There’s some news about everyone’s favorite great-great-great-great-great-(etc.) aunt, Lucy. Also known as AL 288-1 and Dinkinesh, our shared ancestor (or rather, 40 percent of her skeleton) was discovered in 1974 in Ethiopia. This female hominid species Australopithecus afarensis died sometime in her early twenties but is about 3.18 million years old. Yes, that Lucy.
We already know, through plentiful evidence, that she, as well as her contemporaries, were walking upright on two feet even before the introduction of stone tools or enlarged human brains. In a sense, bipedalism was one of humanity’s first steps (literally) toward becoming ourselves.
But exactly how she walked upright is still largely unknown. Lucy’s remains, which are all skeletal, only tell us so much, such as her approximate height (three and a half feet) and weight (60 to 65 pounds). But did Lucy walk fully erect like we do or on bent knees like a chimp or bonobo?
Now we have more solid evidence than ever that Lucy walked on outstretched legs straightened beneath her — and probably ran, too. This research, published on June 13 in the journal Royal Society Open Science, drew from a novel, painstaking computer simulation to render a digital reconstruction of Lucy’s legs and musculature.
An upstanding citizen
A digital reconstruction of Lucy’s leg muscles — 36 in each limb — further solidifies that this icon walked erect while representing a scientific first. This rendering comes courtesy of Ashleigh Wiseman, a paleoanthropology research associate at the University of Cambridge. Wiseman’s first pass at this imaging took three or four months, but then she spent a year tweaking her model.
“If you were to stand up right now, your legs would be entirely straight,” Wiseman tells Inverse. “Our muscle reconstruction confirms that Lucy could do that, too.” It’s not just that we have this mode of mobility in common with our ancestors; understanding this anatomy helps us conceptualize how she walked and ran and how we became upstanding citizens ourselves.
Lucy’s leg muscles didn’t look like ours. Modern humans have much longer muscle proportions. Generally, a modern human thigh’s mass is 50 percent muscle, while the rest is fatty tissue and bone. Lucy’s muscles made up about 75 percent of her thigh mass. That number might make a gym rat swoon, and with good reason: “Typically, greater muscle mass means greater muscle force,” Wiseman says.
“I don’t find that surprising because Lucy lived on the African savannah,” she continues. Lucy spent her life traversing uneven ground and climbing trees. Her fully extended legs afforded her excellent mobility.
Walking, hands free
Humans are the only mammals that can walk upright on two legs. (Remember, birds aren’t mammals.) Walking upright means free hands for holding tools, grabbing food, and carrying babies while on the move. The fact that, at some point, erect walking evolved along with modern humans speaks to how we came to be.
“Understanding where our upright walking came from can help us understand some of those evolutionary pressures that we put on our own legs,” Wiseman says.
Shortly after Lucy’s discovery came what’s called the bent-hip, bent-knee hypothesis, which proposed Lucy walked in a way similar to how chimps walk. Chimps will sometimes stand up and walk on two legs, but their hips and knees will remain in a crouching posture. They never fully extend the hip or knee as we do. Wiseman’s reconstruction hits on the human and the chimp in Lucy.
“Her results suggest Lucy was both more human-like and more chimpanzee-like,” Kevin Hunt, an anthropology professor at Indiana University, writes to Inverse. He continues that while Wiseman’s reconstruction demonstrates Lucy’s erect gait, it also speaks to her “powerful chimpanzee-like limb movements when she assumed postures that would have been useful in trees.”
Muscle and bone
While the paleoanthropological field’s consensus has been leaning toward Lucy’s upright walk, Wiseman wanted to analyze this hypothesis using brand-new tools. Other experts have performed muscle reconstructions on Tyrannosaurus Rex, for example, to simulate how it would have run. But such a technique had never been used before for a hominid, Wiseman says.
She used a tool called polygonal muscle reconstruction, which she and her colleagues developed last year to examine muscles in extinct archosaurs. For this paper, Wiseman drew on more than Lucy’s skeleton. She started with open-access MRI and CT scans of modern humans. MRI scans reveal muscle shape and size, as well as how they connect to different bones. These scans and their cross sections, mapped onto Lucy’s remains, helped guide Wiseman’s muscle reconstructions with her ancient proportions.
In particular, Lucy’s knee extensor muscles, which allow lower leg extension, were strong enough to fully support her frame. “That knee being able to have an erect, still posture is directly related to the muscles that attach on to the femur,” Wiseman says. “It forms a chain, and all of those muscles … contribute towards this erect posture.”
“It helps us understand how muscles that are redirected by being pushed aside by other muscles in a bundle function,” Hunt writes.
Bones and muscles may seem like separate systems, but they work and evolve together. While we have Lucy’s skeletal remains, understanding how her musculature integrated into her skeleton paints a clearer picture of how she walked, which in turn paints a clearer picture of how modern humans eventually evolved.
Animated, not reanimated. Don’t worry.
This reconstruction inspires a new question for Wiseman. “We can see how Lucy could use her muscles, but now I want to see how they were used in action,” she says. “If we were to animate the skeleton, how would her muscles actually perform that movement?” That is to say, how would her various muscles contract and stretch as this digital animation runs or walks? How does this integrated system make her mobile?
Biomechanical animation software is already at Wiseman’s disposal.
Wiseman’s research delves into the 40 percent of a skeleton and helps show us an earlier version of ourselves. “I feel that goes well beyond just the bones, that you start to paint this picture of how and when she lives.”