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For such a petite creature, the 1.2-meter-tall "Ardi" (Ardipithecus ramidus) has made big waves in the paleoanthropology world. The momentous find—announced 15 years ago and formally described in Science this October—has deepened academic debates about when bipedalism evolved, what our last common ancestor with chimpanzees looked like, and how some ancient primates gave way to modern humans.
"This is a fascinating fossil no matter what side you come down on," says William Jungers, a professor and chairman of the Department of Anatomical Sciences at the Stony Brook University Medical Center in Long, Island, N.Y. The 11-paper Science analysis has, indeed, sharpened more differences than it has smoothed over.
The authors of the papers, including Tim White of the University of California, Berkeley, propose that Ardipithecus was "an effective upright walker" and that it "resolves many uncertainties about early human evolution, including the nature of the last common ancestor." But many others in the field propose that some of these statements may be overblown. In fact, Jungers says, "I think some of the things they said might have been for effect."
So, does Ardi represent a true step toward humanity, or should she remain up in the side branches of the evolutionary tree? White and his fellow authors do not propose to have a definitive answer, but through painstaking analysis of the fossil data and surroundings, they conclude in the overview paper that, "There are no apparent features sufficiently unique to warrant the exclusion of Ar. ramidus as being ancestral to Australopithecus," thus proposing she might indeed be an early hominin (the ever-changing nomenclatural group that usually includes living humans and our close extinct relatives, also referred to by White et al. as hominids—although the latter title now often includes the great apes, as well)
But piecing together how Ardipithecus fits into the evolutionary story of humanity may prove even more difficult than reconstructing Ardi's fragmented and fragile bones, and the process has already turned out to be a contentious one.
Because the traditional hallmark of an early human has been the adaptation for upright walking, much of the debate over Ardipithecus's status hinges on how her lower body bones fit together—in particular, the position of her damaged ilium, the winglike upper pelvis bone. Depending on how this bone is oriented, muscles around the hip joints work differently, explains David Begun, a professor in the Department of Anthropology at the University of Toronto. The summary in one of the Science papers, led by Owen Lovejoy of Kent State University in Ohio, argues that by Ardi's time, "the gluteal muscles had been repositioned so that Ar. ramidus could walk upright without shifting its center of mass from side to side" (unlike today's lumbering great apes), but a different interpretation of the ilium could change all of that.
Despite the numerous images and descriptions put forth by the researchers, others are reluctant to take the reconstructions without a grain of salt. Begun says: "Maybe the pieces do fit together nicely, but the reality is they start out with a very damaged specimen, and they end up with something very similar to an australopithecine" (the group that includes "Lucy," the 3.2-million-year-old Australopithecus as well as a 2.7-million-year-old Paranthropus). "It's very difficult not to make them look like something you have in your mind if there's any chance of play," he says. Jungers also notes the perils of reconstruction, which in a case like Ardi's "requires a lot of guesswork."
As the upper pelvis appears like it could belong to an early human, the bottom part looks more like a quadrupedal, nonhuman primate, says Jungers, who recently met with White and examined photos of the bones. White asserts, however, that after working with the fossils himself, there is no way that they could belong to "an animal that wasn't often walking on its hind legs," unless the data "were deliberately ignored, or if we had made them up," he argues.
Even if Ardi's reconstructed hips don't convince everyone, her feet could provide some important insights into the species's locomotion. In a summary to one of the Science papers led by Lovejoy, the authors note that, "although the foot anatomy of Ar. ramidus shows that it was still climbing trees, on the ground it walked upright." Ardi's feet do point to a comfort with life in the trees. Her big toe, which Jungers calls "remarkably primitive," is quite divergent—even more so than the grasping digit in modern-day chimpanzees—which would help with climbing.
None of the known foot components, no matter how well adapted to climbing, preclude Ardipithecus from walking upright on the ground. Jungers, however, thinks "it really doesn't show any adaptations for bipedalism at all." In fact, he says, many components of Ar. ramidus don't make Ardi look that much more adept at walking upright than chimpanzees—a primate that White et al. disavow as a model for early human evolution. In a summary paper led by Lovejoy, the authors describe Ardipithecus as a "facultative upright walker," one that can walk on two legs if needed (to carry something in the forearms, for example) but that isn't necessarily prone to do so.
"What's ironic is that that's how you would describe bipedality in chimpanzees," Jungers says—"they're facultative bipeds." Homo erectus, on the other hand, which lived about 2.6 million years after Ardi, were obligate bipeds, and he points out, "even humans are facultative climbers."
No matter how some of Ardi's bones are assembled or reassembled, the debate about how the species got around on the ground may not be settled by further analysis of this specimen. Even though hundreds of bones were uncovered, the species still lacks a knee joint. "I think a knee joint would seal the deal one way or another," Begun says. And more foot bones wouldn't hurt either, Jungers notes.
Rather than continue the lineage debate below the belt, Ardi's most important features might rest above her shoulders, Jungers says. "We have to abandon bipedality as the hallmark of being a 'hominin' sensu stricto if we hope to keep Ardi in our clade," he wrote in an e-mail to ScientificAmerican.com
"If we had just found the fossils below the neck, it's possible we wouldn't be talking about Ardi as a hominin at all," he says.
The numerous cranial pieces that the research team uncovered might, however, help sway the debate toward the early human camp. In a conversation with White, Jungers says, he was compelled by the dental evidence—especially the upper canine teeth, which were smaller and more humanlike than those of chimpanzees—to consider Ardi as an early step in human evolution. The small canines and minimal size difference between males and females of the species are "indicative of minimal social aggression," the authors of one of the Science summaries (led by White) wrote. If males didn't compete for females through physical aggression, co-author Lovejoy has argued, they might have been more involved in raising offspring—a key component of later human evolution.