When the Sahara desert began expanding 5,500 years ago, cattle herders were forced to migrate southward, following and the grasslands southward. But around 2,000 years ago, archaeological evidence suggests the herders' ended north of Lake Victoria, where new bushland habitat introduced the tsetse fly.

Previously, archaeologists have surmised that the tsetse -- and the sometimes deadly diseases (sleeping sickness and nagana) it carries -- served as a biological barrier to early pastoral people. But new evidence collected at Gogo Falls, just east of Lake Victoria in southwestern Kenya, shows the habitat there was predominately grassland.

In analyzing ancient tooth samples from cattle remains recovered from the site, researchers were able to prove not only the presence of ancient cattle, but that grassland was abundant -- not bushland. The isotopic analysis suggests pastoral Neolithic herders lived at Gogo Falls between 1,600 and 1,900 years ago.

"This study overturns previous assumptions about environmental constraints on livestock management in a key area for southward movement of early herders," study co-author Fiona Marshall, professor of anthropology at Washington University in St. Louis, said in a press release.

"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," Marshall added.

Previous studies have shown people at Gogo Falls ate an unusually diverse diet, including domesticated animals as well as hunted game. Previously, archaeologists argued this diversification was proof of the detrimental effects of the tsetse fly.

"Our findings challenge existing models that explain the settlement's diverse diet as a consequence of depressed livestock production related to tsetse flies," Marshall continued.

"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."

The new evidence suggests that while tsetse flies might have depressed migrations and the spread of herding cultures in other parts of Africa, Lake Victoria may have provided corridors of grassland that sustained early migrations for thousands of years.

The findings may have novel applications for how scientists study the spread of genetic traits linked to early pastoral cultures, like the adaptation for lactose tolerance.

"From a broader perspective, the findings expand our understanding of factors that have influenced and contributed to the distribution of modern populations," Marshall said.

The study was published online this week in the early edition of the journal PNAS.