But sometimes she came down, and walked on two legs. She was, in a sense, taking steps that would later resound across the world.

"Ardi" is the nickname given to a remarkable, shattered skeleton that an international team of scientists believes is a major breakthrough in the study of human origins. The skeletal remains were painstakingly recovered from the Ethiopian desert along with bones from at least 35 other members of a species called Ardipithecus ramidus.

The 15-year investigation of Ardipithecus culminated yesterday in the publication of papers online in the journal Science, as well as dual news conferences in Washington and Addis Ababa, Ethiopia.

"This is huge. This is the biggest discovery really since the 'Lucy' skeleton of the 1970s," said Carol Ward, a University of Missouri paleoanthropologist who was not involved with the research.

Human origins is a field with high stakes and small bones, and the elaborate rollout of the Ardipithecus research will likely trigger debate about the information in fossils so fragile they had to be excavated with dental picks and porcupine quills.

If the scientists are correct, she's a transitional figure, almost a hybrid - a tree creature who could carry food in her arms as she explored the woodland floor on two legs.

Ardi lived more than a million years before Lucy, the name given to a 3.2 million-year-old skeleton found in 1974 that is the best example of Australopithecus afarensis, a small-brained primate that led a two-legged life.

Unlike Ardi, she lacked the grasping big toe that extends laterally from the foot. Lucy's big toe pointed forward, aligned with the other toes, and was used for propulsion.

But Ardi and Lucy had rather similar faces, skulls, hands, and pelvises.

The scientists who found Ardi do not contend that Ardi necessarily evolved into Lucy, or that Ardipithecus ramidus was a direct human ancestor.

"The individual, Ardi, that female individual, is she our ancestor?" said Tim White, a University of California at Berkeley paleoanthropologist who led the research team. "And the answer is, probably not. If she didn't have any kids, tough luck, she's nobody's ancestor."

The Ardi team, however, does make the case that the general body plan of Ardi would evolve into the general body plan of Lucy, and on down the line until the genus Homo appears.

White and colleagues found the first signs of Ardipithecus in 1994. A key moment came Nov. 5, 1994, when a Berkeley graduate student, Yohannes Haile-Selassie of Ethiopia, found fragments of two finger bones. Further digging turned up scraps of a pelvis, feet, hands, and chips from a skull. By January 1995, the scientists realized they had found a paleontological treasure. This was Ardi.

Although Ardipithecus quickly entered the lexicon in the mid-1990s, and scientists knew that this was potentially a major discovery, it was not until yesterday - and after some scientists complained over how long the process was taking - that White and his colleagues gave a detailed description of the species.