How did the 3.2-million-year-old Australopithecus afarensis named Lucy walk?

When the remains of an early human ancestor were found in Ethiopia in 1974, the discovery provided unprecedented insight into a species that lived millions of years before humans walked the Earth.

The rare fossil, representing 40% of a skeleton belonging to a female, has been named “Lucy”, after the Beatles song “Lucy in the Sky With Diamonds”.

Now researchers are using the skeleton to figure out how this ancient human relative moved 3.2 million years ago. The results of the study were published on Tuesday in the journal Royal Society Open Science .

Lucy was shorter than the average human, reaching about 3.3 feet (1 meter) tall, had an ape-like face, and a brain about one-third the size of a human brain.

Analysis of Lucy’s fossil over the past 20 years suggests that she and others of her species walked upright. But the study’s lead author, Dr. Ashleigh LA Wiseman, a research associate at the University of Cambridge in the UK, wanted to go a step further and recreate a component of Lucy that didn’t fossilize: her muscles.

“The defining aspect of what makes us human is the ability to walk on two legs, but understanding how and why this evolved has long been debated,” said Wiseman, who is Leverhulme Trust Early Career Fellow and Isaac Newton Trust. Fellow at the McDonald Institute for Archaeological Research.

“With recent advances in computer modeling, it is now possible to investigate these questions. Of course, in the fossil record, we are left looking at bare bones. But muscles animate the body – they allow you to walk, run, jump and even dance. So if we want to understand how our ancestors moved, we first need to reconstruct their soft tissues.”

Studying Australopithecus afarensis fossils can shed light on the evolution of bipedalism, or upright walking, and when it appeared in early human ancestors. Wiseman’s reconstructions of Lucy’s muscles could also be used to determine how Lucy moved in other ways.

Rebuilding Lucy’s Muscles

Wiseman and his colleagues developed a method called polygonal muscle modeling and initially used it to rebuild the missing soft tissues of extinct reptiles called archosaurs that lived 247 million years ago.

So Wiseman applied the same method to Lucy for the first time to understand the shape and size of her muscles and how she used them to move, judging whether it was like the crouching waddle of an upright chimpanzee or the posture of a human.

Wiseman used scans of the Lucy fossil and human data to build a three-dimensional model of the leg and pelvic muscles of Australopithecus afarensis. After collecting MRI and CT data of muscle and bone structures in modern humans, the researcher digitally created a musculoskeletal model.

So she used scans of Lucy’s fossil to determine how her joints articulated and moved in life. Wiseman stratified 36 muscles in each leg using the “muscle map” of modern human data, combined with “muscle scarring,” or the discernible traces of muscle connection that are detectable in fossils.

Lucy’s skeleton differs from humans in that she had shorter legs and a more plate-like pelvis (when viewed from top to bottom).

Wiseman’s model showed that whereas a modern human’s thigh would be about 50% muscle mass, with the rest attributed to fat and bone, Lucy’s thigh would have been almost 75% muscle. Overall, Lucy’s leg muscles were much larger and took up more space than those of modern humans.

“Lucy lived 3.2 million years ago in the African savannah. She would have needed to walk over uneven terrain and explore a mix of forested environments and open fields,” said Wiseman.

“Greater muscle mass typically means greater muscle strength, and it is not surprising to find that Lucy’s muscle reconstructions demonstrate that she had greater muscle mass than a human being, allowing her to move freely between these different environments.”

standing up

Paleoanthropologists wonder about Lucy’s posture because her skeleton differs from modern humans.

Humans have a stable posture with their legs fully extended, but when chimpanzees stand upright, they cannot straighten their legs. They walk with a crouched posture due to their bent hips and knees, which is why chimpanzees usually walk on all fours.

The 3D model showed that leveraging the knee extensor muscles of Lucy meant that she could stand upright like modern humans.

“I was very surprised to find that the knee extensors (those muscles that produce and keep a straight knee when you stand) were so comparable to humans,” Wiseman said. “That means Lucy could stand and probably walk as efficiently as possible.”

Australopithecines like Lucy lived in an environment that included open grasslands and dense forests, and had bodies adapted to thrive both on the ground and in trees.

“Lucy likely walked and moved in a way that we don’t see in any species alive today,” Wiseman said.

“If Lucy were bipedal like us and walked exclusively on two legs, then she should be able to move similarly to us,” Wiseman said.