early hominid fossils from

The year was 1965

Bryan Patterson, a paleoanthropologist from Harvard University, unearthed a fragment of a fossil arm bone at a site called Kanapoi in northern Kenya. He and his colleagues knew it would be hard to make a great deal of anatomical or evolutionary sense out of a small piece of elbow joint. Nevertheless, they did recognize some features reminiscent of a 2 species of early hominid (a hominid is 5 any upright-walking primate) known as ; Australopithecus, first discovered 40 ent years earlier in South Africa by Raymond < Dart of the University of the Witwater-| srand. In most details, however, Patterson = and his team considered the fragment of = arm bone to be more like those of mod-

ern humans than the one other Australopithecus humerus known at the time.

And yet the age of the Kanapoi fossil proved somewhat surprising. Although the techniques for dating the rocks where the fossil was uncovered were still fairly rudimentary, the group working in Kenya was able to show that the bone was probably older than the various Australopithecus specimens that had previously been found. Despite this unusual result, however, the significance of Patterson's discovery was not to be confirmed for another 30 years. In the interim, researchers identified the remains of so many important early hominids that the humerus from Kanapoi was rather forgotten.

Yet Patterson's fossil would eventually help establish the existence of a new species of Australopithecus—the oldest yet to be identified—and push back the origins of upright walking to more than four million years ago. But to see how this happened, we need to trace the steps that paleoanthropologists have taken in constructing an outline for the story of hominid evolution.

An Evolving Story

SCIENTISTS CLASSIFY the immediate ancestors of the genus Homo (which includes our own species, Homo sapiens) in the genus Australopithecus. For several decades it was believed that these ancient hominids first inhabited the earth at least three and a half million years ago. The specimens found in South Africa by Dart and others indicated that there were at least two types of Australopithecus—A. africanus and A. robus-tus. The leg bones of both species suggested that they had the striding, bipedal locomotion that is a hallmark of humans among living mammals. (The upright posture of these creatures was vividly confirmed in 1978 at the Laetoli site in Tanzania, where a team led by archaeologist Mary Leakey discovered a spectacular series of footprints made 3.6 million years ago by three Australopithecus individuals as they walked across wet volcanic ash.) Both A. africanus and A. robustus were relatively small-brained and had canine teeth that differed from

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