Did the skull found in Chad belong to one of our earliest ancestors?
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More Evidence of Skull's Link to Humans
Remains Believed To Be From Earliest Known Ancestor
By Rick Weiss
Washington Post Staff Writer
Thursday, April 7, 2005; Page A03
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.
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John Hachmann aa #1782
"Those who can make you believe absurdities can make you commit atrocities"
-Voltaire
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