May 28, 2010 — -- When Ardipithecus ramidus -- "Ardi" for short -- was shown off last fall, she took the scientific world by storm. Scientists said the 4.4-million-year-old fossil was the oldest pre-human species ever found.
"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," Tim White of the University of California at Berkeley told ABC News at the time.
"This may be the most important specimen in the history of evolutionary biology," said C. Owen Lovejoy of Kent State University.
But now comes word from other scientists who say, in effect, not so fast.
Writing in today's edition of the journal Science -- which published the original reports on Ardi and later called it the "breakthrough of the year" -- Esteban Sarmiento, a private researcher in New Jersey, says, "Their analysis of shared-derived characters provides insufficient evidence of an ancestor-descendant relationship...."
Sarmiento writes that Ardi probably came before humans and apes split into separate evolutionary family trees.
In a second paper, Thure Cerling of the University of Utah, along with several colleagues, writes that when Ardi was alive, Ethiopia was probably grassy savannah with a few trees -- not the woodlands that White and his team said was key to explaining the way Ardi evolved.
White, who stands by his team's original conclusions, said he was not surprised by the new questions. Part of the regular give-and-take of scientific work is for researchers to publish what they find -- specifically so that others can either confirm it or find contradictory evidence.
"Any time you publish something that challenges long-established views, the people who hold those long-established views are bound to push back," White said today in a conversation with ABC News.
"This is absolutely a normal part of science."
White and his colleagues said Ardipithecus is not the long-sought "missing link" -- the ancestor that scientists say humans and apes have in common -- but they said it comes close.
Ardi: Argument Over Pre-Human Fossil
The most complete skeleton, out of more than 30 found, was female, about four feet tall. They discerned the gender from the shape of the pelvis, which was wide enough for her to have borne a baby in her womb.
White 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. White said Ardi would have been able to climb -- something she would not have had to do if she lived in grasslands the way the doubters suggest.
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 White's team found fossilized wood and seeds around the bones.
The bone fragments were sent 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 more than 15 years, said Lovejoy, to put the pieces together so that a detailed description could be published. The scientists said they could deduce a fair amount from Ardi's skull, jaw, hands, legs and pelvis.
The teeth, for instance, suggested 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.
So is Ardi our great-great-great-grandsomething or not? Many scientists say they would like to see more evidence, but White says the doubters are wrong about what Ardi looked like and where she lived.
"Their reconstruction of a predominantly open grassy environment," he writes, "is inconsistent with the totality of the fossil, geological, and geochemical evidence."
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."