May 28, 2010

Who Or What Is Ardi?

Human evolution had some excitement last fall when a fossil skeleton named "Ardi" was discovered. Now, scientists are having doubts about what exactly the creature was and what kind of landscape it had inhabited.

New findings are leading experts to question whether Ardi really belongs on the human branch of the evolutionary tree, and whether it really lived in woodlands. Another question being raised has implications for theories about what kind of environment spurred early human evolution.

The new findings are being published by the journal Science, which published the original work last year, calling the presentation of the 4.4 million-year-old fossil the magazine's breakthrough of the year.

Ardi, short for Ardipithecus ramidus, is a million years older than the famous "Lucy" fossil discovered by American anthropologist Donald Johnson in November, 1973. Ardi was hailed last October as a window on early human evolution.

Researchers concluded that Ardi walked upright rather than on its knuckles like chimpanzees, for example, and that it lived in woodlands rather than open grasslands. It didn't show much resemblance to chimps, even though it was closer than Lucy to the common ancestor of humans and chimps.

The questioning is not unusual. Big scientific discoveries are normally greeted with much skepticism and criticism. Until more scientists can study fossils and perform their own analyses, broad consensus may be elusive.

Tim White of the University of California, Berkeley, one of the scientists who described Ardi last year, said he is not 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 thinks it came along earlier, before that human branch split off from the ancestors of chimps and gorillas, he told The Associated Press in an interview.

The specific anatomical features of teeth, the skull and other features that the researchers cited just do not make a convincing case for inclusion in the human branch of evolution, he argued.

Some of the features, like in the wrist and where the lower jaw connects to the skull, indicate 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 the interview with AP.

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, told AP that it is too early to tell where Ardi fits in on the evolutionary tree. He said he could not say whether Sarmiento was right or wrong.

"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," said Harcourt-Smith.

Rick Potts, head of the human origins program at the Smithsonian Institution's Natural History Museum, said Ardi is only known 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.

That really makes it difficult to draw conclusions about how the species relates to Lucy and modern humans, said Potts.

"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 told AP.

The other critique focuses on the environment in which Ardi hailed from. Analysis from last year said it was predominantly a woodland dweller. The theory argued against the "savanna hypothesis", which is the idea that early human ancestors began 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 understanding of the evidence shows that Ardi roamed in a savanna covered by no more than 25 percent woodland. So they disagree with last year's theories on the leafy setting.

The basis for their critique focused on evidence like the analysis of ancient soils, tooth enamel from animals found at the site and tiny silica grains found in plants.

In a published rebuttal, White agreed that Ardi's environment included grasslands but said the totality of the evidence shows Ardi preferred living in its wooded areas instead.

He said the skeleton shows adaptations for climbing and "it wasn't climbing grass." And animals found with Ardi's remains are mostly woodland creatures like leaf-eating monkeys, he added.

Potts said he thinks White is right about the environment-side of the dispute. But again, he noted, that's just one site, and not enough evidence for drawing conclusions about the general environmental conditions of early human evolution.


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