Once green, the Sahara expanded 5,500 years ago, leading ancient herders to follow the rain and grasslands south to eastern Africa. But about 2,000 years ago, their southward migration stalled out, stopped in its tracks, archaeologists presumed, by tsetse-infested bush and disease. As the theory goes, the tiny tsetse fly altered the course of history, stopping the spread of domesticated animal herding with a bite that carries sleeping sickness and nagana, diseases often fatal for the herder and the herded. Now, isotopic research on animal remains from a nearly 2,000-year-old settlement near Gogo Falls in the present-day bushy woodlands of...