Ardi: The Human Ancestor Who Wasn't?

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Reuters / Corbis

A reconstructed frontal view of Ardi

At a little over 4 ft. tall, she was small by human standards. But when Ardi, the 4.4 million-year-old hominid fossil found in Ethiopia in 1992, was finally introduced to the world last October in a series of 11 audacious studies in the journal Science, she caused big waves in evolutionary circles. Both TIME and Science named her the "Scientific Breakthrough of the Year." But now Ardi has found herself in a spot of controversy. Two new articles being published by Science question some of the major conclusions of Ardi's researchers, including whether this small, strange-looking creature is even a human ancestor at all.

Neither article challenges the veracity of the evidence published by the team of scientists, led by paleoanthropologist Tim White of the University of California, Berkeley, which painstakingly pieced Ardi together from more than 100 crushed fossil fragments. But they do dispute the conclusions White and his colleagues reached from that evidence.

In the first article, titled "Comment on the Paleobiology and Classification of Ardipithecus ramidus," Esteban Sarmiento, a primatologist at the Human Evolution Foundation, argues that many of the "characters" — the scientific term for physical traits — used by White to place Ardi on the human lineage are also shared by other primates. He argues that the evidence suggests Ardi belongs to a species that evolved before the moment when humans, apes and chimps diverged along different evolutionary paths. That is significant because one of the things that made Ardi interesting scientifically was that she had been identified by White as the earliest known descendant of the last common ancestor of humans and African apes — thus her physiology could offer clues to what makes humans different from their nearest relatives.

"[White] showed no evidence that Ardi is on the human lineage," Sarmiento says. "Those characters that he posited as relating exclusively to humans also exist in apes and ape fossils that we consider not to be in the human lineage."

The biggest mistake White made, according to the paper, was to use outdated characters and concepts to classify Ardi and to fail to identify anatomical clues that would rule her out as a human ancestor. As an example, Sarmiento says that on the base of Ardi's skull, the inside of the jaw joint surface is open as it is in orangutans and gibbons, and not fused to the rest of the skull as it is in humans and African apes — suggesting that Ardi diverged before this character developed in the common ancestor of humans and apes.

White, no surprise, has defended his analysis, publishing a written response of his own in Science. In an e-mail exchange with TIME, he says, "Dr. Sarmiento's views appear to be uniquely his own. Most notable in Dr. Sarmiento's comment is his refusal to recognize as significant the multiple and independently derived features of the Ardipithecus cranium, dentition, and postcranial skeleton. These features uniformly align this primate with all later hominids to the exclusion of any other ape — living or fossil. Has Dr. Sarmiento shown how the Ardipithecus evidence better fits his interpretation than the one we published? Not here."

But Sarmiento's is not the only attack on White's and his colleagues' work. In a separate comment in Science, eight geologists and anthropologists from seven universities question White's conclusion that Ardi lived in a wooded area rather than on open grassland. At the launch of the Ardi papers, White had argued that the wooded terrain Ardi called home disproved the "savanna hypothesis" of bipedalism — the theory that what got our ancestors walking on two legs in the first place was a change in climate that transformed African forest into savanna. In such an environment, goes the reasoning, upright primates would have had the advantage over knuckle walkers because they could see over tall grasses to avoid predators and search for food and carry it back to their homes.

Reached by Internet chat in Kenya, University of Utah geochemist Thure Cerling, lead author of the Science critique, accuses the Ardi team of misinterpreting its data. Carbon measures and other studies on the rock around the Ardi site suggest that tropical grasses contributed up to 77% of Ardi's ecosystem, Cerling says. While acknowledging that there was evidence that woodland existed around Ardi as well, Cerling tells TIME that "early humans had available to them, at no great distance, the resources of the savanna and probably those of the riparian woodland as well." The savanna hypothesis, he adds, remains "a viable idea."

Responding to the critique, White accuses Cerling and his co-authors of "downplaying and ignoring" evidence of forests around Ardi in order to "accommodate their long-held hypothesis that earliest hominid evolution was savanna-driven." He adds that "we have never claimed that there was no grass in Ardi's world," but rather that early hominids simply had "preferences for — and adaptations to — woodland habitats."

While Sarmiento regards the hype around Ardi to have been overblown, Cerling says he still feels the discovery and re-creation of the ancient specimen to be a monumental breakthrough. But, he says, the science was in the evidence collected by White and colleagues, and not in their conclusions. "Many students will thoroughly examine the data and will come to their own independent evaluations," he says. In other words, science works a bit like evolution, and asking whether Ardi will survive as a major advancement is rather like going into the distant past and asking what the fate of her species would be: Only time will tell.