The seven stone, four-foot tall female roamed forests 4.4 million years ago – a million years before the previous oldest discovered fossil.
Her skeleton promises to fill in gaps about how we became human and evolved from apes. It has already reversed some common assumptions of evolution.
Rather than humans evolving from chimps, the new find provides evidence that chimps and humans evolved together from another common more ancient ancestor.
Each has evolved and changed separately along the way, it is believed.
Formally known as Ardipithecus ramidus — which means root of the ground ape — the find is detailed in 11 research papers published in the journal Science.
“This is not that common ancestor, but it’s the closest we have ever been able to come,” said Dr Tim White, an anthropologist and one of the researchers at the University of California.
The lines that evolved into modern humans and living apes probably shared an ancestor six million to seven million years ago, the research suggests.
Ardi has many traits that do not appear in modern-day African apes, leading to the conclusion that the apes evolved extensively since they shared that last common ancestor with humans.
A study of Ardi, under way since the first bones were discovered in 1994 in the Afar region of Ethiopia, indicates her species lived in the woodlands and could climb on all fours along tree branches.
But the development of arms and legs indicates she did not spend much time in the trees, the study claims.
Her pelvis suggests she walked upright and her teeth are closer to humans than primates. While she would have had a muzzle, it did not project out as much as modern apes.
Dr White described her as a “mosaic” that was neither human or chimpanzee.
“The only way we’re really going to know what this last common ancestor looked like is to go and find it,” he said. “Well, at 4.4 million years ago we found something pretty close to it.”
Dr David Pilbeam, palaeoanthropology at Harvard’s Peabody Museum of Archaeology and Ethnology, said: “This is one of the most important discoveries for the study of human evolution.
“It is relatively complete in that it preserves head, hands, feet, and some critical parts in between.”
Until the discovery of Ardi, the earliest well-known stage of human evolution was Australopithecus, the small-brained, fully bipedal “ape man” that lived between four million and one million years ago.
The most famous Australopithecus fossil is the 3.2-million-year-old “Lucy,” found in 1974 about 45 miles north of where Ardi would later be discovered.
Lucy was described as the “mother of man” and the missing link between humans and chimps. Before Ardi, she was thought to be the oldest fossil of a human ancestor that walked on two legs.