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Skull of Oldest Human Ancestor Found in Africa
By Guy Gugliotta
Washington Post Staff Writer
Wednesday, July 10, 2002; 2:04 PM
Anthropologists working in the bleak desert of Northern Chad have unearthed the 7 million- year-old skull of the oldest human ancestor yet found, offering the first opportunity to study a human fossil from the remote slice of time when humans and apes diverged from a common ancestor.
Scientists hailed the discovery – a nearly complete skull, two lower jaw fragments and three teeth – as a major breakthrough in the study of human prehistory, opening researchers to a new region of Africa and a time period about which virtually nothing was known.
"It takes us into another world, of creatures that include the common ancestor, the ancestral human and the ancestral chimp," George Washington University paleobiologist Bernard Wood said. "It is a world of creatures we are unfamiliar with."
Everything scientists know about human evolution is based on a relatively small number of fossilized bones that have been unearthed over many decades of searching. The new discovery pushed the fossil record backward for the third time in the past two years, moving scientists ever closer to a dimly understood moment somewhere between 5 million and 10 million years ago when humans and chimpanzees diverged from a common ancestor.
These earliest humans – represented by a smattering of skulls, bones and bone fragments – dotted the African landscape until distinctly human species arose about 2 million years ago. Modern humans are only about 100,000 years old.
The discovery, reported yesterday, was made in Chad's Djurab Desert by a 40-member multinational team of scientists led by French paleoanthropologist Michel Brunet. Results were reported in today'sThurs issue of the scientific journal Nature.
The team dubbed the new fossil Sahelanthropus tchadensis, referring to the northern Sahel region of Chad where it was found, and, with the blessing of the Chadian government, also decided to call the fossil Toumai, a Goran-language name given to babies born just before the dry season. It means "hope of life."
"It's very emotional to hold the beginning of human lineage in my hand," Brunet said in an interview relayed by Nature from Chad. "Here we are not far from the divergence between chimp and human."
The team said in Nature that Toumai displayed a "unique mosaic" of ape-like and human characteristics. The brain case--between 320 and 380 cubic centimeters--is comparable to that of a chimpanzee.
But tooth enamel was thicker than a chimpanzee's, indicating that Toumai did not dine exclusively on the fruit diet common to apes, and the team said that Toumai's snout did not protrude as far as a chimpanzee's – making it more humanlike.
The telling difference, however, was that Toumai's canine tooth was much shorter than the more prominent canines displayed by all apes past and present: "The fossil is showing the first glimmerings of evolution in our direction," said University of California at Berkeley anthropologist Tim White. "This is by no stretch of the imagination a chimpanzee."
Several scientists noted that the mix of characteristics fit nowhere conveniently in any evolutionary tree yet devised, lending credence to the view that human evolution was a more "bushy" process in which species arose and died out, with modern humans emerging from a jumble of changes that may never be deciphered.
"We've traditionally had this very linear view that evolution was a straight-ahead slog directed by natural selection from primitiveness to perfection," said Ian Tattersall of the American Museum of Natural History. "Clearly it's been much more trial-and-error."
But White cautioned that the fossil record from Toumai's rough contemporaries was too sparse for quick judgments. Neither of the previous "oldest" finds--a 6 million-year-old fossil from Kenya and a 5.5 million-year-old fossil from Ethiopia--is complete enough to provide a worthy comparison:
"All this spin about diversity is completely misdirected," White said. "We hear about all these bushy trees. If you have diversity, where's the second species?" The Kenya find "is three (thigh bones) and some teeth." The Ethiopia find "is 11 bone fragments and no skull."
The team noted that while "it will never be possible to know precisely where or when" humans originated, the new discovery shows that the spread of human ancestors was not confined to Eastern and Southern Africa.
"To study human origins, you need places where the animals lived," said White. "First you had the South African caves and the East African Rift Valley, and now Brunet has found the Chad Basin and opened up a new area and with huge potential."
Brunet and other members of the team began working in the region 20 years ago, and homed in on the Djurab Desert in the 1990s. Seven million years ago, Lake Chad was much larger, and what is now the desert presented a spectacular mix of enviornments--forest, river, lake and savannah.
And a spectacular mix of animals. Since 1994, the team has unearthed more than 10,000 vertebrate fossils in the area, including elephants, antelopes, three-toed horses, giraffes, hyenas, hippopotamus, wild boar, crocodiles, fish and a variety of rodents. It was the animal species that enabled the team to date the site where the fossil was found in July 2001.
The discoverer was team member and university undergraduate Ahounta Djimdoumalbaye, who saw the Toumai skull embedded in a wind-scoured flange of sandstone about 500 miles northwest of N'Djamena, the Chadian capital. Seven more months of excavation yielded the other fragments.
White and several other experts agreed that the new find should put to rest the long-held preeminence of Eastern and Southern Africa in the study of human evolution:
"At this point, nobody can draw a human evolutionary tree without knowing it's completely wrong," said Harvard anthropologist Dan Lieberman. "We've been assuming that the fossils in East and South Africa are sufficient to answer the questions we have about human evolution. Guess what. They're not."
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