Radiometric dating human fossils

"This find is far more important than Lucy," said Alan Walker, a paleontologist from Pennsylvania State University who was not part of the research.

"It shows that the last common ancestor with chimps didn't look like a chimp, or a human, or some funny thing in between." (Related: "Oldest Homo Sapiens Fossils Found, Experts Say" [June 11, 2003].) Ardi Surrounded by Family The Ardipithecus ramidus fossils were discovered in Ethiopia's harsh Afar desert at a site called Aramis in the Middle Awash region, just 46 miles (74 kilometers) from where Lucy's species, Australopithecus afarensis, was found in 1974.

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In the end, the research team recovered more than 125 pieces of the skeleton, including much of the feet and virtually all of the hands—an extreme rarity among hominid fossils of any age, let alone one so very ancient.

"Finding this skeleton was more than luck," said White.

Ardi instead shows an unexpected mix of advanced characteristics and of primitive traits seen in much older apes that were unlike chimps or gorillas (interactive: Ardi's key features).

As such, the skeleton offers a window on what the last common ancestor of humans and living apes might have been like.

According to the researchers, the pelvis shows a similar mosaic of traits.

The large flaring bones of the upper pelvis were positioned so that Ardi could walk on two legs without lurching from side to side like a chimp.Radiometric dating of two layers of volcanic ash that tightly sandwiched the fossil deposits revealed that Ardi lived 4.4 million years ago.Older hominid fossils have been uncovered, including a skull from Chad at least six million years old and some more fragmentary, slightly younger remains from Kenya and nearby in the Middle Awash.Modern chimps and gorillas have evolved limb anatomy specialized to climbing vertically up tree trunks, hanging and swinging from branches, and knuckle-walking on the ground.While these behaviors require very rigid wrist bones, for instance, the wrists and finger joints of Ardipithecus were highly flexible.As a result Ardi would have walked on her palms as she moved about in the trees—more like some primitive fossil apes than like chimps and gorillas.

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