"We saw something that talked about special knuckle walking adaptations in modern African apes," Dr Richmond said.

The hand will give us new insight into Australopithecus behaviour

"I could not remember ever seeing anything about wrists in fossil hominids. I thought 'Oh my god, I don't think anyone's looked at this.'

"Across the hall was a cast of the famous fossil Lucy. We ran across and looked at it and bingo, it was clear as night and day. It was a eureka moment."

Lucy's stiff wrists suggest that her ancestors - and ours - walked on their knuckles.

"We have found evidence in the wrist joint that sheds new light on arguably the most fundamental adaptation in humans ... which is why did humans start walking upright?" Dr Richmond said.

Flexible wrists

"Walking upright is the hallmark of humanity. It is the feature that defines all of our ancestors to the exclusion of our ape relatives."

Lucy, who lived in Africa between 4.1 and 3 million years ago, did walk upright. Her hip and leg bones make that clear.

But she did not have the flexible wrists that allowed later humans to throw spears and make tools.

Dr Richmond said it took a year of "tedious work" to prove that other pre-humans, notably Australopithecus anamensis, also had the stiff wrist.

The finding in no way suggests that humans descended from apes.

Long fingers

Experts say humans, chimps, gorillas and other apes descended from a common ancestor and evolved independently.

Humans came to walk upright, leaving their hands free to gather and carry things, while apes did not.

"It is thought that knuckle walking is something that allows these animals to retain long fingers that allow them to climb in trees, but to also walk using their forearms," Dr Richmond said.

"But for that you need a very stable wrist joint. Chimpanzees and gorillas have a special locking mechanism that prevents the wrists from cocking back very far."

Lucy has it too. Dr Richmond said it was not until Australopithecus africanus emerged, about 2.5 million years ago, that the wrist became mobile like that of modern humans.

The research is published in Nature.