A new hominid skeleton found in east Africa suggests that ancestors of modern humans may have walked upright much earlier than previously thought, researchers say.

These anatomically arranged elements show the partial skeleton of a male Australopithecus afarensis specimen found in Ethiopia, which walked upright much like modern humans. (Yohannes Haile-Selassie/Cleveland Museum of Natural History/National Academy of Sciences)

The partial skeleton was unearthed in a mud flat in the Afar region of Ethiopia. The scientists who discovered the fossil say it's the earliest known example of Australopithecus afarensis — a mainly bipedal hominid that is widely believed to be an early ancestor of modern humans.

It dates from 3.6 million years ago — making it 400,000 years older than the famous "Lucy" skeleton that was found in the same general area in 1974.

Unlike Lucy — who researchers estimate was barely 1.1 metres tall — this new specimen suggests an individual between 1.5 and 1.8 metres in height.

Scientists have named the fossil Kadanuumuu — which means "big man" in the Afar language.

'Fully bipedal'

"This individual was fully bipedal and had the ability to walk almost like modern humans," said lead author Yohannes Haile-Selassie of the Cleveland Museum of Natural History.

"As a result of this discovery, we can now confidently say that Lucy and her relatives were almost as proficient as we are walking on two legs, and that the elongation of our legs came earlier in our evolution than previously thought," he said in a statement.

Examination of the Lucy skeleton has led some researchers to say she came from a group of hominids that were making the evolutionary transition from tree climbers to upright walkers.

But the team behind the Kadanuumuu fossil says the evidence now suggests that the transition was actually made hundreds of thousands of years earlier. They think the "big man" spent his time on the ground.

Not all scientists are convinced that the new specimen definitely belongs to A. afarensis, pointing out that the find lacks skull bones or teeth. The fossil includes a shoulder blade, a collarbone, part of an arm, part of the rib cage, a thigh bone, a shin bone, and fragments of the pelvic region.

An artist's conception shows the life-sized model of Lucy, the 3.2-million-year-old Australopithecus afarensis skeleton found in 1974. (Michael Stravato/Associated Press)

Analysis of the new fossil shows that the shoulder and rib cage are different from those of chimpanzees. "These findings further confirm what we concluded from the 'Ardi' specimen — that chimpanzees have undergone a great deal of specialized evolution since we shared a last common ancestor with them," co-author Owen Lovejoy of Kent State University told the National Geographic Society.

The discovery of the partial skeleton of Ardi — or Ardipithecus ramidus — was announced last year to great fanfare. Ardi is a 4.4-million-year-old hominid that was described as "a mosaic creature" that was neither chimp nor human.

The "big man" find was made by an international team led by Lovejoy and Haile-Selassie. The research was funded by the National Science Foundation, the Leakey Foundation, the Wenner-Gren Foundation and the National Geographic Society.

The team's study was released in the latest edition of the Proceedings of the National Academy of Sciences.

With files from The Associated Press