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You’ll Never Guess Who Walked In!

Ardi redefines the branch between apes and hominins

Pat Shipman

More Surprises

When Tim White, Gen Suwa of Tokyo University, and Berhane Asfaw described the fossils now known as Ardipithecus ramidus in 1994, they classified the new specimens as Australopithecus. Because the specimens showed some unusual features that differed from other hominins, they defined a new species, Au. ramidus, derived from the Afar word ramid, meaning root.

A comparison of teeth and pelvisesClick to Enlarge ImageIn all biological sciences, naming a new species requires that the scientists fully describe the specimens anatomically and then provide a diagnosis, stating clearly in what ways the type specimen (the object to which the name is formally attached) differs from other known species. At the time, all they had were teeth and jaws, two cranial fragments and a few arm bones—but these pieces were enough to indicate that these 4.4 million-year-old remains were something new. Au. ramidus had small canine teeth, like hominins and unlike apes. Small canine teeth that don’t project below (or above) the rest of the tooth row are a key feature separating apes from hominins. Relative to the cheek teeth, the canines of Au. ramidus were still large but were not projecting. Tellingly, the first milk molar—the tooth that falls out in humans at about age five—was small, narrow and very similar to that of a chimpanzee.

At the end of the paper, the team made a quiet pronouncement that raised a few eyebrows:

We have taken a conservative position here regarding the placement of …[these] fossils at the family and genus levels. The major anatomical/behavioural threshold between known great apes and the Hominidae is widely recognized to be bipedality and its anatomical correlates…. The anticipated recovery at Aramis of additional postcranial remains, particularly those of the lower limb and hip, may result in reassessment of these fossils at the genus and family level.

All taxonomic assessments are subject to revision if new specimens show features not previously known, so why include such a statement at all? I think that the authors had named a new species—only a new species, not a new genus—because they had limited material, none of which answered the crucial question: How did this creature move? Depending on what the rest of this fossil creature looked like, they anticipated that they might need to shift ramidus onto the ape branch of the evolutionary tree (even though the teeth looked homininlike) or—if its pelvis and legs showed evidence of two-legged locomotion—into a new hominin genus. Perhaps they had one of those nonscientific hunches that the postcranial bones would be downright surprising.

In less than nine months’ time, an extraordinary but brief corrigendum appeared in Nature, written by the same authors. The name of the fossils was revised from Australopithecus ramidus to Ardipithecus ramidus (abbreviated as Ar. ramidus to prevent confusion). The new generic name, Ardipithecus, comes from the word for “ground” or “floor” in Afar. I know of no other instance in which the naming of a new genus has been published as a correction to a previous paper. More commonly, a new genus is named as part of a detailed reanalysis of already-known material and/or with a thorough description of additional, new fossils. That was not the case here.

The major distinction between Australopithecus and Ardipithecus mentioned in the corrigendum is that the dentition of the new genus is less dominated by large cheek teeth than the former. The implied explanation for the need to revise the name at all was a bald statement that a partial skeleton, including a jaw full of teeth and many postcranial bones, had been found 50 meters from the type specimen and at the same stratigraphic level. “Analysis of this specimen has begun,” the authors deadpanned, “and will provide further features with which to characterize Ardipithecus.

The paleoanthropological community buzzed with excitement at this revision. First of all, the note was slipped into a major scientific journal in an unusual way. Second, the paper was as clever a way of saying “We’ve just found something amazing—wait until we tell you!” as anyone could imagine.

For nearly 15 years, the Aramis team held their peace and worked fervently on analyzing and understanding their new specimens, which continued to pile up. The most spectacular was the partial skeleton of Ardi. About the only comment made in public about the locomotion of Ardipithecus was a quip made by Tim White at a conference, “If you wanted to find something that moved like these things, you’d have to go to the bar scene in Star Wars.”

In October 2009, the team published their work in yet another unconventional way. They took over most of an issue of Science with 11 papers (more than 50 published pages of text and figures) written by 47 coauthors on Ardipithecus’ anatomy, inferred behavior, geology, dating, associated animals, plants and invertebrates, and preservation. At least as many pages of additional information appeared only in the supporting online material.

The decision to put all of this work into an issue of Science was influenced by a concern to make the facts accessible. Says White, “With the click of a mouse, Science makes available—to anybody with an Internet connection—a massive amount of material and data. This is effectively monographic treatment, for free, now.” A conventional monograph is planned for the future.

In a very real sense, the biggest surprise of all was what Ardipithecus ramidus was like. It was like nothing ever seen before.

Ardipithecus may not be the elusive last common ancestor of chimps and humans, but it is far and away the best known of the oldest species. Another species in the genus, Ar. kadabba, lived at least 5.8 million years ago. Together, these new materials give us stunning insights into what the last common ancestor was like.

Practically every paleoanthropologist in the world has drawn a large Y on the blackboard for a class, labeling the lefthand branch “chimpanzees” and the righthand one “modern humans,” and putting an X on the stem just below the divergence and labeling it “last common ancestor.” Based on genetic distances between chimps and humans, that last common ancestor probably lived 6 to 7 million years ago.

One of the key points Ardipithecus has revealed is how our thinking has been influenced by the very shape of this common Y diagram. Because we humans have foolishly thought of chimpanzees and other apes as primitive, we have been misled into thinking that the last common ancestor would somehow be a sort of half-chimp and half-human. Some even fell into the trap of thinking that the last common ancestor was basically a chimp, forgetting that chimpanzees have had as long to evolve since that ancestor as we have. (Students who are a bit confused on this point are always asking, “But if we are descended from the apes, why aren’t there new humans walking out of the jungle today?”)

“We have seen the ancestor,” says Tim White succinctly, “and it is not a chimpanzee.”

The ancestor also was not Lucy. As the best-known specimen of Australopithecus afarensis—a species that lived between 3.6 and 2.9 million years ago—Lucy became iconic of early hominins. Lucy was an especially petite and diminutive female, standing about 1.1 meters tall, weighing about 29 kilograms, with a reconstructed braincase of about 380 cubic centimeters. When she was discovered in 1974, she seemed incredibly primitive and apelike, so the notion took root in many people’s minds that our earliest ancestors ought to be small, like Lucy.

In contrast, Ardi is surprisingly large, which would have had a profound impact on her ecological niche. She stood perhaps 1.2 meters tall, weighed a hefty 50 kilograms, and had a brain size similar to Lucy’s, variously estimated between 300 and 350 cubic centimeters. She was no chimpanzee, but she was within the same size range (both bodily and brainily) as modern chimpanzees and bonobos.

Like a chimp, Ardi had long arms, but her fingers were only moderate in length, and her thumbs were intermediate between a chimpanzee’s relatively short thumbs and humans’ relatively long thumbs. Like a chimp, she had a divergent big toe and long toes that could be used to grasp tree branches during climbing.

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posted by Michael Waldrep
March 17, 2010 @ 2:14 PM

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