This and other stone tools are stored in a small rock building where American Robert Blumenschine and Tanzanian Fidelis Masao lead teams of scientists who have collected ancient animal bones, pieces of fossilized reeds and grasses from the lake that once lapped the valley floor. From the rafters hang crocodile and hippo skulls. There is even a boxed collection of "modern herbivore dung."

"Their activities were extremely variable across a diverse landscape," Blumenschine says. "The places where we find the biggest concentration of activities were well-wooded, for safety and a higher abundance of plant food and for scavenging opportunities from leopard and cat kills."

"That's how I would look at it," Blumenschine says.