The 110-pound, 4-foot female roamed forests a million years before the famous Lucy, long studied as the earliest skeleton of a human ancestor.
This older skeleton reverses the common wisdom of human evolution, said anthropologist C. Owen Lovejoy of Kent State University.
Rather than humans evolving from an ancient chimp-like creature, the new find provides evidence that chimps and humans evolved from some long-ago common ancestor -- but each evolved and changed separately along the way.
"This is not that common ancestor, but it's the closest we have ever been able to come," said Tim White, director of the Human Evolution Research Center at the University of California, Berkeley.
The lines that evolved into modern humans and living apes probably shared an ancestor 6 million to 7 million years ago, White said in a telephone interview.
But Ardi has many traits that do not appear in modern-day African apes, leading to the conclusion that the apes evolved extensively since we shared that last common ancestor.
A study of Ardi, under way since the first bones were discovered in 1994, indicates the species lived in the woodlands and could climb on all fours along tree branches, but the development of their arms and legs indicates they didn't spend much time in the trees. And they could walk upright, on two legs, when on the ground.
Formally dubbed Ardipithecus ramidus -- which means root of the ground ape -- the find is detailed in 11 research papers published Thursday by the journal Science.
"This is one of the most important discoveries for the study of human evolution," said David Pilbeam, curator of paleoanthropology at Harvard's Peabody Museum of Archaeology and Ethnology.
"It is relatively complete in that it preserves head, hands, feet and some critical parts in between. It represents a genus plausibly ancestral to Australopithecus -- itself ancestral to our genus Homo," said Pilbeam, who was not part of the research teams.
Scientists assembled the skeleton from 125 pieces.
Lucy, also found in Africa, thrived a million years after Ardi and was of the more human-like genus Australopithecus.
"In Ardipithecus we have an unspecialized form that hasn't evolved very far in the direction of Australopithecus. So when you go from head to toe, you're seeing a mosaic creature that is neither chimpanzee, nor is it human. It is Ardipithecus," said White.
White noted that Charles Darwin, whose research in the 19th century paved the way for the science of evolution, was cautious about the last common ancestor between humans and apes.
"Darwin said we have to be really careful. The only way we're really going to know what this last common ancestor looked like is to go and find it. Well, at 4.4 million years ago we found something pretty close to it," White said. "And, just like Darwin appreciated, evolution of the ape lineages and the human lineage has been going on independently since the time those lines split, since that last common ancestor we shared."