in Eastern Africa.”