Scientists announce discovery of earliest prehuman skeleton

'Ardi' lived 4.4 million years ago

By Carolyn Johnson
Globe Staff / October 1, 2009

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More than a hundred crushed fossil fragments unearthed in Ethiopia have been painstakingly pieced together to reconstruct "Ardi," the earliest skeleton of a prehuman ever found, providing an extraordinary new glimpse into the largely mysterious period when the ancestors of humans and chimpanzees split, paleontologists said today.

At simultaneous press conferences held in Addis Ababa and Washington, D.C., scientists announced the results of 17 years of excavation and study: the partial skeleton of a 4.4 million-year-old female specimen who would have stood about four feet tall and weighed 110 pounds. Ardi is nicknamed for her species, Ardipithecus ramidus, and offers scientists a bevy of body parts -- skull, teeth, pelvis, hands, legs, and feet -- to examine for clues about the deepest roots of our evolution.

Ardi's skeleton will be studied for years to come. But in a special issue of the journal Science to be published tomorrow, 11 papers representing the work of an international team of 47 scientists reveal the initial findings and interpretation -- including the conclusion that she was less like a chimpanzee than expected.

"This is a spectacular collection of fossils from an especially important ancestor; this is one of the biggest finds in the last 50 years of human evolutionary studies," said Carol V. Ward, a professor of anatomy at the University of Missouri-Columbia who specializes in studying apes that lived more than 5 million years ago, and was not involved in the research. "You have an animal ... that is quite clearly a close cousin of ours, if not an ancestor, but [from] earlier in time than most fossils ever found. It is really an outstanding opportunity and is just unparalleled."

The new creature was farther back on the human side of the primate family tree than Australopithecus afarensis, best known by the famous 3.2 million-year-old Lucy skeleton. Ardi is discernibly more primitive than Lucy, scientists found, and might even have been an ancestor of Australopithecus. But perhaps most striking is its bizarre mixture of traits.

Ardi's pelvis is a mosaic -- some features are specialized for climbing, but some indicate the creature would have walked upright, a hallmark of the human lineage. Her hands and feet are like those of primitive extinct apes, with a grasping toe, and -- unlike humans and Australopithecus -- no arch in the foot. There is no evidence she walked on her knuckles.

"A lot of people were happy to hypothesize that as you went back, into that first half of human evolution since the last common ancestor, as you found these fossils they'd be increasingly chimpanzee-like," said Tim D. White, a paleontologist at the University of California, Berkeley, and a leader of the research team. "We have something getting pretty close to it in time, and it turns out it doesn't look chimpanzee-like; it's an unexpected combination of characteristics, some of which are new in evolution and put this pretty firmly on our side of the family tree and some others that are very primitive."

Ardi's fossilized remains were very fragile, sometimes disintegrating when they were touched -- and splintered into many pieces as if they had been trampled by a large animal, such as a rhinoceros, White said. But after years of excavation, reconstruction, and analysis he and others think her remains provide further evidence that contrary to the widespread belief, depicted in the familiar cartoon of a chimpanzee slowly "evolving" into a caveman holding a club, that our last common ancestor was not in fact chimplike.

Donald Johanson, the founding director of the Institute of Human Origins at Arizona State University and the scientist who discovered Lucy, said the find had been eagerly awaited by scientists, and was likely to spark vigorous debate as outside scientists began to examine an interpret the finding themselves.

"I think it's a siginficant discovery ... and will generate an enormous amount of controversy," said Johanson, anticipating the storm of debate over the coming years as scientists try to understand whether this creature walked on two feet or how to understand its strange mixture of traits. "I think it's very important to say that this supports the long held idea that we did not evolve from things that look like modern apes."