PHOTOS: Oldest "Human" Skeleton Refutes "Missing Link"

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Editor's note: At the request of illustrator J.H. Matternes and Science/AAAS, we are removing the illustrations of Ardipithecus ramidus that were once featured here. The illustrations may be viewed in a scientific paper.

October 1, 2009--In 1994 a research team led by Tim White of the University of California, Berkeley; Berhane Asfaw, former director of the National Museum of Ethiopia; and Giday WoldeGabriel of the Los Alamos National Laboratory announced the discovery of the first fossils of a new human ancestor, Ardipithecus ramidus. The researchers presented tantalizing evidence that the species was a biped living in woodland conditions more than a million years before the famous "Lucy" fossil of the species Australopithecus afarensis.

The research, to be published in an October 2, 2009, special issue of the journal Science, reveals that our earliest ancestors underwent a previously unknown phase of evolution, shedding new light on the nature of the last common ancestor of chimpanzees and humans.

What did our earliest known ancestors look like? The skeletal material associated with Ardipithecus ramidus now provides the best evidence. (Above, a digital reconstruction of skull parts from two individuals is shown.)

"Ardi" stood about 47 inches (120 centimeters) tall and weighed about 110 pounds (50 kilograms). The face of "Ardi" did not project as much as those of modern apes, but was not as flat and massive as the later australopithecines. Researchers who studied the species suggest this difference is related to the small size of the species' incisor teeth compared to those of chimps. Based on the relatively small size of its brow ridge and canine teeth, scientists suggest this fossil is of a female.

While A. ramidus was quite apelike in appearance, researchers who studied the fossil skeleton suggest the species lacked the adaptations of living apes for climbing vertically, hanging from branches, and walking on its knuckles. Instead, it was a "careful climber" in the trees, and supported its weight on the palms of its hands while using its divergent big toe for grasping.

The feet of the new ancestor were more like those of Old World monkeys, which have feet that are more rigid and provide a better lever for walking. The lower part of A. ramidus's pelvis, however, retained an apelike condition, probably to accommodate strong leg muscles still useful for climbing.

Read more about Ardi >>
— Image courtesy Science/AAAS
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