Old bones paint new picture of human ancestry

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. They come from an area where scientists have found over 1,900 fossils representing over 60 mammal species.

Ethiopian paleontologist Yohannes Haile-Selassie, a graduate student at the University of California, Berkeley, found the bones. He says that the teeth have some characteristics that are exclusive to hominids - the human ancestral group.

The back teeth are bigger than a chimpanzee's and the front teeth narrower, indicating they ate less fruit and leaves and more fibrous food.

"These fossils are strong evidence that lines leading to chimpanzees and humans had already split well before 5 million years ago," he says.

He also explains that the toe bone shows the kind of structure that is "consistent with an early form of terrestrial bipedality." Until now, the earliest fossils definitively considered pre-human date to 4.4 million years ago.

Three laboratories did the geological analysis of the fossil-bearing rocks - Los Alamos National Laboratory in New Mexico, Berkeley Geochronology Center in California, and at Miami University in Ohio. Giday WoldeGabriel at Los Alamos says "there is no doubt about the antiquity of these fossils - these are solid dates."

But it is the environmental implications that surprise team members. "The expectation was that we would find hominids in savannah grassland sites that date back to about 8 million years ago. That hasn't happened. All older hominids have been found in forested environments," says University of Illinois chemist Stanley Ambrose, who analyzed soils found with the fossils.

Analysis by Dr. WoldeGabriel and his colleagues show that, 5 million years ago, the newly discovered creatures roamed a region that was cooler, wetter, more forested, and up to 5,000 feet higher than it is today.

Mr. Haile-Selassie points out that it will take many more discoveries of very old fossils even to begin to answer the many questions the bones raise. This will be difficult, since hominid fossils 5 million or more years old are very rare.

Yet he observes: "Nobody thought we would find any hominids there at all. Now that we did, everybody is hoping that we will find a skeleton."

(c) Copyright 2001. The Christian Science Monitor