"This may be the most important specimen in the history of evolutionary biology," said C. Owen Lovejoy, an anthropologist at Kent State University in Ohio, in an interview with ABCNews.com.
Lovejoy was one of more than 40 researchers from around the world who analyzed the Ardi fossils.
Ardipithecus is not the long-sought "missing link" -- the ancestor that scientists say humans and apes have in common -- but comes close. And it helps show that both human beings and apes have evolved from something, about six million years ago, that did not look much like either.
"Six months ago, we would have said our common ancestor looked something like a chimp," said Tim White of the University of California at Berkeley, a senior researcher on the project. "Now all that has changed.
"What we found in Ethiopia at 4.4 million years ago is the closest we've ever come to that ancestor along our own line," White said.
The most complete skeleton, out of more than 30 found, was female, about four feet tall. They discerned the sex from the shape of the pelvis, which was wide enough for her to have borne a baby in her womb.
There are older specimens from other locations, but they are not complete enough for the analysis that has now been applied to Ardipithecus.
The scientists said the fossils show that Ardipithecus walked upright, and that her teeth resemble modern human teeth more closely than they do those of a chimpanzee.
Curiously, though, her feet were capable of grasping, something chimps need in order to climb in trees. She would have been able to climb trees, but she probably did not swing from branches the way modern chimps do.
The back of her skull is small, indicating she had a small brain.
The Ardipithecus bone fragments came from a layer of rock beneath the Afar region of Ethiopia. Afar is now desert, but the scientists said it was woodland 4 million years ago. They found fossilized wood and seeds around the Afar bones.
The fossil hunters, from Ethiopia and the United States, sent the bone fragments they found to a team in Japan. There, 3-D computer models were made of each piece, and the pieces were digitally reassembled, a bit like a jigsaw puzzle.
The first fragments were found in 1992, and more in later years. It took this long, said Lovejoy, to put the pieces together so that a detailed description could be published. The results are online in this week's edition of the journal Science.
Ardipithecus is 1.2 million years older than Lucy (Australopithecus afarensis), the famous pre-human fossil found in Africa in 1974. Lucy, like Ardi, walked upright and had a small brain, but was clearly closer to modern human beings -- probably not capable, for instance, of climbing routinely in trees.
So what would life have been like for a primitive being more than four million years ago? Scientists say they can deduce a fair amount from Ardi's skull, jaw, hands, legs and pelvis.
The teeth, for instance, suggest that Ardipithecus was probably an omnivore -- eating anything, plant or animal, that it could find. It did not have the pointed teeth found in modern chimpanzees, useful for eating fruit.
The shape of the large canine teeth in the front of the jaw is important. Male teeth were not larger than females'. It provides clues about social structure, suggesting that the males of the species did not fight each other for the females' attention.
Instead, said Lovejoy, "It is likely that the males went looking for food and brought it back to the females, possibly in return for sex, though that's another story."
He added, "This was probably a species for which male aggressiveness was not something that led to evolutionary success."
Scientists have believed since Charles Darwin's time that apes and human beings have common origins. But they have been hampered by the lack of fossils to trace the evolutionary path.
White and Lovejoy said that is why Ardipithecus is so important.
"This," said Lovejoy, "fills a huge gap."