One Giant Step For Mankind

Meet your newfound ancestor, a chimplike forest creature that stood up and walked 5.8 million years ago

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As is often the case with discoveries like this, Haile-Selassie was not specifically looking for the things he found. He had set out to better understand how the ancient ecosystems worked and evolved. "I didn't even think about finding hominids," he says. "All I wanted to do was collect enough vertebrate bones so that I could write my dissertation." In December 1997, though, at a place called Alayla, he spotted a piece of jawbone lying on the rock-strewn ground. "I picked up the mandible less than five minutes after we got there," he recalls, "but didn't realize I had something really special until a year later, when we found some more bones and I started the serious analysis."

In all, the team eventually found 11 specimens--from at least five different individuals--in a cluster of sites, including Haile-Selassie's partial lower jaw with associated teeth, several hand and foot bones, and pieces of three arm bones and a collarbone. Luckily, the fossils were trapped in sediments that were sandwiched between layers of volcanic ash, whose age can be accurately gauged by a technique known as argon-argon dating. (This layering is still visible in places that have not been so heavily eroded, enabling the scientists to trace the area's geologic history.) The verdict, confirmed by a second dating method and by the other primitive animals found with the hominid remains: most of the fossils are between 5.6 million and 5.8 million years old, although one toe bone is a few hundred thousand years younger.

It was the detailed anatomy of these fragmentary fossils, especially the teeth, that convinced Haile-Selassie that he had discovered a new human ancestor. Although apelike, the lower canines and upper premolars, in particular, display certain traits found only in the teeth of later hominids--the term scientists use to describe ourselves and our non-ape ancestors. They also differ in shape from the teeth of all known fossil and modern apes. Even the way in which the teeth had been worn down was telling. Explains Haile-Selassie's thesis adviser, Berkeley paleontologist Tim White: "Apes all sharpen their upper canines as they chew. Hominids don't." The new creature's back teeth are larger than a chimp's too, while the front teeth are narrower, suggesting that its diet included a variety of fibrous foods, rather than the fruits and soft leaves that chimps prefer.

When Haile-Selassie compared the newly discovered bones and teeth with those of Ardipithecus ramidus, a 4.4 million-year-old hominid found in the Middle Awash in the early 1990s that was the previous record holder, he realized that the two creatures were very similar. But the older one's teeth, while different from an ape's, do have a number of characteristics that are decidedly more apelike than those of the younger hominid.

On the basis of these minor but distinctive differences, Haile-Selassie decided to classify the new human ancestor as a subspecies, or variant, of ramidus and has given it the name Ardipithecus ramidus kadabba. (The name is derived from the local Afar language. Ardi means ground or floor; ramid means root; and kadabba means basal family ancestor. In accordance with the sometimes bizarre nomenclature of science, the younger creature now gets renamed Ardipithecus ramidus ramidus.)

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