Study of bone strength reveals new clues about human evolution
By comparing the strength of fossilized limb bones, a research team led by the Keck School of Medicine of USC uncovered new evidence about the movement patterns of early human ancestors
image:
On the right is a depiction of an early Homo human ancestor characterized by relatively greater bone strength of the femur in the thigh compared to the humerus in the arm. To the left is a depiction of an older human ancestor, Australopithecus, characterized by more equivalent strength in the bones of the arm and thigh.
view moreCredit: Image: Cullen Townsend
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.
Journal
Science Advances
Method of Research
Experimental study
Article Title
Proportional limb strengths signal an adaptive shift in arboreality in early human evolution
Article Publication Date
16-Sep-2026
Little Foot study points to major shift in how human ancestors moved
A major change in how our ancestors moved through their environment took place between about three million and 1.8 million years ago
University of the Witwatersrand
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In the foreground (right) is a depiction of an early Homo human ancestor characterized by relatively greater bone strength of the femur in the thigh compared to the humerus in the arm. In the background (left) is a depiction of an older human ancestor, Australopithecus, characterized by more equivalent strength in the bones of the arm and thigh. The authors’ findings document a shift in relative bone strength between these two ancestors, signaling a fundamental change in how they used their upper limb during daily interactions with the surrounding environment. In early Homo ancestors, it appears that the upper limb was no longer appreciably used in weight-bearing activities, such as arboreal movements, that appear to have characterized Australopithecus ancestors.
view moreCredit: Kris J Carlson
New research on the limb bones of the famous Little Foot skeleton from the Wits Sterkfontein Caves suggests that a major change in how our ancestors moved through their environment took place between about three million and 1.8 million years ago.
The study, published in Science Advances, finds that Australopithecus, the group of hominins that includes Little Foot, combined regular tree-climbing with walking on two legs on the ground. However, by about 1.8 million years ago, early members of the genus Homo appear to have shifted much more strongly towards a life on the ground.
The findings are based on comparisons of the relative strength of arm and leg bones in fossil human ancestors, modern humans and living apes.
Professor Kristian Carlson of the University of Southern California, who is affiliated with the Evolutionary Studies Institute at Wits University, says the results point to a fundamental difference between Australopithecus and later Homo.
“Early human ancestors such as Australopithecus appear to have used trees as much as modern chimpanzees and gorillas do today, but when they were on the ground they were walking on two legs,” Carlson says.
“That combination is important because there is no living species that provides a good behavioural comparison for them.”
The researchers added new data from the approximately 3.67-million-year-old StW 573 skeleton, known as Little Foot, from the Wits Sterkfontein Caves in South Africa, and fossils from Dmanisi in Georgia dating to about 1.8 million years ago.
They compared the strength of bones in the arms and thighs, as well as strength patterns within the lower limbs.
Little Foot and other Australopithecus fossils show relatively strong arms compared with their thighs, a pattern closer to African apes than to modern humans. This suggests their arms were still regularly exposed to the loads associated with climbing and moving in trees.
But Little Foot’s lower limb tells a different story. The relationship between the strength of the thigh and lower-leg bones is closer to the modern human pattern.
“This is one of the most surprising results,” Carlson says. “The Australopithecus individual has a modern human-like signal within the lower limb, while the relationship between the arm and thigh is much more African ape-like.”
He says the combination highlights how unusual Australopithecus locomotion was.
“These ancestors were not simply moving like chimpanzees, nor were they moving like modern humans. They seem to have combined substantial use of trees with regular bipedal walking on the ground.”
In contrast, the Dmanisi fossils and other early Homo and Homo erectus individuals show limb-strength patterns much closer to those of modern humans. The researchers interpret this as evidence that, by about 1.8 million years ago, the human lineage had become far more committed to walking on two legs on the ground and was using trees much less.
The researchers propose that the difference may represent a “threshold” between the adaptive patterns of Australopithecus and Homo, similar to other major changes used to distinguish stages of human evolution.
What caused the shift remains uncertain. The authors suggest it could be linked to changes in ranging, food gathering and other pressures that favoured stronger lower limbs and greater dependence on life on the ground.
They also note that the change took place during a broad period in which brain size was increasing in the human lineage.
“We are not saying that one change caused the other,” Carlson says. “But the timing is intriguing. We hope the possible relationship between changes in limb use, ranging behaviour and brain size will stimulate further discussion.”
The study includes researchers from Wits University’s Evolutionary Studies Institute and the Sterkfontein Caves research programme, together with international collaborators.
Journal
Science Advances
Article Publication Date
16-Sep-2026
