Between the trees and ground, there was Ardi

She stood 4ft (122cm) tall but she was no lightweight - her muscular body weighed nearly eight stone. She could climb trees easily with the help of long arms, huge hands and grasping toes, but "Ardi" could also walk fully upright on two legs - the first known human ancestor with a bipedal gait.

With a hairy body and snout-like face, Ardi must have looked more ape than human when she roamed her woodland habitat in East Africa some 4.4 million years ago.

But Ardi's uncertain role in the story of human origins has now become clearer following an exhaustive investigation into the 110 fragments of fossilised bones belonging to her species.

It is now clear that Ardipithecus ramidus is the earliest unequivocal member of the long lineage extending to anatomically modern humans, Homo sapiens, from the last common ancestral species we shared with chimpanzees, our closest living relatives whose DNA is 99 per cent similar to our own.

In addition to the many fragments of bones, scientists have found that a partial skeleton of a female Ardipithecus - Ardi, as she is affectionately called - is the oldest, most complete set of fossilised remains belonging to the many ancestors descended from that elusive common ancestor, a so-far undiscovered species that is believed to have lived between about 6 million and 7 million years ago.

Following studies into every aspect of the anatomy and habitat of Ardipithecus, scientists yesterday presented the results of their research, spanning 17 years, in the form of 11 separate scientific papers published in the journal Science.

The investigation, involving fossil collection in the field and sophisticated analysis in the laboratory, has revealed how this early human ancestor bridged the divide between the purely tree-dwelling past of our more distant primate relatives and the grassland savannah habitat of our fully bipedal ancestors.

Among the most surprising findings was that the conventional view of human evolution, that our ancestors must have been knuckle-walking creatures, has been exploded. It is now clear that modern-day chimps have diverged from our last common ancestor just as much, and as dramatically, as modern humans.

"Charles Darwin was very wise on this matter," said Professor Tim White of the University of California Berkeley, one of the leaders of the research. "He said that we have to be really careful. The only way we're really going to know what this last common ancestor looked like is to go and find it.

"Well, at 4.4 million years ago we found something pretty close to it. And just like Darwin appreciated, evolution of the ape lineages and the human lineage has been going on independently since the time those lines split, since that last common ancestor we shared," he said.

Ardi may have been bipedal, but the way she walked on her two legs was not exactly the way scientists had imagined bipedalism to have evolved. Professor White once quipped to a colleague that if you want to find something that moved like Ardi must have moved, you had to go to the bar in Star Wars.

"Ardipithecus is not a chimp. It's not a human. It's what we used to be. You're seeing a mosaic creature, that is neither chimpanzee, nor it is human. It is Ardipithecus," he said.

The story of Ardipithecus began in December 1992 when a former graduate student of Professor White's called Gen Suwa saw a glint of light coming from a patch of desert scrub near the village of Aramis in the Afar region of north-eastern Ethiopia. It was a reflection from the polished surface of a molar tooth belonging to a hominid - the lineage descended from the last common ancestor.

Within a few years the scientists had amassed a rich collection of fossilised bones belonging to Ardipithecus ramidus and dating showed that at 4.4 million years it was more than a million years older that the previously oldest member of the lineage, a species known as Australapithecus afarensis, whose most famous fossil was "Lucy", discovered in 1974. Like Lucy, Ardi had a relatively large brain, not much bigger than a chimp's and about a third of the size of modern man. Ardi's face had a muzzle, but it jutted out less than that of a chimp and she had lost the long, dagger-like canine teeth possessed by apes.

Ardi's cranial base - the distance between the back and the front of the skull - is short, indicating that her head was balanced on top of her spine, like other upright walkers, rather than to the front of the spine as in quadrupedal apes.

But whereas Lucy was fully bipedal and had lost the adaptations that allow apes to climb trees easily, such as an opposable big toe, Ardi still retained the anatomical features in its feet and arms that point to a partial tree-dwelling existence.

Her teeth are protected by moderately thick enamel, thinner than the tough enamel seen in later hominids such as Lucy which ate tough, abrasive food, but not as thin as the enamel found in modern-day chimps, which have a diet rich in soft fruits. This finding suggests that Ardi had an intermediate, omnivorous diet of fruits, roots, insects, eggs and perhaps small mammals.

Her hands were capable of grasping objects, which is believed to have been an essential attribute that allowed primates to become so unusually intelligent. It allowed them to pick things up, to manipulate them and, in the case of chimps and humans, to use them as tools.

But it would be another couple of million years before Ardi's descendants developed the large brains and higher intelligence that separates humans from the rest of the animal kingdom. It is now clear that the expansion in brain size came long after the evolution of upright walking on two legs.

ARDIPITHECUS RAMIDUS

* The oldest known ancestor of modern humans.
* The bridge between tree-dwelling and bipedal apes.
* At 4.4 million years it was more than a million years older than the previously oldest member of the lineage.
* A brain a third the size of modern humans.

- INDEPENDENT