In a dusty, barren area in the Great Rift Valley of Ethiopia, about 140 miles northeast of Addis Ababa, lies a place that holds unique renown among paleontologists. Over the eons, seasonal rains have washed out and exposed bits of the past, including a spectacular, if scattered, assemblage of human ancestors. Here, a triangular area, 310 miles per side, cradles an estimated six million years of prehistory, in rare pockets within mile-thick sediment. "Whenever it rains, new fossils are revealed," explains Yohannes Haile-Selassie, curator of physical anthropology at the Cleveland Museum of Natural History.