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When the remains of an ancient human ancestor were found in Ethiopia in 1974, the discovery provided an unprecedented look at a species that lived millions of years before humans walked the Earth.
The rare fossil, which represents 40% of a skeleton belonging to a female Australopithecus afarensis, is named “Lucy,” for the Beatles song “Lucy in Heaven With Diamonds.”
Now, researchers are using the skeleton to figure out how this ancient relative moved 3.2 million years ago. The study’s findings were published Tuesday in the journal Royal Society Open Science.
Lucy is shorter than the average human, standing about 3.3 feet (1 meter) tall, has an ape-like face and a brain about one-third the size of a human brain.
Edwin Remsberg/Alamy Stock Photo
Lucy’s fossil includes 40% of her skeleton, one of the most complete Australopith fossils found to date.
Fossil analysis of Lucy over the past 20 years has suggested that she and the rest of her species walked upright. But lead study author Dr. Ashleigh LA Wiseman, a research associate at the University of Cambridge in the United Kingdom, wanted to take things a step further and recreate a part of Lucy that didn’t fossilize: her muscles.
“The defining aspect of what makes us human is the ability to walk on two legs, but understanding how and why it evolved has long been debated,” said Wiseman, who is a Leverhulme Trust Early Career Fellow. and Isaac Newton Trust Fellow at the McDonald Institute for Archaeological Research.
“With recent advances in computational modeling, it is now possible to investigate these questions. Of course, in the fossil record we are left looking at bones. But muscles give life to the body – they allow you to walk , run, jump and dance. So, if we want to understand how our ancestors moved, we first need to reconstruct their soft tissues.
Studying Australopithecus afarensis fossils may provide insight into the evolution of bipedalism, or upright walking, and when it evolved in early human ancestors. Wiseman’s reconstructions of Lucy’s muscles can also be used to determine how Lucy behaved in other ways.
Wiseman and his colleagues developed a method called polygonal muscle modeling and initially used it regenerate lost soft tissues of extinct reptiles called archosaurs that lived 247 million years ago.
Wiseman then applied the same technique to Lucy for the first time to understand the shape and size of her muscles and how she used them to move, assessing whether it was like the stooping waddle of an upright chimpanzee or the stance of one person.
Wiseman used Lucy’s fossil scans and data from humans to build a three-dimensional model of the leg muscles and pelvis of Australopithecus afarensis. After collecting data from MRI and CT scans of muscle and bone structures in modern humans, the researcher digitally created a musculoskeletal model.
Then, he used scans of Lucy’s fossil to determine how her joints articulated and moved in life. Wiseman layered the 36 muscles in each leg using a “muscle map” from modern human data, combined with “muscle scarring,” or the visible traces of muscle connection seen in fossils.
Dr Ashleigh Wiseman/University of Cambridge
Lucy’s muscle modeling, dubbed “AL 288-1,” was compared next to human muscle maps.
Lucy’s skeleton differs from humans in that she has shorter legs and a more plate-like pelvis (when viewed from the top down).
Wiseman’s model showed that while the modern human thigh is about 50% muscle mass, with the rest being fat and bone, Lucy’s thigh was about 75% muscle. In general, Lucy’s leg muscles were larger and took up more space than modern humans.
“Lucy lived 3.2 million years ago in the African savannah. She had to walk on uneven ground and explore a mixed forest environment and open grassland,” said Wiseman.
“Greater muscle mass usually means greater muscle force, and it’s no surprise to see that reconstructions of Lucy’s muscles show that she has greater muscle mass than a human, giving -allow him to move freely between these different environments.”
Standing straight
Paleoanthropologists have wondered about Lucy’s posture because her skeleton differs from that of modern humans.
Humans have a stable stance with fully straightened legs, but when chimpanzees stand up straight, they cannot straighten their legs. They walk in a hunched posture due to bent hips and knees, which is why chimps often walk on their knees.
The 3D model showed that Lucy’s action knee extensor muscles meaning he could stand up straight like modern humans.
“I was surprised to find that the knee extensors (muscles that make and maintain a straight knee when you stand up straight) are comparable to humans,” Wiseman said. “It means Lucy can stand and probably walk as well as we can.”
Australopiths like Lucy lived in an environment that included both open grasslands and dense forests, and had bodies adapted to thrive both on the ground and in trees.
“Lucy probably walked and moved in a way that we don’t see in any living species today,” Wiseman said.
“If Lucy was a biped like us and walked exclusively on two legs, then she should be able to move in as similar ways as possible,” Wiseman said.
