Scientists Challenge ‘Breakthrough’ on Fossil Skeleton – By JOHN NOBLE WILFORD (NEW YORK TIMES)

May 28th, 2010 Print Print Email Email

The fossil skeleton known as Ardi, hailed in some quarters as the scientific “breakthrough” of 2009, has now drawn critics who dispute claims that the species lived in dense woodlands rather than grassy plains, which have been long considered the favored habitat of early prehumans and perhaps account for their transition to upright walking.

Another scientist has stepped forward to challenge Ardi’s classification as a member of the human lineage after the divergence from African apes. Its primitive anatomy, he contends, suggests a species predating the common ancestor of the human and chimpanzee family trees.

Two critiques are being published Friday in the journal Science, along with responses from the research team that reported last October the first detailed description and interpretation of the 4.4-million-year-old skeleton of Ardipithecus ramidus, or Ardi. The specimen, an adult female, probably stood four feet tall and was more than a million years older than Lucy, the famous skeleton of the species Australopithecus afarensis.

An international team led by Tim D. White of the University of California, Berkeley, discovered the fossils in 1992. It took 17 years to reconstruct and analyze the skeleton and related specimens and also to study the habitat in which the species had lived, in what is now Ethiopia. The team’s comprehensive report appeared in Science, which called it the “breakthrough of the year.”

It was perhaps inevitable that a discovery of such magnitude should draw critical fire, as Dr. White himself acknowledged this week in an e-mail message. “It was bound to generate some give and take,” he said. “So from that point of view, this is just part of normal science.”

The question of Ardi’s habitat was raised by Thure E. Cerling, a geochemist at the University of Utah, and seven other geologists and anthropologists. They said they used the White team’s own data for soils and silica from ancient plants, and found it did not support an interpretation that Ardi lived in thick woods.

Instead, Dr. Cerling’s group said, “We find the environmental context of Ar. ramidus at Aramis to be represented by what is commonly referred to as tree- or bush-savanna, with 25 percent or less woody canopy cover.”

The critics said that a landscape with a minimum of 60 percent trees and shrubs was required to meet the definition of a closed-canopy woodland. In other words, the findings did not, as Dr. White’s team inferred, overturn what is known as the savanna hypothesis associated with the evolution of upright walking — bipedalism — as a distinctive characteristic setting prehumans apart from ancestral apes.

Members of Dr. Cerling’s group said they were not advocating this conventional hypothesis, simply noting that Ardi data supported it rather than contradicted it.

In its published response, the White team said the critics ignored “the totality of the fossil, geological and geochemical evidence” presented in its original papers. There were indeed grasses at the site, the team noted, but the abundant fossils there were of mammals adapted to wooded life, and this established Ardi as “a denizen of the closed habitats,” not the open savanna.

Francis H. Brown, also a Utah geologist and an experienced researcher of early human origins who was a co-author of the Cerling paper, said in a recent interview, “We’re trying to set the record straight — we don’t think open savanna grassland is what Ardi lived in, nor is it a closed wooded environment.”

Another scientist, Esteban E. Sarmiento of the Human Evolution Foundation in East Brunswick, N.J., challenged the identification of Ar. ramidus as a hominid — a species of the human lineage that arose from an ancestor in common with the branch leading to modern chimpanzees.

“Sufficient support for this claim, however, is lacking,” Dr. Sarmiento, a vertebrate zoologist, wrote in a one-page article in Science. He cited the Ardi skeleton’s primitive aspects and molecular and anatomical studies that he said suggested Ar. ramidus “predates the human/African ape divergence.”

In its rebuttal, Dr. White’s team noted that Dr. Sarmiento based his argument on biomolecular estimates of the hominid-ape divergence at three million to five million years ago. These dates are unreliable, the team wrote, and other fossil discoveries have pushed the divergence time back sometime before six million years ago. Dr. White said in his e-mail message that Dr. Sarmiento had failed “to recognize as significant the multiple and independent features of the Ardipithecus cranium, dentition and skeleton,” which he added “uniformly align this primate with all later hominids, including Lucy, to the exclusion of any other ape, living or fossil.”

A few anthropologists have expressed doubts, as yet unpublished, about Ardi’s place in the human lineage. Richard G. Klein, a Stanford University anthropologist and another co-author of the Cerling paper, said in an interview, “I frankly don’t think Ardi was a hominid, or bipedal.”

Daniel E. Lieberman, a paleoanthropologist at Harvard who was neither a critic nor defender in the dispute, said he was “convinced that Ardi is a hominid.” But he added, “Everybody has questions about the kind of hominid it is, and about what this has to say about the last common ancestor of hominids and chimps.”

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