Scientists who three years ago discovered a nearly complete 7 million-year-old skull in central Africa have dug up additional evidence supporting the conclusion that the skull belonged to the earliest known human ancestor.
The new findings -- two jaw bones and an upper premolar tooth -- lend credence to the proposition that the creature was probably among the first hominid, or human-like, primates to live after humans and chimpanzees diverged from each other a little more than 7 million years ago.
Researchers said the new fossils, along with a sophisticated computer reconstruction of the previously discovered skull, solidify the remains' stature as among the most important paleological finds of the past several decades. Together they paint a picture of an ancestral primate with a chimpanzee-sized body and brain but a face and teeth more like those of modern humans.
The new work also offers tantalizing clues that this extremely early human, Sahelanthropus tchadensis, may have already been walking upright -- though a definitive answer to that important question will probably have to await the discovery of a leg or hip bone.
The fresh findings are "very cool," said John G. Fleagle, a distinguished professor of anatomical science at Stony Brook University in New York. For scientists who have been questioning whether S. tchadensis was really an early human or belonged on the branch of the family tree that led to modern apes instead, the new evidence "clearly follows the hominids, and pulls away from the chimps and gorillas," Fleagle said.
The initial find of the skull, along with two lower jaw fragments and three teeth, was reported in July 2002 by Michel Brunet of the University of Poitiers in France and his colleagues. They found the fossil remains in Chad's Djurab Desert, about 1,500 miles west of Kenya's Rift Valley, where so many fossils of early humans have been found.
Though dry today, the Djurab region was home to a large lake and rich biological diversity several million years ago, as evidenced by the plentiful fossil remains of ancient hippopotami, elephants, antelopes, crocodiles and rodents. Dating methods place those fossils as about a million years older than the 6 million-year-old remains of an early human unearthed in 2001 in Ethiopia, which were then the oldest such bones ever found.
The Chad skull, believed to be of a male primate who has been nicknamed Toumai (a Goran language name that means "hope of life"), was described three years ago as probably more human than ape, with a low bony brow and a flattened face that had a snout less prominent than in chimpanzees. But in part because the skull was partially crushed, questions lingered.
Foremost among several tentative lines of evidence for Toumai being a hominid is his canine tooth, which is far shorter than the prominent canines sported by all apes, both today and in the past. The newly discovered premolar tooth is also more human than ape, the team noted in a report published today in the journal Nature.
But equally compelling, scientists said, is a computer-assisted reconstruction of the skull described in an accompanying Nature article.
That aspect of the work, led by Christoph P.E. Zollikofer and Marcia S. Ponce de Leon of the University of Zurich-Irchel in Switzerland, started with a CAT scan of the skull, which had become deformed during its 7 million-year-old interment. It looks as though it had been squeezed centrally from both ears, with the right side of the cranium shifting upward and the left pushed downward. "The thing was sort of squashed and unlike anything that had been seen before, so you could only say so much about it," Fleagle said.
With a detailed three-dimensional X-ray image in hand from the CAT scan, the team was able to move the pieces around -- on a computer monitor -- until they lined up and fit together in what appears to have been their original form. That image reveals a skull wider than initially anticipated, with round eye sockets that look more human than ape. The relatively vertical face and other cranial and dental features "support the conclusion that Sahelanthropus is a hominid," the team concluded.
In addition to fitting all the pieces together, the researchers did an experiment: They tried to get the virtual pieces to fit into the general outline of a hominid skull, and also tried to get them to fit into an outline of a 3-D ape skull.
In the first case, the pieces fit together almost perfectly. In the second, there were overlaps and gaps -- evidence that the reconstruction was correct.
"The digital restoration is excellent," said anthropologist Tim D. White of the University of California. "The original interpretation [that Toumai was a hominid] is probably correct."
The researchers offer preliminary evidence that Toumai may have walked upright. Most compelling, the reconstruction of the skull suggests the cranium lined up vertically with the neck and spine, as in modern humans. By contrast, animals that walk on all fours, such as chimpanzees and gorillas, keep their heads turned upward to keep their faces perpendicular to the ground.
But there is not enough preserved anatomy to settle that question definitively, the team concluded. Also unknown is whether Toumai's offspring survived the thousands of millennia to become modern humans or died out, as many early human lines may have done.
In that case, the actual forebear of today's humans may remain unknown.