When did humans learn how to walk upright?
Through digitally recreating the muscles of an early human ancestor, research has shed new light on how humans evolved to walk upright.
Source - https://www.jpost.com/archaeology/article-747250
Illustrative depiction of the Australopithecus Afarensis (photo credit: Wikimedia Commons)
The study, conducted by Dr. Ashleigh L.A. Wiseman of Cambridge University’s McDonald Institute for Archaeological Research and published in the peer-reviewed journal The Royal Society, examines the evolution of bipedalism - the ability to walk upright - through turning to one of the most famous early human ancestors, the Australopithecus Afarensis known as Lucy.
Lucy - humanity's most famous ancestor
Lucy is comprised of several hundred fragments of fossilized bone which, when combined, make up the skeletal structure of about 40% of the female body of the hominin species Australopithecus Afarensis.
Malapa Hominin 1 (MH1) left, Lucy (AL 288-1 (Centre), and Malapa Hominin 2 (MH2) right. Image compiled by Peter Schmid courtesy of Lee R. Berger, University of the Witwatersrand. (credit: Wikimedia Commons)
The species dates back to some 2.3 million years ago and although thought to have been ape-like in appearance with a smaller brain, they had already developed the same ability to walk upright that humans have today.
"Lucy likely walked and moved in a way that we do not see in any living species today." Dr. Ashleigh L.A. Wiseman
However, while scientists agree that Lucy and the rest of her species walked upright, they disagree when it comes to how exactly she did this, and what this looked like.
According to Wiseman, some scientists have long believed that the Australopithecus Afarensis walked in a crouching waddle, similar to chimpanzees, while others believed that they walked entirely upright, similar to Homo Sapiens.
Now, through a digital reconstruction of the hominin's lower body muscles, Wiseman hopes to put the debate to rest once and for all.
Recreating long-lost muscle mass
In order to reconstruct the soft tissue composition of Lucy's body, Wiseman used a method known as digital polygonal muscle modeling, in which a three-dimensional model of each muscle in a body segment is re-created, guided by comparative data from similar, still existing species.
Once each segment has been re-created, the entire "attachment site" that connects each segment is also re-created. The various segments are then connected by threading a line through the midpoint of each new 3D muscle, creating a correctly configured muscular system for each limb.
In this instance, the extant species used for comparison was the Homo Sapien and the muscles were mapped using MRI and CT scans of the muscle and bone structures of a modern woman and man.
Lucy, a partially intact fossilized female Australopithecus Afarensis (credit: Wikimedia Commons)
Wiseman then used existing virtual models of Lucy’s skeleton to rearticulate the joints, defining the axis from which each joint was able to move and rotate, replicating how they moved during life.
Using this method, Wiseman was able to successfully recreate 36 muscles in each leg, most of which were much larger and stronger, and occupied more space, than the leg muscles of a modern human.
Carving a path through human evolution
Most notably, major muscles in Lucy's calves and thighs were double the size of the muscles found in modern humans, and while muscles make up just 50% of the total mass in the modern human's thigh, with the rest being fat, they made up 74% of the Australopithecus thigh.
Another notable finding of the study was the way in which the early human ancestors' knees functioned, and their range of movement.
In order for humans to stand, the knee must extend, and it was observed that the knees of the Australopithecus Afarensis did just that. According to the study, the knee extensors of the species indicated that they could have, and probably did, walk and stand upright with an erect position of the knee, as humans do today.
However, Wiseman notes in the findings, it is unclear if upright walking was frequently practiced or if it was a rarer occurrence, and further research will be conducted to this end in the future.
The research also indicated that while Lucy's bipedalism was similar to humans in some ways, she was also likely to have had a range of movement types that humans today are not capable of.
"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, " Wiseman explains of the research findings.
"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."
Three-dimensional volumetric muscle reconstruction of the Australopithecus afarensis pelvis and limb, with estimations of limb leverage - Ashleigh L. A. Wiseman - https://doi.org/10.1098/rsos.230356
Abstract
To understand how an extinct species may have moved, we first need to reconstruct the missing soft tissues of the skeleton, which rarely preserve, with an understanding of segmental volume and muscular composition within the body. The Australopithecus afarensis specimen AL 288-1 is one of the most complete hominin skeletons. Despite 40+ years of research, the frequency and efficiency of bipedal movement in this specimen is still debated. Here, 36 muscles of the pelvis and lower limb were reconstructed using three-dimensional polygonal modelling, guided by imaging scan data and muscle scarring. Reconstructed muscle masses and configurations guided musculoskeletal modelling of the lower limb in comparison with a modern human. Results show that the moment arms of both species were comparable, hinting towards similar limb functionality. Moving forward, the polygonal muscle modelling approach has demonstrated promise for reconstructing the soft tissues of hominins and providing information on muscle configuration and space filling. This method demonstrates that volumetric reconstructions are required to know where space must be occupied by muscles and thus where lines of action might not be feasible due to interference with another muscle. This approach is effective for reconstructing muscle volumes in extinct hominins for which musculature is unknown.