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Ardi (right) lived in a forest in Africa. Her fossil skeleton shows that she walked upright and yet had an opposable toe, good for climbing trees. (Tim White 2009, From Science Oct. 2 issue; Illustration by J.H. Matternes)

The Human Family's Earliest Ancestors

Studies of hominid fossils, like 4.4-million-year-old "Ardi," are changing ideas about human origins

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In 2000, Martin Pickford of the College of France and Brigitte Senut of the National Museum of Natural History in Paris announced their team had found an even older hominid—13 fossils representing a species that lived six million years ago in the Tugen Hills of Kenya. Two of the fossils were thighbones, including one that provided the oldest direct evidence of upright walking in a hominid. They named this creature Orrorin tugenensis, drawing on a Tugen legend of the “original man” who settled the Tugen Hills. Informally, in honor of its year of discovery, they called it Millennium man.

Hot on the heels of that discovery came the most surprising one of all—a skull from Chad, about 1,500 miles west of the Great Rift Valley of eastern Africa where many of the most ancient hominids have been found. A Chadian student named Ahounta Djimdoumalbaye picked up a ball of rock on the floor of the Djurab Desert, where windstorms blow sand dunes like waves on a sea and expose fossils buried for millions of years. When Djimdoumalbaye turned over the stone, he stared into the vacant eye sockets of an ape-like face—the skull of a primate that lived six million to seven million years ago on the shores of an ancient lake. It had traits that suggested it was a hominid—a small lower face and canines and a skull that seemed to sit atop its spine, as in upright walkers. Paleontologist Michel Brunet, then of the University of Poitiers in France, introduced it as the oldest known hominid, Sahelanthropus tchadensis. (Its nickname is Toumaï, which means “hope of life” in the Goran language.) But proving that a skull walked upright is difficult, and questions linger about whether Sahelanthropus is a bona fide hominid or not.

Taken together, fossils discovered over the past 15 years have provided snapshots of several different creatures that were alive in Africa at the critical time when the earliest members of the human family were emerging. When these snapshots are added to the human family album, they double the time researchers can see back into our past—from Lucy at 3.2 million years to Toumaï at almost 7 million years.

One of the most sought-after fossils of that distant era was Lucy’s direct ancestor. In 1994, 20 years after Lucy’s skeleton was discovered, a team in Kenya led by Meave Leakey (the wife of Richard Leakey) found teeth and parts of a jaw as well as two pieces of shinbone that showed the creature walked upright. The fossils, named Australopithecus anamensis, were 4.1 million years old.

“This has been a fascinating 40 years to be in paleoanthropology,” says Johanson, “one of the great times to be in this field.” But, he adds, “there’s still enormous confusion” about the murky time before 4 million years ago.

One thing that is clear is that these early fossils belong in a class by themselves. These species did not look or act like other known apes or like Lucy and other members of Australopithecus. They were large-bodied ground dwellers that stood up and walked on two legs. But if you watched them move, you would not mistake them for Lucy’s species. They clung to life in the trees, but were poised to venture into more open country. In many ways, these early species resemble one another more than any fossils ever found before, as if there was a new developmental or evolutionary stage that our ancestors passed through before the transition was complete from ape to hominid. Indeed, when the skulls of Toumaï and Ardi are compared, the resemblance is “striking,” says paleoanthropologist Christoph Zollikofer of the University of Zurich in Switzerland. The fossils are too far apart in time to be members of the same species, but their skulls are more like each other than they are like Lucy’s species, perhaps signaling similar adaptations in diet or reproductive and social behavior.

The only way to find out how all these species are related to one another and to us is to find more bones. In particular, researchers need to find more overlapping parts of very early fossils so they can be compared directly—such as an upper end of a thighbone for both Ardi and Toumaï to compare with the upper thighbone of O. tugenensis.

At Aramis, as soon as the clan leaders gave the Middle Awash team their blessing, White began dispatching team members like an air traffic controller, directing them to fan out over the slope near Ardi’s grave. The sun was high in the sky, though, making it hard to distinguish beige bone among the bleached out sediments. This time, the team found no new hominid fossils.

But one morning later that week, the team members drove up a dry riverbed to a site on the western margin of the Middle Awash. Only a few moments after hiking into the fossil beds, a Turkish postdoctoral researcher, Cesur Pehlevan, planted a yellow flag among the cobbles of the remote gully. “Tim!” he shouted. “Hominid?” White walked over and silently examined the molar, turning it over in his hand. White has the ability to look at a tooth or bone fragment and recognize almost immediately whether it belongs to a hominid. After a moment, he pronounced his verdict: “very good, Cesur. It’s virtually unworn.” The molar belonged to a young adult A. kadabba, the species whose fossils began to be found here in 1997. Now the researchers had one more piece to help fill in the portrait of this 5.8-million-year-old species.

“There’s your discovery moment,” said White. He reflected on the fossils they’ve bagged in this remote desert. “This year, we’ve got A. kadabba, A. anamensis, A. garhi, H. erectus, H. sapiens.” That’s five different kinds of hominids, most of which were unknown when White first started searching for fossils here in 1981. “The Middle Awash is a unique area,” he said. “It is the only place on the planet Earth where you can look at the full scope of human evolution.”

Ann Gibbons is a correspondent for Science and the author of The First Human: The Race to Discover Our Earliest Ancestors.

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