It has long been a mystery how long ago cattle-raising people from Central Africa managed to reach southern Africa. It was assumed that the herders were stopped by sleeping sickness, which is carried by tsetse flies and kills cattle, goats, sheep and people.
The tsetse fly’s natural range stretches across Africa and through all the woodland and rain forest areas below the Sahara to roughly as far south as the Zambia-Zimbabwe border.
But a recent study, published in Proceedings of the National Academy of Sciences by chemists and anthropologists from several American universities, raises the possibility that more than 2,000 years ago, there were fly-free corridors leading south from East Africa that small groups of herders might have been able to use. Once through, they could have spread out across southern regions free of sleeping sickness (also called African trypanosomiasis). They would have had little competition for grazing, because that part of Africa was populated by hunter-gatherers like the San people, who were formerly called Bushmen.
Analysis of isotopes in animal teeth in the ashes of cooking fires in an ancient settlement in Kenya near Lake Victoria indicated that the animals ate grass almost exclusively.
Because tsetse flies inhabit bushes and trees, the area must have somehow been denuded of them, at least temporarily — perhaps by heavy grazing or fire, said Fiona Marshall, an anthropologist at Washington University in St. Louis and one of the authors of the study.
Early migration by herders would explain how a genetic mutation that originated in East Africa and helps people digest cow’s milk could have become as widespread as it is in southern Africans, Dr. Marshall said.
-New York Times