The skull itself also raises questions about how similar Ardi was to our other ancestors, such as Lucy. The authors of the Science papers make note of the smaller lower face in Ardipithecus, which doesn't project as much as a chimpanzee's and is shaped more like that of Australopithecus. But outside researchers focus on the similarity in size to other nonhuman primates, such as extinct Miocene epoch apes.
White, however, prefers to take the specimen in full, calling pointedly piecemeal analysis "entirely hypothetical, and actually unrealistic." He grants that "if only an intermediate hand phalanx had been found, then it would not have been possible to ascertain the phylogenetic relationships of the species," but he concludes that, "the characters of the dentition, skull and postcranial skeleton…are all uniquely shared by Ar. ramidus and later hominids, to the exclusion of all other extant and extinct apes," he wrote in an e-mail. "Even without the cranium and dentition," he maintains, "the same case would still be supported because of the shared derived traits in the hip and the foot."
He and his collaborators do not insist on upright walking as the only indicator that Ardi and her clade were indeed early humans, but he notes that so far it is part of the picture. He says that although their group's definition of the family "Hominidae" was "not based on bipedality, per se," the designation "appears consistent with both bipedality and SCC [sectorial canine complex] loss happening close to the time of divergence" of the human and chimpanzee lines.
A place in the evolutionary trees
Long an evolutionary mystery, the last common ancestor shared by humans and chimpanzees may be at least partially clarified by the discovery of Ar. ramidus, argued the authors of the 600-plus pages of material submitted to Science about the species.
Ardi does help to settle some important debates about this crucial creature, such as whether our early ancestors walked on their front knuckles like modern chimpanzees. (Now it appears that they probably did not.) But as Jungers points out, the notion that humans evolved from chimpanzees (or even a chimpanzeelike creature) is already an antiquated one. Likewise, to paint a picture of a last common ancestor that lived some six million to eight million years ago through 4.4-million-year-old Ardi would be a difficult task, notes Begun. "In the same way Tim [White] argues that it's naive to assume" chimpanzees haven't evolved in millions of years, Begun says, it may be naive to assume Ardi bears much resemblance to a common ancestor.
Begun and others are perhaps slower to propose a place for Ardi in the direct human line than are the project researchers, who note that even though the species is "substantially more primitive than Australopithecus" (as they wrote in a summary led by White), "it appears…to have occupied the basal adaptive plateau of hominid natural history" (as another summary, led by Lovejoy, noted).
But as difficult as it has been to claim Ardi as a close relative, it has also been difficult to dismiss her. "I don't think its unfair to say that Ardi's precise phylogenic position is unclear and debatable at this time," Jungers comments. Even White notes that "the three most likely possibilities" are that Ardipithecus is either in the human line, chimpanzee line or predates both. "We assign it to the hominid [aka hominin] clade based on a series of newly evolved characters that it shares exclusively with all other members of that clade—Australopithecus species and Homo sapiens," he says.
From studying the published data in Science, Begun found "very little in the anatomy of this specimen that leads directly to Australopithecus, then to Homo sapiens," he says. "This could very easily be a side branch."
Broader analyses of Ardi's place in the primate family tree and her role in the move to upright walking may have to wait until the original fossils and their casts become available for other researchers to examine. "We're raring to go to see how Ardi fits in," Jungers says. White himself seems anxious to let others see for themselves the evidence that he is confident in: "We welcome these investigators to have a close comparative look at the fossils before drawing conclusions on something as important as bipedality."
Opportunities to have a close look at the originals are being granted on a case-by-case basis. A more complete analysis of the find is forthcoming in publications on the larger Middle Awash Project (to be published by the University of California Press).*
Regardless of the eagerness to lay eyes on Ardi and the other specimens as well as lingering questions about the species's status as a hominin, most researchers applaud the significant work involved in excavating and analyzing the fossils. "What those guys did was pretty amazing," Jungers says. The extensive documentation of Ar. ramidus' context has "set a new standard," which, he says, is "truly extraordinary."
*Correction (11/19/09): This paragraph was changed after publication to reflect the current Middle Awash Project policies for viewing.