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An Archeological Facelift
6 April 2005 (All day)
A computer reconstruction of a 7-million-year-old skull found among the shifting sand dunes of Chad's Djurab Desert suggests that the owner is the oldest known member of the human family.
The skull of Sahelanthropus tchadensis, nicknamed Toumai, was a dramatic find, offering the first glimpse of a primate alive at the dawn of humankind. But although the team that unearthed it--led by paleontologist Michel Brunet of the University of Poitiers, France--claimed the skull belonged to the oldest known hominid, rivals soon argued that Toumai looked more like a gorilla ancestor than a human (Science, 12 July 2002, p. 171).
To perform the reconstruction, Brunet's team brought the skull to neurobiologist Christoph Zollikofer and anthropologist Marcia Ponce de Leon, both of the University of Zurich (UZ), Switzerland. The skull had been crushed under a sand dune and distorted, but the researchers were able to erase the ravages of time in the computer, using three-dimensional computer graphics tools to rebuild it piece by piece. The resulting face is taller, with a bit more snout than seen in the original.
Zollikofer and Ponce de Leon then identified 39 landmarks on the skull, which they used to compare it directly with the skulls of fossil hominids, two chimpanzee species, and gorillas. They found that the shape of Toumai's skull "falls exactly within the hominids," says Zollikofer. No matter how they tried, they could not force the pieces of the skull to fit into the shape of a chimpanzee or gorilla skull without deforming it grossly. "It is impossible to reconstruct Toumai as an ape," he says.
The new analysis, published 7 April in Nature, also suggests that Sahelanthropus might have walked upright, a traditional marker of being a hominid. A virtual line from the top to the bottom of Toumai's eye orbit makes roughly a right angle with another virtual plane at the base of the skull. That right angle is also seen in humans, reflecting that the head sits directly atop a vertical spine, pointing the eyes forward during upright walking.
"What a facelift! This beautiful reconstruction is the outcome of high technology combined with a deep understanding of anatomy," says Tel Aviv University paleoanthropologist Yoel Rak. But some caution that although the new evidence, including new fossils of teeth and jaw fragments, helps build the case that Toumai was a hominid, its identity is not certain. "I'd be happy to put it down as [a very early] hominid," says anatomist Fred Spoor of University College London. "But it's a time we know so little about that I am still skeptical."