Old Bones Paint New Picture of Human Ancestry ; Fossils May Be the Oldest Yet on Trail That Led from Apes to Humans - and Show Forest, Not Plains, as Habitat

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A handful of very old bones is prodding scientists to rethink key assumptions about the evolution of our earliest ancestors.

They come from sediments in Ethiopia that date back 5.2 million to 5.8 million years. That's close to the time that paleontologists think the evolutionary tracks leading to modern humans and chimpanzees diverged.

A toe bone among the 11 fossil fragments indicates the creatures walked on their hind legs. Scientists have considered that trait to be a key indicator of the human evolutionary line. They have also speculated that the trait arose when climate change forced our forest dwelling ancestors to adapt to a dryer, more open savannah environment.

The new findings, reported yesterday in the journal Nature, challenge both assumptions.

First, the fossils come from sediments accumulated in a cool, high-altitude forest, suggesting that bipedal walking developed before the supposed climate change. This, in turn, raises a second question: whether bipedal walking was a trait our ancestors shared with other creatures not on the human evolutionary track.

The findings encourage scientists to consider the possibility that bipedal walking "might have occurred in lineages of apes that are now extinct," as well as in the human line, explains Nature senior editor Henry Gee, himself a paleontologist.

"It is possible," he adds, that some of the very old, supposedly pre-human fossils being found "might not be hominids at all." For example, a 6 million-year-old "hominid" fossil recently reported from Kenya has yet to be clearly classified.

The current discoveries are discussed in two separate papers in Nature. They result from ongoing research in the international Middle Awash project, named for the study area in Ethiopia. The project, carried out under the auspices of the Ethiopia's Ministry of information and Culture, is supported by the US National Science Foundation and several US universities. About 45 scientists from a dozen countries are involved.

The fossil fragments include a jawbone with teeth, several hand and foot bones, arm bone fragments, and a collar bone piece. They apparently represent at least 5 individuals probably about the size of a modern chimpanzee. …


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