Soft tissues rarely preserve in the fossil record, rather scientists are mostly left with just the skeletal material. Yet, muscles animate the body. They allow an animal to move, walk and run. To understand how an extinct species may have moved, scientists first need to reconstruct the missing soft tissues of the skeleton with an understanding of volume and the composition within the body. In the new study, University of Cambridge’s Dr. Ashleigh Wiseman 3D-modeled the leg and pelvis muscles of Australopithecus afarensis using MRI and CT scans of ‘Lucy,’ the famous fossil specimen discovered in the Hadar region of Ethiopia in the 1970s.
A group of Australopithecus afarensis. Image credit: Matheus Vieeira.
Australopithecus afarensis was an early human species that lived in East Africa over 3 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 species of Australopithecus — with 40% of her skeleton recovered.
Dr. 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.
She 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.
Paleoanthropologists agree that Lucy was bipedal, but disagree on how she walked. Some argue that she moved in a crouching waddle, similar to chimpanzees when they walk on two legs. Others believe that her movement was closer to our own upright bipedalism.
Research in the last 20 years have seen a consensus begin to emerge for fully erect walking, and the new work adds further weight to this.
Lucy’s knee extensor muscles, and the leverage they would allow, confirm an ability to straighten the knee joints as much as a healthy person can today.
“Lucy’s ability to walk upright can only be known by reconstructing the path and space that a muscle occupies within the body,” Dr. Wiseman 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 likely walked and moved in a way that we do not see in any living species today.”
“Australopithecus afarensis would have roamed areas of open wooded grassland as well as more dense forests in East Africa around 3 to 4 million years ago. These reconstructions of Lucy’s muscles suggest that she would have been able to exploit both habitats effectively.”
Workflow diagram outlining the process for this study. Image credit: Ashleigh Wiseman, doi: 10.1098/rsos.230356.
Lucy was a young adult, who stood at just over 1 m tall and probably weighed around 28 kg. Lucy’s brain would have been roughly a third of the size of ours.
To recreate the muscles of this hominin, Dr. Wiseman started with some living humans.
Using MRI and CT scans of the muscle and bone structures of a modern woman and man, she was able to map the muscle paths and build a digital musculoskeletal model.
She then used existing virtual models of Lucy’s skeleton to rearticulate the joints — that is, put the skeleton back together.
This work defined the axis from which each joint was able to move and rotate, replicating how they moved during life.
Finally, muscles were layered on top, based on pathways from modern human muscle maps, as well as what little muscle scarring was discernible. The traces of muscle connection detectable on the fossilized bones.
“Muscle reconstructions have already been used to gauge running speeds of T. rex, for example,” Dr. Wiseman said.
“By applying similar techniques to ancestral humans, we want to reveal the spectrum of physical movement that propelled our evolution — including those capabilities we have lost.”
The study was published in the journal Royal Society Open Science.
_____
Ashleigh L.A. Wiseman. 2023. Three-dimensional volumetric muscle reconstruction of the Australopithecus afarensis pelvis and limb, with estimations of limb leverage. R. Soc. open sci 10 (6): 230356; doi: 10.1098/rsos.230356
