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Untangling Ancient Roots: Earliest hominid shows new, improved face

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1:48pm, April 6, 2005
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Two new lines of evidence bolster the claim that the oldest known member of the human-evolutionary family lived in central Africa between 6 million and 7 million years ago.

In 2001, at a site in Chad, anthropologist Michel Brunet of the University of Poitiers in France and his coworkers found jaw fragments, isolated teeth, and the nearly complete skull of a creature that the researchers identified as a hominid and assigned to the category Sahelanthropus tchadensis. The skull combines a cranium suitable for a chimp-size brain with facial and tooth structures resembling those of later human ancestors (SN: 7/13/02, p. 19: Evolution's Surprise: Fossil find uproots our early ancestors).

After the discovery, a group of researchers led by Milford H. Wolpoff of the University of Michigan in Ann Arbor initiated a controversy by contending that Sahelanthropus looks more like a fossil ape than a hominid (SN: 10/19/02, p. 253: Available to subscribers at Fossil skull spurs identity dispute).

Brunet's team now reports the discovery of two more jaw pieces and another tooth from Sahelanthropus. The researchers unearthed the specimens in Chad at three locations, including the site of the prior finds.

The new fossils cement Sahelanthropus' position as a hominid that lived shortly after modern humans' evolutionary family diverged from chimpanzee ancestors, the scientists argue in the April 7 Nature.

In an accompanying paper, Brunet and his colleagues—with Christoph P.E. Zollikofer of the University of Zurich—present a computerized, three-dimensional reconstruction of the ancient creature's skull. The digital version corrects crushed sections of the actual fossil.

"This is a state-of-the-art technique for taking a squished specimen and putting it back together digitally," remarks Tim White of the University of California, Berkeley. White regards the Chad fossils as those of a hominid. Only further discoveries can establish whether Sahelanthropus represented a separate genus or belonged to a previously identified group of nearly 6-million-year-old African hominids dubbed Ardipithecus, he says.

However they're labeled, the new fossils exhibit key hominid traits, Brunet holds. For instance, one jaw piece retains a canine tooth that's smaller than the teeth of chimps but comparable to those of hominids that came after Sahelanthropus. Some newfound cheek teeth also contain thinner enamel layers than corresponding teeth from chimps do.

The virtual skull further establishes Sahelanthropus' hominid status, in Brunet's view. It features a relatively flat face and a forward-positioned opening for the spinal cord, both typical hominid characteristics. The angle of the creature's neck was similar to that of later hominids that are known to have walked upright, the scientists say.

More-revealing tests of the ancient creature's walking behavior await the discovery of fossils from its legs, pelvis, and feet.

The new reports leave Michigan's Wolpoff unconvinced that Sahelanthropus was a hominid. The jaws and teeth of apes and early hominids look much alike, so the new fossil finds don't clarify the creature's evolutionary identity, he says.

Moreover, the reconstructed skull contains bony attachments for neck muscles that were far too large to accommodate a regular upright stance, Wolpoff says. Neck muscles of that size might have attached to unusually long arms on a ground-dwelling ape, not a hominid, he suggests.

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