Human ancestor Lucy ‘had knee joints that allowed her to walk fully upright’

Lucy, the world’s most famous early human ancestor, had knee joints that allowed her to walk fully upright as well as strong leg and pelvic muscles suited to living in trees, research suggests.

For the first time, scientists at the University of Cambridge digitally reconstructed the lower limb muscles of the extinct ape-like relative who lived in Africa more than three million years ago.

The results, published in the journal Royal Society Open Science, suggest Lucy could straighten her knee joints, giving her the ability to stand upright and walk on two legs with an erect posture, similar to modern-day humans.

The team also found that her leg muscles were bigger and more powerful than those seen in modern humans, enabling her to live in trees like apes.

Lucy’s unique lower body muscle structure would have helped her adapt to life in the open grasslands as well as dense forests, according to the researchers.

But it also means she would have walked and moved in a way that cannot be seen in any living species today.

Still, the team said, the findings may help shed light on how physical movement evolved in humans, “including those capabilities we have lost”.

Dr Ashleigh Wiseman, from the University of Cambridge’s McDonald Institute for Archaeological Research, said: “We are now the only animal that can stand upright with straight knees.

“Lucy’s muscles suggest that she was as proficient at bipedalism as we are, while possibly also being at home in the trees.”

Lucy belongs to the extinct species Australopithecus afarensis, a group of small-bodied and small-brained early human relatives.

Her skeleton, which is about 40% complete with 47 out of 207 bones, was discovered in 1974 in northern Ethiopia.

It includes parts of her arms, legs, spine, ribs and pelvis, as well as her lower jaw and several other skull fragments.

At the time of her discovery, Lucy – whose name comes from the Beatles classic Lucy In The Sky With Diamonds – was the most complete known early human ancestor.

Over the years, experts have disagreed on how Lucy walked. Some believe she may have crouched and waddled like chimpanzees while others say her gait was closer to modern-day humans.

At the time of her death, she would have been a young adult just over one metre tall and weighed under 30kg, while her brain would have been roughly a third of the size of a human’s.

More recent research suggests she may have walked fully erect, so to find out more, Dr Wiseman constructed a digital 3D model of Lucy’s leg and pelvis muscles.

She recreated 36 muscles in each leg and found that the major muscles in Lucy’s calves and thighs were more than twice the size of those in modern humans.

The team realised muscles made up 74% of the total mass in Lucy’s thigh, compared to 50% in humans.

Dr Wiseman said: “Lucy’s ability to walk upright can only be known by reconstructing the path and space that a muscle occupies within the body.”

She added: “Australopithecus afarensis would have roamed areas of open wooded grassland as well as more dense forests in East Africa around three to four million years ago.

“These reconstructions of Lucy’s muscles suggest that she would have been able to exploit both habitats effectively.”