Ardi evolved from the common ancestor we share with chimpanzees and was equally at home walking on the ground and swinging through the trees

The remains of a female who lived and died at the dawn of humanity have been uncovered in Ethiopia, giving the clearest picture yet of the origin of our species.

The partial skeleton, the oldest from a human ancestor ever ­discovered, belonged to a female who walked on two legs but was adept at climbing trees and moving through the forest canopy some 4.4m years ago.

Experts have described the find as the most important regarding human evolution in the past century.

The female, named Ardi by the researchers who worked on her, belongs to a new species Ardipithecus ramidus and may be the earliest human ancestor ever discovered that was capable of walking upright.

The finding sheds light on a critical but unknown period of evolution at the root of the human family tree, shortly after our ancestors split from chimpanzees more than 6m years ago.

Remnants of the skeleton, skull, pelvis, hands, feet and other bones were excavated from the reddish-brown sediments of an ancient river system near the village of Aramis in northern Ethiopia, along with fragments from at least 35 other individuals.

Fossil hunters first glimpsed the new species in 1992 when a tooth belonging to Ardipithecus was spotted among pebbles in the desert near Aramis. Over the next two years, the researchers scoured the area on hands and knees and slowly uncovered pieces of bone from the hand, ankle and lower jaw, and finally a crushed skull.

A total of 47 researchers then spent a further 15 years removing, preparing and studying each of the fragments ahead of the publication tomorrow of an in-depth description of the species in 11 papers in the US journal Science.

Their investigation shows Ardi stood four feet (1.2m) tall and weighed a little under eight stone (50kg), making her similar in size and weight to a living chimpanzee. But many of Ardi's features are far more primitive than those seen in modern apes, suggesting chimpanzees and gorillas have evolved considerably after they split from the common ancestor they shared with humans.

The discovery of Ardi provides vital clues about the earliest human ancestor that lived at the fork in the evolutionary road that led to humans on one side and chimps on the other.

"Darwin was very wise on this matter. Darwin said 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," said Tim White, a lead author on the study and professor of human evolution at the University of California, Berkeley. "Well, we haven't found it, but we've come closer than we've ever come, at 4.4 million years ago."

The remains of animals, seeds and pollen uncovered at the excavation site reveal it to have been a woodland where colobus monkeys swung in trees full of swifts, doves and lovebirds, and spiral-horned antelope, elephants, shrews and early forms of peacock roamed the forest floor below.

The discovery is being seen as more important than Lucy, the 3.2m-year-old skeleton of a potential human ancestor which proved at a stroke that early humans walked upright before evolving large brains. The remains of Lucy, who belongs to the species Australopithecus afarensis, were uncovered in another part of Ethiopia in 1974.

"We thought Lucy was the find of the century but, in retrospect, it isn't," palaeontologist Andrew Hill at Yale University told Science. "It's worth the wait."

Measurements of Ardi's skeleton reveal she had a brain the size of a chimp's, but very long arms and fingers, and opposable toes that would have helped her grasp branches while moving through the forest.

Though Ardi would have spent much of her time in the trees, her pelvis was adapted to walking upright when she came down to the forest floor. Her unusual skeleton led White to comment of her species that "if you wanted to find something that moved like these things, you'd have to go to the bar in Star Wars."

Analysis of Ardi's teeth points to a diet of figs and other fruit, leaves and small mammals. Remarkably, both male and female Ardipithecus had very small incisors and canines, which are enlarged in modern apes. The finding suggests that unlike chimpanzees, baboons and gorillas, the male did not bare its teeth in battles over females and was already part of a more cooperative social group. It was probably involved in the parenting process.

"Natural selection has led to the reduction of this male canine tooth very, very early in time, right at the base of our branch of the family tree."

It may take years to confirm exactly where Ardi fits in the history of human evolution. One possibility is that she is a direct ancestor of Lucy's species, Australopithecus.

"The most important thing in the broader sense is that we now no longer have to guess about where we came from ... We now have an evidentiary basis for understanding that we didn't get here in the form we see today, we evolved," said White.

Chris Stringer, head of human origins at the Natural History Museum in London, said: "This is as important as the Lucy skeleton in terms of what it tells us about an even earlier stage of human evolution."

"The assumption among many researchers is that while humans have evolved a lot, chimps haven't changed much, so we can use them as a model of the common ancestor we shared. But why shouldn't chimps have changed? Everything evolves."

"We are really trying to establish what set us off on our evolutionary path," he added. "What would start the process off? That is one of the great mysteries."

• This article was amended on 6 October 2009. Some references in the original called Ardi a woman. This has been corrected.