ST LOUIS, MISSOURI—It had been thought that the tsetse fly, which carries sleeping sickness and nagana and thrives in bushy woodlands, stopped the spread of herders of domesticated animals into southern Africa some 2,000 years ago. Fiona Marshall of Washington University has led a team of researchers who analyzed the isotopes in animal teeth from a 2,000-year-old settlement near Gogo Falls in southern Kenya. The people who lived there ate a varied diet that included domestic and wild food sources. The region is now made up of bushy woodlands, but the results of the study suggest that there had been abundant grassland vegetation for the animals to eat in what may have been a grass-woodland transition zone. So did the human residents consume wild food because tsetse flies damaged their livestock herds? “Our findings challenge existing models that explain the settlement’s diverse diet as a consequence of depressed livestock production related to tsetse flies. 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. 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,” Marshall explained. Changes in rainfall, grazing by wild herbivores, and burning of land by the herders may have maintained the savanna for the livestock and provided a corridor through the Lake Victoria basin for the migration of pastoralists into southern Africa. To read about a fascinating archaeological discovery made in southern Africa, see "First Use of Poison."