3D muscle reconstruction reveals 3.2 million-year-old ‘Lucy’ could stand upright like modern humans

Cross section of a polygonal muscle modeling approach, guided by muscle scarring data and magnetic resonance imaging data. Credit: Dr. Ashley Weisman

Digital modeling of the soft tissue of the mythological fossils indicates that Australopithecus afarensis possessed strong leg and pelvic muscles suitable for tree-dwelling, but knee muscles that allowed fully upright walking.

A researcher at the University of Cambridge has digitally reconstructed the lost soft tissues of an early human ancestor – or hominin – for the first time, revealing the ability to stand upright as we do today.

Dr. Ashley Weisman has 3D modeled the leg and pelvic muscles of human legs Australopithecus afarensis Using the “Lucy” survey: the famous fossil specimen discovered in Ethiopia in the mid-1970s.

Australopithecus afarensis He was an early human[{” attribute=””>species that lived in East Africa over three million years ago. Shorter than us, with an ape-like face and smaller brain, but able to walk on two legs, it adapted to both tree and savannah dwelling – helping the species survive for almost a million years.

Named for the Beatles classic ‘Lucy in the Sky with Diamonds’, Lucy is one of the most complete examples to be unearthed of any type of Australopithecus – with 40% of her skeleton recovered.

A digitization of the muscle attachment areas used to build the model of Lucy’s muscles, next to the completed 3D muscle model. Credit: Dr. Ashleigh Wiseman

Wiseman was able to use recently published open-source data on the Lucy fossil to create a digital model of the 3.2 million-year-old hominin’s lower body muscle structure. The study is published in the journal Royal Society Open Science.

The research recreated 36 muscles in each leg, most of which were much larger in Lucy and occupied greater space in the legs compared to modern humans.

For example, major muscles in Lucy’s calves and thighs were over twice the size of those in modern humans, as we have a much higher fat-to-muscle ratio. Muscles made up 74% of the total mass in Lucy’s thigh, compared to just 50% in humans.

A 3D polygonal model, guided by imaging of scan data and muscle scarring, to reconstruct the lower limb muscles of Australopithecus afarensis fossil AL 288-1, known as ‘Lucy’. In this model, the muscles were color-coded. Credit: Dr. Ashley Weisman

Paleoanthropologists agree that Lucy walked on two legs, but they disagree about how she walked. Some have argued that it moved in a crouching rib, similar to chimpanzees—our common ancestor—when they walk on two legs. Others think its locomotion was akin to walking on two legs.

Research in the past 20 years has seen a consensus emerge around fully erect walking, and Wiseman’s work adds even more weight to this. Lucy’s knee extensor muscles, and the leverage they allow, confirm the ability to straighten knee joints as well as a healthy person can today.

“Lucy’s ability to walk upright can only be known from a reconstruction of the trajectory and the space occupied by muscles within the body,” said Wiseman, of the University of Cambridge’s MacDonald Institute for Archaeological Research.

Completed views (abdominal, dorsal, lateral, and medial) of the polygonal muscle modeling approach in AL 288-1, where 36 muscles were generated for each lower limb. The polygonal muscle of AL 288-1 is shown compared to the 3D muscle of a human that was sectioned from the MRI scan data. Credit: Dr. Ashley Weisman

“We are now the only animal that can stand upright with straight knees. Lucy’s muscles indicate that she was as adept at walking on two legs as we are, while she is also likely at home in the trees.” Lucy likely walked and moved in a way that we don’t see, Wiseman said. in any organism today.

“Australopithecus afarensis It may have roamed the open wooded grassland areas as well as the denser forests of East Africa some 3 to 4 million years ago. Lucy’s muscle reconstructions indicate that she was able to exploit both habitats effectively.”

Lucy was a young woman, just over a meter tall and probably weighed about 28 kg. Lucy’s brain would have been about a third the size of ours.

To recreate the muscles of the hominins, Wiseman started with some living humans. Using MRI and CT scans of the musculoskeletal structures of modern women and men, I was able to plot “muscle pathways” and build a digital musculoskeletal model.

A 3D polygonal model, guided by imaging of scan data and muscle scarring, to reconstruct the lower limb muscles of Australopithecus afarensis fossil AL 288-1, known as ‘Lucy’. Credit: Dr. Ashley Weisman

Wiseman then used existing virtual models of Lucy’s skeleton to “reshape” the joints—that is, to put the skeleton back together again. This work identified the axis from which each joint was able to move and rotate, replicating how they moved through life.

Finally, layers of muscle were layered on top, based on trajectories from recent human muscle maps, as well as discernible small ‘muscle scarring’ (traces of muscle contact detectable on fossilized bone). “Without the science of open access, this research would not have been possible,” Weissman said.

These reconstructions can now help scientists understand how this human ancestor functioned. “Muscle reconstruction has already been used to measure the running speeds of a T-Rex, for example,” Wiseman said. “By applying similar techniques to human ancestors, we want to reveal the spectrum of physical movement that drove our evolution – including those abilities we’ve lost.”

Reference: “Three-dimensional volumetric reconstruction of the muscle.” Australopithecus afarensis Pelvis and Limb, with Limb Pressure Estimates” By Ashley La Wiseman, Jun 14, 2023 Available Here. Royal Society for Open Science.
DOI: 10.1098/rsos.230356