Ardi, short for Ardipithecus ramidus, is the newest fossil skeleton out of Africa to take its place in the gallery of human origins. At an age of 4.4 million years, it lived well before and was much more primitive than the famous 3.2-million-year-old Lucy, of the species Australopithecus afarensis. By replacing Lucy as the earliest known skeleton from the human branch of the primate family tree, the scientists said, Ardi opened a window to “the early evolutionary steps that our ancestors took after we diverged from our common ancestor with chimpanzees.”

Ardi’s feet had yet to develop the arch-like structure that came later with Lucy and on to humans. The hands were more like those of extinct apes. And its very long arms and short legs resembled the proportions of extinct apes, or even monkeys.

David Pilbeam, a professor of human evolution at Harvard University who had no role in the discovery, said in an e-mail message that the Ardi skeleton represented “a genus plausibly ancestral to Australopithecus” and began “to fill in the temporal and structural ‘space’ between the apelike common ancestor and Australopithecus.”

In some ways the specimen’s features are surprising, Dr. Hill added, “but it makes a very satisfactory animal for understanding the changes that have taken place along the human lineage.”

Dr. White, Berhane Asfaw of the Rift Valley Research Service in Ethiopia and other team members concluded that “despite the genetic similarities of living humans and chimpanzees, the ancestor we last shared probably differed substantially from any extant African ape.”

As Dr. Hill of Yale said, “It is always new specimens, particularly those from little known time periods or geographic areas, that provoke the greatest changes in our ideas.” Looking ahead, Dr. White lamented that there were so few sites in Africa known to have fossil deposits six million to seven million years old. “We are getting so close to that common ancestor of hominids and chimps, and we’d love to find an earlier skeleton,” he said.