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 million to 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, PhD , 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 to 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.
Measuring bone strength
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 and 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. But when researchers looked at the legs specifically, comparing the strength of thigh and shin bones, they found a human-like pattern. This indicates Australopithecuswas already using its lower limbs for upright walking, while still using its upper limbs for movement in trees. In contrast, the early Homo individuals had relatively stronger thighs compared with their arms, resembling a modern human pattern.
The findings contribute to a longstanding debate over how much time Australopithecus spent on the ground versus in trees. While some researchers have argued that these early ancestors spent the majority of their time on the ground walking upright, the new evidence suggests they continued to spend substantial time in trees, Carlson said.
Bipedalism and brain size
Other studies have shown how bone features changed over many generations as bipedalism evolved. This study adds a different kind of evidence by showing how individuals used their limbs during their lifetimes—and supporting the idea that a major shift in movement occurred by roughly two million years ago.
The researchers suggest this behavioral shift is particularly intriguing when considered alongside another major change in human evolution: the dramatic increase in brain size that began around the same time. Scientists have proposed many explanations for the increase, including language, tool use and changes in how human ancestors found food. Carlson and his colleagues posit that the move toward walking greater distances on the ground may have contributed to both the shift in relative limb strength and the increase in brain size.
"We speculate that the shift toward more walking may have placed new demands on the body and brain, which could help explain why these changes happened around the same time," Carlson said.
About this research
In addition to Carlson, the study's other authors are Tea Jashashvili from the Keck School of Medicine of USC and the Department of Biological Sciences at USC Dornsife College of Letters, Arts and Sciences; Ronald J. Clark and Dominic Stratford and Kathleen Kuman from the University of the Witwatersrand, Johannesburg, South Africa; Christopher B. Ruff and Adam D. Sylvester from John Hopkins University; Jason L. Heaton from the University of Alabama-Birmingham; Travis R. Pickering and A.J. Heile from the University of Wisconsin—Madison; M. Loring Burgess from Harvard University; Lauren Sarringhaus from James Madison University; Timothy M. Ryan from Pennsylvania State University; Amelie Beaudet from the University of Poitiers, France; Robin H. Crompton from the University of Liverpool, United Kingdom; and David Lordkipanidze from Tbilisi State University and the Georgian National Museum, Tbilisi, Georgia.
This work was supported by Standard Bank and JP Morgan Chase; the Palaeontological Scientific Trust; the National Research Foundation (South Africa) African Origins Platform, Strategic Research Infrastructure Grant [#75430] and Centre of Excellence in Paleosciences; the National Science Foundation [BCS-2609570, SBR-8919155, SBR-8919749, BCS-1316104 and BCS-1419564]; the Wenner-Gren Foundation for Anthropological Research; the L.S.B. Leakey Foundation; and the Keck School of Medicine of USC.
