Researchers analyzed the likelihood of bipedal movement in hominin ancestors using muscle reconstruction technology.
By Roseanne J. Keolakhonevong
You may be familiar with “Lucy,” the renowned 3.2 million-year-old Australopithecine fossilized skeleton discovered in Ethiopia in 1974. Being one of the most complete fossils of her species of bipedal primate ever discovered (around 40 percent), she has been instrumental in reconstructing the evolutionary history of Homo sapiens.
Although multiple lines of evidence have confirmed that Australopithecus afarensis was a bipedal hominin, the efficiency and frequency of this bipedalism is still an interest to researchers.
Lucy continues contributing to science
Lucy’s fossilized remains have been researched for decades, leading us to a wealth of knowledge regarding the anatomy and biomechanics of this ancestral species. The academic significance of the Lucy specimen was therefore the perfect candidate for Dr. Ashleigh Wiseman and her team at the University of Cambridge. They used three-dimensional modeling technology to reconstruct 36 pelvic and lower limb muscles of Australopithecus afarensis. They then compared these recreations to modern humans in order to get a clearer understanding of how Australopithecus afarensis stood and walked on two feet.
The problem and the solution
Soft tissues, like muscles, rarely fossilize. This is unfortunate, as the locomotion of a species can be determined via analysis of the musculoskeletal system. This is due to the direct correlation between the structure and function of muscle and bone. Lucy’s muscles did not preserve, but the attachment scars her muscles left on her bones did.
Dr. Wiseman and her team took imaging scans of Lucy’s fossilized skeleton and used them to create detailed 3D models of her bones. Using the identified attachment points and knowledge of muscle anatomy in living species, the scientists created models of 36 muscles that would have been present when Lucy was alive. They next integrated these reconstructed muscles and bones together, creating a complete model of her pelvis and lower limbs. They then compared this complete model to the same structures in modern humans.
Capable of human-like bipedal movement and more
The researchers found significant similarities in the moment arms of both Australopithecus afarensis and humans. Moment arms, in simple terms, are measurements that help us understand the role of muscles and joints. Although anatomical differences are expected between two anatomically distinct species, the researchers found that the moment arms across the hip muscle groups were similar enough between the two species for them to posit that Lucy was capable of a range of motion that would have allowed her to walk upright. This range of motion also means that it is not conclusive whether or not her species was limited to bipedal movement.
Dr. Wiseman also found that the hip extensors were comparable to those of modern humans, meaning that most likely Australopithecus afarensis had human-like bipedal leverage through the hip (which contradicts earlier ideas that Australopithecus afarensis had similar posture to a chimpanzee).
The knee extensors in the 3D model showed that her knees were able to fully extend, a necessary trait for efficient bipedal standing and walking. Furthermore, the moment arm measurements of these knee extensors provided evidence that the knee could not only fully extend, but was fully erect when standing or walking (just like in Homo sapiens).
Some of the results from the study also showed that Australopithecus afarensis was capable of a broader range of motion than modern humans. So, although their anatomy clearly shows they were able to stand and walk with an erect bipedal posture, their wider range of motion in various joints would allow for other types of locomotion as well. Dr. Wiseman and her team provided more confirmatory evidence that Australopithecus afarensis was capable of standing and walking just like us. However, further studies must be conducted in order to determine how often these ancestors would have stood or walked on two feet.
This study was published in the peer-reviewed journal Royal Society Open Science.
Wiseman, A. L. A. 2023. Three-dimensional volumetric muscle reconstruction of the Australopithecus afarensis pelvis and limb, with estimations of limb leverage. Royal Society Open Science 10(6), 230356. https://doi.org/10.1098/rsos.230356
About the Author
Roseanne J. Keolakhonevong is a high school chemistry teacher in Iowa. When she is not working or spending time with her family, she is reading and writing about current scientific discoveries and advancements. She has a special interest in the fields of human evolution and astrophysics.