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Primatologists go ape over Ardi

LONDON—When Tim White of the University of California, Berkeley, agreed to speak at a human origins meeting* at the Royal Society here, he sent no abstract and provided only a one-word title: “Ardipithecus.”

The 2 October 2009 cover of Science: Partial skeleton of Ardipithecus ramidus, a hominid species living about 4.4 million years ago in Ethiopia. This female stood about 1.2 meters high© T. White, 2008

But that one word was enough to earn him a spot on the podium and a prominent role at the meeting. After 15 years of study, White and his colleagues have just published their massive, 108-page report on Ardipithecus ramidus, at 4.4 million years the oldest partial skeleton of a putative human ancestor (see the Science special issue of 2 October, cover at right). The 2-day meeting marked White’s first in-person presentation of Ardipithecus to a roomful of independent researchers; co-author anthropologist C. Owen Lovejoy of Kent State University in Ohio also made the trip. Their emphasis on Ardi’s non-apelike features, however, drew a tart response from some primatologists in the room.

The roughly 50-kilogram female, which the Ardipithecus team concludes walked upright although it also spent time in the trees, has a decidedly un-chimplike anatomy. White and colleagues therefore have asserted that living apes are not good models for understanding the last common ancestor (LCA) of humans and chimpanzees—a claim that has stung many primatologists. “I assumed … chimps might be helpful in tackling the challenges of human evolution,” said chimpanzee expert William McGrew of the University of Cambridge in the United Kingdom. But after Ardi, “all this chasing after chimpanzees was deemed to be irrelevant” to human evolution. McGrew, the first speaker, challenged a statement in one of the papers in Science that “no modern ape is a realistic proxy for characterizing early hominid evolution—whether social or locomotor.”

White, Lovejoy, and their colleagues base this conclusion on many features of Ardi’s postcranial anatomy. Although some researchers had assumed that the LCA had arms adapted for swinging in the trees and walked on its knuckles like chimps, A. ramidus lacks these and many other features possessed by modern apes such as chimps and gorillas. The team argues that Ardi’s anatomy suggests that its behavior and social structure also differed from that of modern chimps. For example, one of the papers, authored by Lovejoy, suggests that A. ramidus males were probably much less aggressive toward each other than chimp males are and that they pair-bonded with females.

McGrew told the attendees that although he agreed that “the LCA was not a chimpanzee,” the behavior of living chimps such as those at left can still inform hypotheses about the LCA’s behavior. McGrew listed the considerable evidence for chimp behaviors that possibly mirror those of extinct and modern humans, such as the use of complex tools, aiming and throwing objects, the construction of sleeping nests or platforms, and evidence for considerable spatial cognition such as the ability to remember the locations of thousands of trees in a forest. At least some of these behaviors, McGrew said, are shared by chimps and modern humans because they have deep evolutionary roots. “So to hypothetically credit the LCA with the ability of [chimps] is not unreasonable,” McGrew concluded.

A chimpJupiterimages

At the end of the day, when the meeting was thrown open for discussion among the roughly 200 attendees, White countered McGrew’s argument, pointing to what he saw as the dangers of using a chimp model for the LCA’s behavior. “If we try to model the LCA or even the earliest hominids based on living chimps, which have these adaptations to [swinging in the trees], to moving through that canopy so well and so quickly that they can take down a red colobus monkey, we could be very misled. Ardipithecus probably couldn’t do that, and the LCA probably couldn’t do that.”

Yet White says he’s not trying to toss out all chimp research. “The fact that a few highly derived descendants managed to survive until today obviously enhances our appreciation of their evolution,” he told Science. “It also provides perspective on ours. The study and conservation of chimpanzees and bonobos are surely justifiable on their own merits, even though [early] hominid skeletal anatomy, behavior, and ecology do not match those seen now in these specialized persistent ape lineages.”

And Carol Ward, a paleoanthropologist from the University of Missouri School of Medicine in Columbia, points out that White and Lovejoy “aren’t saying that [studies of] chimps aren’t useful but that we didn’t evolve from chimps, so they are useful as referential models rather than actual representatives of what our ancestors were like.” Chris Stringer of the Natural History Museum in London, who co-organized the meeting with Alan Walker of Pennsylvania State University, University Park, also sees a role for chimp work in evolutionary studies. “There is such a lot of excellent and important data on ape behavior,”  Stringer told Science, “that we would be foolish to throw it out as providing potential models for the behavior of early hominins.”

*The First 4 Million Years of Human Evolution, London, 19-20 October 2009.