This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Bipedalism, or walking on two legs, is one of the key features that distinguishes humans from other apes. But when and why our ancestors transitioned from a life often spent in trees to a life spent almost exclusively on the ground remains a subject of debate.
New evidence from fossils representing two groups of human ancestors—Australopithecus and early Homo—adds an important piece to the puzzle. In the study, just published in the journal Science Advances, an international team led by the Keck School of Medicine of USC analyzed fossil data from seven human ancestors ranging from about 1.5–3.7 million years old. The researchers measured the strength of arm and thigh bones, which adapt to the forces placed on them during life, to learn about how human ancestors used their bodies and moved through their environments.
Animals that spend more time moving in trees tend to have relatively strong arm bones, while humans who walk upright on the ground tend to have stronger thigh bones. The research team found that Australopithecus, an early ancestor that lived roughly two to four million years ago, had a distinct approach that resembles both modern apes and humans. These individuals had strong arms, suggesting they spent substantial time in trees, while the strength of their leg bones showed a more human-like pattern, suggesting they walked like humans on the ground.
"Australopithecus combined an ape-like upper limb strength with a human-like pattern in the legs, suggesting they had a unique movement strategy that has no modern comparison," said Kristian J. Carlson, Ph.D., a professor of clinical medical education at the Keck School of Medicine and the study's lead author. The early Homo individuals, descendants of Australopithecus that lived about 1.8–2.3 million years ago, had relatively strong leg bones and weaker arm bones—a pattern more similar to modern humans.
The findings suggest that the transition from Australopithecus to Homo included a major shift in how human ancestors moved and lived. "We're proposing that relative limb strength is a 'threshold trait'—a difference that marks an important and fundamental shift in behavior between Australopithecus and Homo," Carlson said. To estimate bone strength, the researchers collected computed tomography (CT) scans, which use X-rays to create a series of detailed images of the bones and their internal structure.
Using these images, they calculated the thickness, quantified the structure of bone shafts and estimated how resistant the bones would have been to bending and twisting. They compared the strength of the upper arm, thigh and shin bones within each individual. Australopithecus had relatively strong arms compared with its thighs, similar to modern apes.
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