3.2 million year old lucy could stand erect, muscle reconstruction shows

A groundbreaking achievement has been made by a Cambridge University researcher who successfully reconstructed the missing soft tissue of an early human ancestor, known as a hominin. Dr. Ashleigh Wiseman employed advanced 3D modeling techniques to recreate the leg and pelvis muscles of Australopithecus afarensis, a species that existed over three million years ago. The basis for this reconstruction was scans of the renowned fossil specimen named “Lucy,” discovered in Ethiopia in the mid-1970s.

Australopithecus afarensis, despite being shorter than modern humans and possessing ape-like facial features and a smaller brain, exhibited the remarkable ability to walk upright. This adaptation allowed them to thrive in both tree-dwelling and savannah environments, enabling their survival for nearly a million years.

Referred to as “Lucy” in reference to the Beatles’ famous song “Lucy in the Sky with Diamonds,” this fossil remains one of the most complete examples of Australopithecus ever unearthed, with 40% of the skeleton preserved. By utilizing recently available open source data on the Lucy fossil, Dr. Wiseman successfully generated a digital model showcasing the lower body muscle structure of this ancient hominin, which dates back 3.2 million years. The findings of this pioneering research have been published in the esteemed scientific journal Royal Society Open Science.

Through this innovative approach, Dr. Wiseman managed to recreate 36 muscles in each leg, most of which were significantly larger in Lucy compared to modern humans. These muscles occupied a greater space within the legs, providing valuable insights into the physical capabilities and locomotion of Australopithecus afarensis.

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

One intriguing revelation from Dr. Wiseman’s study is the stark contrast between Lucy’s leg muscles and those of modern humans. In particular, the major muscles in Lucy’s calves and thighs were more than twice the size of their counterparts in present-day humans, primarily due to the higher fat to muscle ratio in our species. Astonishingly, muscles accounted for a staggering 74% of the total mass in Lucy’s thigh, while in humans, this proportion is merely 50%.

The locomotion of Australopithecus afarensis, including Lucy, has been a subject of debate among paleoanthropologists. Differing viewpoints exist regarding how she walked. Some argue for a crouching waddle, reminiscent of chimpanzees, our shared ancestors, when they move on two legs. Others contend that her gait was more akin to our own upright bipedalism.

However, over the past two decades, a consensus has gradually emerged favoring the notion of fully erect walking in Australopithecus afarensis. Dr. Wiseman’s research serves as additional evidence supporting this perspective. The presence of well-developed knee extensor muscles in Lucy, along with the leverage they would provide, signifies her ability to straighten her knee joints to a degree comparable to that of a healthy modern individual.

Dr. Wiseman, affiliated with the McDonald Institute for Archaeological Research at Cambridge University, emphasized the crucial role of reconstructing the path and spatial occupancy of muscles within the body to ascertain Lucy’s capacity for upright walking.

A 3D polygonal model, guided by imaging scan data and muscle scarring, reconstructing the lower limb muscles of the Australopithecus afarensis fossil AL 288-1, known as ‘Lucy’. In this model, the muscles have been color coded. Credit: Dr Ashleigh Wiseman

According to Dr. Wiseman, our ability to stand upright with straight knees sets us apart as the sole species capable of this posture. In comparing Lucy’s muscles to our own, it becomes evident that she possessed a similar proficiency in bipedalism while potentially retaining adaptability for arboreal locomotion. The way Lucy walked and moved likely differed from any extant species observed today, showcasing a unique combination of characteristics.

Around 3 to 4 million years ago, Australopithecus afarensis, including Lucy, inhabited diverse environments encompassing open wooded grasslands as well as denser forests in East Africa. The muscle reconstructions of Lucy indicate her capacity to effectively navigate and exploit both of these habitats.

Lucy, a young adult, stood at a height slightly over one meter and weighed an estimated 28 kilograms. Her brain size would have been approximately one-third the size of our own.

To reconstruct the muscles of this hominin, Dr. Wiseman began by examining living humans. By utilizing MRI and CT scans of the muscle and bone structures of a modern woman and man, she meticulously mapped the “muscle paths” and constructed a digital musculoskeletal 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
Completed views (ventral, dorsal, lateral and medial) of the polygonal muscle modeling approach in AL 288-1, in which 36 muscles were created per lower limb. The polygonal muscles of AL 288-1 are shown in comparison to 3D muscles of the human which were segmented from MRI scan data. Credit: Dr Ashleigh Wiseman

Next, Dr. Wiseman proceeded to employ existing virtual models of Lucy’s skeleton to reconstruct the joints, effectively reassembling the skeleton. This step involved defining the axes along which each joint could move and rotate, replicating their range of motion during Lucy’s lifetime.

Subsequently, the muscles were layered onto the reconstructed skeleton, utilizing pathways derived from maps of modern human muscles and any discernible “muscle scarring” present on the fossilized bones. Dr. Wiseman emphasized the critical role of open access science in enabling this research to take place.

These reconstructions now provide valuable insights into the walking mechanics of this ancient human ancestor. Dr. Wiseman highlighted how muscle reconstructions have been previously employed to estimate the running speeds of creatures such as the T-Rex. Similarly, by applying similar techniques to ancestral humans, the aim is to uncover the range of physical movements that propelled our evolution, including those capabilities that have since diminished or been lost.

Source: University of Cambridge

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