Image caption The fossil is about 3.2 million years old

New fossil evidence seems to confirm that a key ancestor of ours could walk upright consistently - one of the major advances in human evolution.

The evidence comes in the form of a 3.2 million-year-old bone that was found at Hadar, Ethiopia.

Its shape indicates the diminutive, human-like species Australopithecus afarensis had arches in its feet.

Arched feet, the discovery team tells the journal Science, are critical for walking the way modern humans do.

"[The bone] gives a glimpse of foot anatomy and function," explained William Kimbel, director of the Institute of Human Origins at Arizona State University, US.

"It is the fourth metatarsal bone, which resides on the outside of the middle part of your foot, and which helps support the well-developed arches of the foot that we see in the soles of modern human feet.

"The bone that was recovered from the Hadar site has all the hallmarks of the form and function of the modern human foot," he told the BBC.

Arch types

Palaeo-scientists knew A. afarensis spent some of its time standing tall; that much has been clear since 1974 when they first examined a skeleton of the species, famously dubbed "Lucy", also found near the village of Hadar in the Ethiopian rift valley.

Image caption The area around Hadar continues to reveal remarkable information about human evolution

But the absence of important foot bones in all of the specimens uncovered to date has made it difficult for researchers to understand precisely how much time Lucy and her kin spent on their feet, as opposed to moving through the branches of trees.

Human feet are very different from those of other primates. They have two arches, longitudinal and transverse.

These arches comprise the mid-foot bones, and are supported by muscles in the soles of the feet.

This construction enables the feet to perform two critical functions in walking. One is to act as a rigid lever that can propel the body forwards; the other is to act as a shock absorber as the feet touch the ground at the end of a stride.

In our modern ape cousins, the feet are more flexible, and sport highly mobile large toes that are important for gripping branches as the animals traverse the tree tops.

Professor Kimbel and colleagues tell Science journal that the feet of A. afarensis' say a lot about the way it lived.

Image caption The position of the fourth metatarsal in a human foot

It would have been able to move across the landscape much more easily and much more quickly, potentially opening up broader and more abundant supplies of food, they say.

"Lucy's spine has the double curve that our own spine does," Professor Kimbel said.

"Her hips functioned much as human hips do in providing balance to the body with each step, which in a biped of course means that you're actually standing on only one leg at a time during striding.

"The knees likewise in Lucy's species are drawn underneath the body such that the thighbone, or femur, angles inwards to the knees from the hip-joints - as in humans.

"And now we can say that the foot, too, joins these other anatomical regions in pointing towards a fundamentally human-like form of locomotion in this ancient human ancestor."

A. afarensis is thought to have existed between about 2.9 million and 3.7 million years ago, and the Hadar area has yielded hundreds of fossil specimens from the species.

Long road

Commenting on the latest research, Professor Chris Stringer, a palaeoanthropologist at London's Natural History Museum, said scientists were gradually filling in the detail of this creature's position in the human origins story.

Image caption An artist's impression of what Lucy might have looked like

"Bipedalism in Lucy is established, but there has been an issue about how much like our own that bipedalism was," he told BBC News.

"Was it a more waddling gait or something more developed?

"And certainly there's evidence in the upper body that the Australopithecines still seemed to have climbing adaptations - so, the hand bones are still quite strongly curved and their arms suggest they're still spending time in the trees.

"If you are on the ground all the time, you need to find shelter at night and you are in a position to move out into open countryside, which has implications for new resources - scavenging and meat-eating, for example.

"If the Australopithecines were on that road, they were only at the very, very beginning of it."

Jonathan.Amos-INTERNET@bbc.co.uk