Begun and his colleagues say the fossil could represent the oldest known human ancestor. They further suggest that the fossil means our ancestors diverged from apes in Southern Europe — not Africa.

Both are bold and highly disputed claims. But the fossil itself is a rare specimen of an ape from around the time of the last common ancestor of chimpanzees and humans.

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In a pair of reports published Monday in the journal PLOS One, the scientists describe the fossil and the possible savanna environment in which the species lived. The researchers claim that the Eastern Mediterranean could “just as likely” be the location of ape and human diversification, as well as human ancestor origins, as tropical Africa.

Other experts on human prehistory disagree, pointing to extensive fossil evidence that hominins, meaning non-ape humans and their ancestors, originated in Africa and migrated north.

“David Begun has repeatedly proposed that the African ape and human clade arose in Europe and that gorillas, chimps and humans arose from an early European member of this group that migrated into Africa,” said Jay Kelley, a paleontologist at Arizona State University's Institute of Human Origins. “This 'back into Africa' scenario has garnered few if any adherents.” The “near consensus,” Kelley said in an email, is that “the hominin lineage arose in Africa.”

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The jaw in question belonged to a primate that anthropologists had previously named Graecopithecus freybergi. In the new study, researchers at the University of Tübingen in Germany used a CT scan to peer inside the jaw. They also analyzed an upper premolar tooth from another primate, dated via paleomagnetic studies to 7.2 million years ago, discovered in the Balkans in Bulgaria. The authors of the new studies suggest that this loose tooth could have come from another member of the same species.

The scans of the jaw showed some similarities with human ancestors. Despite the chipped teeth, the roots buried in the jawbone remained intact. “The canine root was very short,” Begun said. “The premolar roots were simplified, partly fused. Both of those characteristics we find only in members of human lineage.”

For the loose tooth, the thickness of the enamel ruled out other, better-documented ancient apes, the scientists said, such as Ouranopithecus. But they could not prove with absolute certainty that the tooth came from Graecopithecus freybergi.

“I really appreciate having a detailed analysis of the Graecopithecus jaw — the only fossil of its genus so far,” said Richard Potts, a paleoanthropologist who directs the Smithsonian Institution’s Human Origins Program. “But I think the principal claim of the main paper goes well beyond the evidence in hand.”

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Potts, who was not involved in this study, noted that “the heyday of ape diversity occurred in Eurasia between 12 and 10 million years ago.” This diverse group of primates migrated to lower latitudes — namely, Africa and Asia — to escape cooling northern temperatures and more powerful seasonal changes. There was “little to support” the idea that hominins appeared before leaving Europe.

“I don't think it's a strong case for hominin origins in Asia rather than Africa. Possible, yes,” said New York University's Susan C. Antón, a professor of anthropology who was not affiliated with the recent study. But because all of the later hominins are found in Africa, the simplest explanation, she said, “is an African origin.”

Kelley questioned the significance of the fused premolar root. Some of the earliest hominins didn’t have these fused tooth roots, whereas some of the later hominins did, he said. Given this discrepancy, it's a feature “that may have evolved independently in several different lineages,” he said.

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“The evidence that canine root reduction indicates the hominin status of Graeco is also not very convincing,” Potts said. Only one canine root was studied. Plus, he said, there's no way to consider the root in the context of the entire tooth — the canine crown had snapped off. “So there’s little basis for accepting the exceptional claim that a 7.2 million year old fossil from Greece is the oldest known human ancestor!”

“I'm the first to admit that what we have is less than ideal,” Begun said. “We need better-preserved jaws, and some limb bones to tell us if it was bipedal.” The site in urban Athens where the jaw was discovered is now too developed to hunt for fossils. But where the lone tooth was found in the Balkans, to the north across the Aegean Sea from Athens, ancient rocky outcrops remain.

Begun said he plans to travel to Bulgaria to look for additional fossils. “If you have one tooth,” he said, “that means there has to be other specimens out there.”