3.2 million-year-old human ancestor 'Lucy' had massive leg muscles to stand up straight and climb trees

A sculptor's rendering of the hominid Australopithecus afarensis in an exhibition that included the 3.2 million-year-old fossilized remains of "Lucy." (Image credit: Dave Einsel via Getty Images)

Our 3.2 million-year-old ancestor "Lucy" could stand and walk upright just like modern humans do, new 3D muscle modeling reveals.

The finding bolsters a growing consensus among researchers that Australopithecus afarensis — the extinct species to which Lucy belongs — walked erect rather than with a chimpanzee-like, crouching waddle.

The hominin's reconstructed pelvis and leg muscles also suggest that she could climb trees, meaning the species likely thrived in both forest and grassland habitats in East Africa 3 million to 4 million years ago.

"Lucy's muscles suggest that she was as proficient at bipedalism as we are, while possibly also being at home in the trees," Ashleigh Wiseman, a research associate at the University of Cambridge's McDonald Institute for Archaeological Research in the U.K. who conducted the modeling study, said in a statement. "She would have been able to exploit both habitats effectively."

Lucy's fossils are the best-preserved Australopithecus remains ever unearthed, with 40% of her skeleton recovered from Ethiopia's Hadar region in the mid-1970s. Her bones indicate that she stood 3.4 feet (1 meter) tall and weighed between 29 and 93 pounds (13 to 42 kilograms). Her discovery pointed to the possibility that human ancestors could walk upright long before they evolved bigger brains.

 Related: Human ancestor 'Lucy' gets a new face in stunning reconstruction

While soft tissue is not visible in the fossil record, scientists can piece together what the extinct species' muscles may have looked like by using modern humans (Homo sapiens) as analogs. Our bone structure and muscle attachments can inform how muscles were layered on Lucy's skeleton.

In a study published Wednesday (June 14) in the journal Royal Society Open Science, Wiseman used a digital modeling approach to recreate 36 muscles in each of Lucy's legs. 

Wiseman used scans from modern humans as a reference to layer muscles onto a virtual model of Lucy's skeleton. (Image credit: Dr Ashleigh Wiseman)

The reconstruction shows that Lucy could straighten her knee joints and extend her hips in a similar way to modern humans, suggesting that the species could stand and walk upright.

The model also reveals the proportions of fat and muscle in Lucy's legs, showing they were far more muscular than a modern human's and similar in composition to a bonobo's (Pan paniscus). While a human thigh is about 50% muscle, Lucy's were likely 74% and less fatty. Some of her calf and thigh muscles occupied twice as much space in her legs as they do in human legs today.

Lucy's knees demonstrated a wider range of motion in the extension-flexion axis than a human's. This, combined with her muscle mass, suggests that A. afarensis could utilize a wide range of habitats, from dense forests to grassy savannas. This type of locomotion is not seen in any modern animal, Wiseman said. "Lucy likely walked and moved in a way that we do not see in any living species today."

While the finding is based on an incomplete skeleton, and it remains unknown how often A. afarensis adopted an upright posture, the results of the analysis support the current consensus of Lucy's physical abilities. 

The model revealed that Lucy's legs were far more muscular than a modern human's and more similar in composition to a bonobo's. (Image credit: Dr Ashleigh Wiseman)

"The current paper is not a game changer in our thinking," said Fred Spoor, a professor and researcher at the Natural History Museum in the U.K., who was not involved in the research.

However, reconstructing the muscles is a novel and exciting method to confirm bipedalism, Spoor told Live Science in an email. "This approach is certainly promising," he said. "It goes beyond the sometimes somewhat simplistic interpretations of paleontologists when it comes to inferring what movements and locomotor pattern characterized an extinct species."

Muscle modeling has already helped researchers gauge the walking speed of a Tyrannosaurus rex and could shed light on similar traits in archaic humans. "By applying similar techniques to ancestral humans, we want to reveal the spectrum of physical movement that propelled our evolution," Wiseman said. 

Sascha Pare
Trainee staff writer

Sascha is a U.K.-based trainee staff writer at Live Science. She holds a bachelor’s degree in biology from the University of Southampton in England and a master’s degree in science communication from Imperial College London. Her work has appeared in The Guardian and the health website Zoe. Besides writing, she enjoys playing tennis, bread-making and browsing second-hand shops for hidden gems.

  • Kristenalena26
    Lucy’s weight is wrong in your article. Physical anthropologist would not be so variable as to say anywhere from 29 to 93 pounds. It’s clearly supposed to be 92 to 93.
  • litzwritten
    Wow science really is a organized system of ignorance. Nothing ever evolves out of its skin celestial and terrestrial skins don't crossover from 1 to the other humans do not come from apes primate like science so often promulgate this is all from a mind which wishes to suffocate conscious of treating humans like animals same as science looks to disprove a GOD rather than prove one the world is only ideas of what will please the masses and that's all ever was and ever will be but the fight comes from idiots in power trying to under cut the masses so they can sit around and act important all day people whom are in reality of less value than those they would under cut money is their only worth take from them and they will kill themselves that goes for anyone of those in that long line of people who been ripping off the masses now for a long time
  • BacBob
    Lucy is a fraud. Why continue the lie?

    This 1974 find by Donald Johanson was a fantastic discovery of a 40 percent chimpanzee skeleton. The fingers and toes were both long and curved, similar to those of modern-day apes and helpful for swinging from trees. Of course, that would not match what the Darwinists need to proclaim as “the missing link,” so the hip bones were artificially reconstructed to give the appearance that Lucy actually walked upright and the obvious ape-like skull was adjusted to look more like man’s.

    One has to wonder if the museum also mentions the 1978 Laotoli footprint findings of Paul Abell and Mary Leakey. They found fossils of actual human footprints in Tanzania from the same era as the Lucy fossils, estimated at 3.6 million years ago.

    The simple conclusion is that man was already walking the Earth during the time of the chimpanzee that our museum is using as its centerpiece on “Human Origins.”
  • Deedee1
    Exactly! Not only is it a fraud, but it was condemned as a fraud by a German scientist. One major factor is the skeleton, which proved that the spinal column of Lucy is that of an ape. Apes spinal connection to the brain is centered, as opposed to human spinal columns are located toward the back of the head. Science also has a knack of "filling in" for missing vertebrae that would conclude a primate, miraculously changing into a human being. Skeletal structure and DNA is a major factor that is missing when when they make these determinations. Lucy and others are conjecture at best. Animals, particularly primates have existed along side of man for tens of thousands of years, while common sense is missing concerning their existence dually. Thanks.