Father of Us All?

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ILLUSTRATION FOR TIME BY C.F. PAYNE

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In the Rift Valley, to the east, torrential downpours erode the jagged hillsides and bring ancient fossils to the surface. In the Djurab, Brunet and his colleagues realized, the relentless winds would have a similar erosive effect. So, accompanied by some 40 scientists from 10 countries, Brunet's Franco-Chadian Paleontological Mission began to dig in the Djurab in 1994.

It didn't take long for it to become clear that his reasoning was dead on. To date, the team has found 10,000 fossils in the area, including those of all sorts of animals and, seven years ago, at least one ancient hominid — a 3.5 million-year-old jaw that some researchers believe may come from the same species as Lucy (though Brunet has other ideas). Then, on July 19 of last year, a student named Ahounta Djimdoumalbaye unearthed the astonishing skull. "He is by far our best fossil hunter," says Brunet. "[Our colleague] Patrick Vignaud and I said to him that he would be the one to make a huge discovery."

The cranium was somewhat squashed, and blowing sand had eroded some of its detail, but it was nearly complete. And in a game where an entire jawbone is regarded as a rare treasure, the fossil was almost a miracle. Over the next seven months, the team found pieces of what it believes are at least five individuals of the same species, including two lower-jaw fragments and three isolated teeth. Without a telltale foot, leg or other skeletal feature, the team could not be positive that the animal walked upright, but its skull is similar in important ways to those of hominids that did.

Ideally, the researchers would have preferred to find the bones sandwiched between layers of volcanic ash containing potassium and argon, as these can be precisely dated by tests involving radioactive decay. Unfortunately, the geology at Toros-Menalla did not cooperate. But the scientists found something nearly as good. The site was replete with fossils from all sorts of other primitive animals, including fish, crocodiles, rodents, elephants, giraffes, aardvarks and more--42 types in all. Many were identical to specimens that have been radiometrically dated with great precision elsewhere. As a result, a team led by Vignaud confidently pinpointed the skull's age at between 6 million and 7 million years, probably much closer to the latter.

That puts the new Sahelanthropus tchadensis at a crucial evolutionary crossroads. Scientists have long believed that apes and humans share a common ancestor. But recently, comparisons of fossil and modern primates and analyses of modern ape and modern human dna have independently indicated that a single ancestral ape gave rise to both chimps and hominids between 5 million and 7 million years ago. That presumed great-great-great-grandape almost certainly swung from trees in the African forest. If so, then Sahelanthropus, or Toumai, could well have been the very first hominid, or at least one of the first, to begin the evolutionary march that ultimately led to Homo sapiens.

But in the contentious field of human paleontology, "could well have been" leaves plenty of room for heated argument. There seems little doubt, at least, that Toumai was truly a hominid. Though the skull and brain are no bigger than a chimp's, that is no surprise. Our characteristically large brains did not evolve until about 2 million years ago, well after Lucy's time. But features like a short face with a massive brow ridge, a mouth and jaw that protrude less than in most apes, and relatively small canine teeth make it clear that this creature was not a chimpanzee.

Where exactly Toumai falls in the evolutionary scheme of things, though, depends largely on whom you ask. A number of distinguished paleontologists, including Bernard Wood, Ian Tattersall of the American Museum of Natural History in New York City, and Chris Stringer of London's Natural History Museum, perceive the face to be jarringly modern — more modern even than Lucy's species, Australopithecus afarensis, which is between 3.6 million and 2.9 million years old — and thus quite different from what they expected to see in such an ancient hominid.

Over the years since Lucy was found, several even older hominids, including Ardipithecus ramidus ramidus (4.4 million years old) and Ardipithecus ramidus kadabba (5.8 million), have been put forward as the most ancient of our direct ancestors. But Toumai is older still. If it is as modern looking as Wood believes, Lucy and the others may not be our direct ancestors at all but instead dead-end side branches of the family tree, like the Neanderthals. That would make them not our great-great-great-grandparents but rather ancient uncles and aunts whose lineages have long since gone extinct. One possibility is that Sahelanthropus gave rise to intermediate descendant species that have not yet been discovered. These descendants would have led to Homo habilis or Homo rudolfensis, both of which are contenders for the first member of our genus, which arose about 2 million years ago.

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