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

An intersection of polygonal muscle modeling approach guided by muscle scar and MRI data. Credit: Dr. Ashley Wiseman

Digital modeling of the soft tissue of the legendary fossil shows that Australopithecus afarensis had powerful leg and hip muscles for tree-dwelling, but knee muscles that allowed it to walk fully upright.

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

“Lucy’s muscles suggest she’s a bipedal expert like us.” — Dr. Ashley Wiseman

Dr. Ashley Wiseman 3D modeled the hominin’s leg and hip muscles Australopithecus afarensis Using Lucy’s scans: A famous fossil specimen discovered in Ethiopia in the mid-1970s.

Australopithecus afarensis Early man[{” 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.

Lucy 3D Muscle Model

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 scan data and muscle scars, reconstructs the lower limb muscles of the Australopithecus afarensis fossil AL 288-1, known as ‘Lucy’. In this model, the muscles are color coded. Credit: Dr. Ashley Wiseman

They agree that Lucy was bipedal, but disagree on how she walked. Some have argued that when chimpanzees — our common ancestor — walked on two legs, she moved in a stooped waddle. Others believe her locomotion was closer to our own upright bipedal gait.

Over the last 20 years of research, a consensus has begun to emerge for fully upright walking, and Wiseman’s work adds further weight to this. Lucy’s knee extensor muscles and the leverage they allow ensure her ability to straighten the knee joint as much as a healthy person can today.

“Lucy’s ability to walk upright can only be determined by reconstructing the path and space occupied by the muscles on the body,” says Wiseman of the Macdonald Institute for Archeology at the University of Cambridge.

AL 288-1 vs human muscles

Complete views of the polygonal muscle modeling approach (ventral, dorsal, lateral and medial) in AL 288-1, where 36 muscles were created for the lower limb. Polygonal muscles of AL 288-1 are shown in comparison to human 3D muscles segmented from MRI scan data. Credit: Dr. Ashley Wiseman

“Now we are the only animal that can stand upright with straight knees. Lucy’s muscles suggest that even at home in the trees, she had mastered bipedalism like the rest of us. “Lucy probably walked and moved in a way not seen in any living creature today,” Wiseman said.

Australopithecus afarensis 3 to 4 million years ago roamed East Africa in open wooded grasslands and dense forests. These reconstructions of Lucy’s muscles suggest that she could have effectively exploited both habitats.

Lucy was a young adult who was over one meter tall and weighed 28 kg. Lucy’s brain would have been about a third of ours.

To recreate this hominin’s muscles, Wiseman started with some living humans. Using MRI and CT scans of the muscular and skeletal systems of a modern woman and man, he was able to map “muscle pathways” and create a digital musculoskeletal model.

Weissman then used existing virtual models of Lucy’s skeleton to “rearrange” the joints — that is, put the skeleton back together. This work defined the axis from which each joint could move and rotate, reflecting how they moved in life.

Finally, muscles were superimposed based on the trajectories of modern human muscle maps, as well as small “muscle scars” (traces of muscle attachment found on fossil bones). “Without open access science, this research would not be possible,” Wiseman said.

These reconstructions can now help scientists understand how these human ancestors walked. “For example, muscle reconstruction has already been used to measure the running speed of T-rex,” Wiseman said. “By applying similar techniques to our ancestors, we want to reveal the spectrum of physical movement that drove our evolution — including skills we’ve lost.”

Reference: “Three-Dimensional Measurement of Muscle Reconstruction Australopithecus afarensis Hip and joint, with joint leverage assessments” By Ashley LA Wiseman, 14 Jun 2023, Royal Society Open Science.
DOI: 10.1098/rsos.230356

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