Last fall, a fossil skeleton named "Ardi" shook up the field of human evolution. Now, some scientists are raising doubts about what exactly the creature from Ethiopia was and what kind of landscape it inhabited.
New critiques question whether Ardi really belongs on the human branch of the evolutionary tree, and whether it really lived in woodlands. That second question has implications for theories about what kind of environment spurred early human evolution.
The new work is being published by the journal Science, which last year declared the original presentation of the 4.4 million-year-old fossil to be the magazine's breakthrough of the year.
Ardi, short for Ardipithecus ramidus, is a million years older than the famous "Lucy" fossil. Last October, it was hailed as a window on early human evolution.
Researchers concluded that "Ardi" walked upright rather than on its knuckles like chimps, for example, and that it lived in woodlands rather than open grasslands. It didn't look much like today's chimps, our closest living relatives, even though it was closer than Lucy to the common ancestor of humans and chimps.
Such questioning isn't unusual; big scientific discoveries are typically greeted that way. Until more scientists can study the fossil and other work can be done, broad consensus may be elusive. The 2003 discovery of diminutive fossil "hobbits" in Indonesia, for example, has spurred a long-running debate about whether the hobbits were a separate species or not.
Wrong evolutionary tree?
Tim White of the University of California, Berkeley, one of the scientists who described Ardi last year in Science, said he isn't surprised by this week's debate.
"It was completely expected," he said. "Any time you have something that is as different as Ardi, you're probably going to have it."
Esteban Sarmiento of the Human Evolution Foundation in East Brunswick, N.J., wrote in the new analysis that he's not convinced Ardi belongs on the evolutionary tree branch leading to modern humans.
Instead, he said in an interview, he thinks it came along earlier, before that human branch split off from the ancestors of chimps and gorillas.
The specific anatomical features of teeth, the skull and elsewhere that the researchers cited just don't make a convincing case for membership on the human branch, he argued. Some, like certain features in the wrist and where the lower jaw connects to the skull, indicate instead that Ardi arose before humans split off from African apes, he said.
In a written rebuttal in Science and in a telephone interview, White disagreed with Sarmiento's conclusion. "The evidence is very clear that in Ardipithecus, there are characteristics shared only by later hominids ... and humans," White said.
If Ardi were really ancestral to chimps, certain features of its teeth, pelvis, and skull would have had to later evolve back to their more ape-like conditions, an "evolutionary reversal that's highly unlikely," White said in an interview.
Two other experts, however, said in interviews that they think it's too early to tell where Ardi fits on the evolutionary tree.
Will Harcourt-Smith, a research associate at the American Museum of Natural History and member of the anthropology department at Lehman College in New York, said he could not say whether Sarmiento was right or wrong.
"It's early days" in the analysis of Ardi, he said. "Until there is a more complete description of the skeleton, one has to be cautious about interpreting the initial analyses one way or another... I still think it's open season."
Harcourt-Smith said he did disagree with Sarmiento's assertion that Ardi was probably too old to belong to the human branch of the evolutionary tree.
Rick Potts, head of the human origins program at the Smithsonian Institution's Natural History Museum, said Ardi is known chiefly from just one site. And it lived during a dimly understood period of evolution when there might have been "a lot of experimentation," he said.
Potts said that makes it hard to draw conclusions about how the species relates to Lucy and modern humans.
"I think it's just too soon to tell exactly where it stands in relationship to the branching point of humans from other African apes," he said.
Questions about environment
The second critique focuses on Ardi's environment. Last year's analysis said it was predominantly a woodland setting. So that argued against the "savanna hypothesis," the idea that early human ancestors started to walk upright because they lived on grassy plains and savannas.
In this week's critique, geochemist Thure Cerling of the University of Utah and other scientists said their reading of the evidence shows Ardi roamed in a savanna with no more than 25 percent covered by a woody canopy. So they disagreed with last year's emphasis on the leafy setting.
The critique focused on evidence like analysis of ancient soils, tooth enamel from animals found at the site and tiny silica grains found in plants.
In a published rebuttal and the interview, White agreed that Ardi's environment included grasslands but said the totality of the evidence shows Ardi preferred living in its wooded areas instead.
For example, the skeleton shows adaptations for climbing and "it wasn't climbing grass," he said. And animals found with Ardi's remains are mostly woodland creatures like leaf-eating monkeys, he said.
Potts said he thinks White is right about the environment of the site in dispute.
But again, he said, that's just one site, and not enough for drawing conclusions about the general environmental conditions of early human evolution — if indeed White is also right about Ardi's place on the family tree.