First hominin muscle reconstruction shows 3.2 million-year-old ‘Lucy’ could stand as erect as we can

Newswise — A Cambridge University scholar has digitally restored the absent fleshy matter of a primitive human predecessor – or hominin – for the initial time, uncovering a capacity to assume an upright position akin to our present-day stance.

Dr. Ashleigh Wiseman has utilized 3D modeling to reconstruct the leg and pelvic muscles of the hominin Australopithecus afarensis, employing scans of 'Lucy': the renowned fossil find unearthed in Ethiopia during the mid-1970s.

Australopithecus afarensis, an ancient human species, inhabited East Africa more than three million years ago. Despite being shorter in stature compared to us, this species possessed an ape-like facial structure and a relatively smaller brain. However, it exhibited the ability to walk upright on two legs, adapting to both tree-dwelling and life in the savannah. These adaptations played a crucial role in the species' survival for nearly a million years.

Nicknamed after the Beatles' iconic song "Lucy in the Sky with Diamonds," Lucy stands as one of the most extensively preserved specimens of any Australopithecus species, with approximately 40% of her skeletal remains having been discovered.

Utilizing recently published open-source data on the Lucy fossil, Wiseman successfully constructed a digital representation of the lower body muscle structure of this 3.2 million-year-old hominin. The findings of this study have been published in the journal Royal Society Open Science.

The research endeavor involved the reconstruction of 36 muscles in each leg, revealing that the majority of these muscles were considerably larger in Lucy and occupied more significant space within her legs in comparison to present-day humans.

As an illustration, notable muscles in Lucy's calves and thighs were more than double the size of those found in contemporary humans, primarily due to the disparity in fat-to-muscle ratio. In Lucy's thigh, muscles accounted for 74% of the total mass, whereas in humans, that proportion is only 50%.

Paleoanthropologists share a consensus that Lucy possessed the ability to walk upright, but they hold differing viewpoints regarding the specific manner in which she moved. Some argue that her walking style resembled a crouching waddle, akin to how chimpanzees—our shared ancestor—walk on two legs. Conversely, others maintain that her mode of movement was more akin to our own upright bipedalism.

Over the past two decades, research has been accumulating, leading to a growing consensus favoring the notion of Lucy engaging in fully erect walking. Wiseman's research contributes to this trend by providing additional evidence. The knee extensor muscles in Lucy's anatomy, along with the leverage they would provide, confirm her capacity to straighten the knee joints to a similar extent as a healthy individual today.

Wiseman, from Cambridge University's McDonald Institute for Archaeological Research, emphasized that understanding Lucy's ability to walk upright can only be achieved through the process of reconstructing the trajectory and spatial allocation of muscles within the body.

According to Wiseman, "We are now the only animal that can stand upright with straight knees. Lucy's muscle structure implies that she possessed the same level of proficiency in bipedalism as we do, while potentially being adept in arboreal environments as well. Lucy likely exhibited a unique style of walking and movement that is not observed in any present-day species."

Australopithecus afarensis, including Lucy, likely inhabited regions encompassing open wooded grasslands and dense forests in East Africa approximately 3 to 4 million years ago. The muscle reconstructions of Lucy indicate her ability to effectively adapt and thrive in both of these habitats.

Lucy, being a young adult, stood at a height slightly above one meter and likely weighed around 28 kilograms. Her brain size would have been approximately one-third the size of our modern human brains.

To reconstruct the muscles of this hominin, Wiseman initiated the process by examining living humans. By employing MRI and CT scans of the muscle and bone structures of a contemporary woman and man, she successfully mapped the "muscle paths" and constructed a digital musculoskeletal model.

Subsequently, Wiseman utilized preexisting virtual models of Lucy's skeleton to "rearticulate" the joints, essentially reconstructing the alignment of the skeletal components. This process enabled the determination of the axes along which each joint could move and rotate, replicating their movements during Lucy's lifetime.

Lastly, muscles were added, layer by layer, utilizing pathways derived from modern human muscle maps. Additionally, any discernible "muscle scarring," which refers to the traces of muscle connections visible on the fossilized bones, was taken into account. Wiseman emphasizes that this research would not have been feasible without the accessibility of open access science.

These reconstructions now provide scientists with valuable insights into how this human ancestor, Lucy, walked. Wiseman points out that muscle reconstructions have previously been employed to estimate the running speeds of dinosaurs such as the T-Rex. Similarly, by utilizing similar techniques on ancestral humans, the aim is to unveil the range of physical movements that drove our evolution, including capabilities that we may have lost over time.