Paleontologist Gen Suwa was walking across the pebble-covered desert of north- central Ethiopia under the searing midday sun, peering carefully around him for ancient bones. Then he saw it: the telltale gleam of a fossil tooth partially exposed on the rocky ground. "I knew immediately that it was a hominid tooth," says the University of Tokyo scientist, "and one of the oldest ever found."

It was more than that. Suwa had uncovered nothing less than a new chapter in the history of human evolution. He and his colleagues report in the current , Nature that the archaic molar, along with other...