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.
Image: Cullen Townsend
One of the persistent problems for creationism is that our ancestors left evidence of their existence millions of years before the supposed biblical “Creation Week”. Worse still for the claim that humans were created separately from the other apes, those ancestors possessed combinations of characteristics that make sense as stages in an evolutionary history. The creationist insistence that a fossil must be either “just an ape” or “fully human” cannot accommodate the very mixtures of ancestral and derived features that evolution predicts.
A new study published in Science Advances adds another piece to this picture. Led by Kristian J. Carlson of the Keck School of Medicine of the University of Southern California, the researchers examined CT scans of limb bones from seven fossil hominins, dating from approximately 3.7 million to 1.5 million years ago. By analysing the structure of their bone shafts, they estimated resistance to bending and twisting and compared strength across the limbs.
Bone responds to the mechanical loads placed upon it during life, so its internal structure offers clues to habitual activity. In the sampled Australopithecus individuals, relatively strong upper arms suggest substantial climbing, while the relationship between thigh-bone and shin-bone strength was more human-like. Early Homo, by contrast, showed relatively stronger thighs compared with upper arms, consistent with greater reliance on terrestrial walking. These findings support a change in locomotor behaviour, although seven individuals cannot establish every step or the precise pace of that transition.
The evolutionary significance lies in the combination. Upright walking could coexist with continued use of the trees: acquiring one capability did not require the immediate abandonment of another. An ancestor with this mixture was a functioning animal in its own environment, with no need to anticipate the anatomy or lifestyle of its distant descendants. Evolution has no destination, and modern humans were never a goal towards which these populations were consciously or inevitably progressing.
For young-Earth creationism, the chronological difficulty is equally stark. Even the youngest fossils examined are about 150 times older than a generously extended 10,000-year biblical chronology. Their bone structure provides evidence about how these individuals lived; their geological ages place those lives in a past that the creationist narrative simply does not allow. Together, they present a much more substantial account of human origins than an assertion that fully formed humans appeared by supernatural decree a few thousand years ago.
Background: How bones record a lifetime of movement. A fossil bone preserves more than its owner’s size and shape. Its internal architecture can also retain clues to the forces it experienced during life — helping researchers investigate how an extinct animal moved.The paper in Science Advances was accompanied by a Keck School’s press release published via EurekAlert!.
Bone is living tissue
Although we tend to think of bones as permanent scaffolding, living bone is continually maintained and altered. Cells called osteoclasts remove bone tissue, while osteoblasts produce new bone. During growth and later life, these processes help the skeleton respond to mechanical loading, including forces generated by muscles and by supporting body weight.
Modelling changes a bone’s size or shape through bone formation and removal that need not occur at the same site. Remodelling replaces existing tissue through linked cycles of removal and formation. Together, these processes maintain the skeleton and contribute to its response to use.
Why look inside a bone?
The shaft of a long bone has a dense outer wall, called cortical bone, surrounding an internal cavity. Its resistance to bending and twisting depends partly on how this tissue is distributed. For a given amount of material, placing more of it farther from the centre generally increases resistance to these forces — an engineering principle also exploited by hollow tubes.
CT scans allow researchers to examine cross-sections without cutting through a valuable fossil. Measurements of the bone wall and its distribution provide estimates of structural strength. These are calculations based on the preserved geometry, rather than tests of how much force the fossil itself can withstand.
Why compare arms and legs?
A strong arm bone alone is difficult to interpret: its owner might simply have been large. Comparing strength across different bones in the same individual helps reveal how loading was distributed between the limbs. Comparisons with living species then provide a framework for interpreting the fossil pattern.
In the study discussed here, Australopithecus combined relatively strong upper arms with a more human-like relationship between thigh-bone and shin-bone strength. This supports substantial climbing alongside upright walking. The sampled early Homo individuals had relatively stronger thighs compared with their arms, suggesting greater reliance on movement over the ground. [Institutional research report](https://phys.org/news/2026-09-bone-strength-reveals-clues-human.html).
Ancestry and activity leave overlapping signals
An inherited feature might persist after the behaviour it originally supported became less important. Because bone-shaft structure responds to loading during development, it provides additional evidence about actual limb use. This approach also informed an earlier study of the skeleton known as “Lucy”, which found evidence consistent with both climbing and bipedalism.
The distinction is not absolute: internal structure also reflects inheritance, growth and other influences. Changes caused by activity within one lifetime are not themselves evidence that acquired characteristics were inherited by the next generation.
What bones cannot tell us
Bone structure is not a precise diary. Researchers must consider body size, age, development, health and preservation, and different activities can produce overlapping mechanical demands. These measurements cannot reveal exactly how many hours an individual spent climbing, or reconstruct every detail of its gait.
Walking upright did not mean abandoning the trees
Bipedalism and climbing are compatible abilities. Early hominins could walk on two legs while retaining substantial use of trees. Their evolution was therefore a mosaic: different features and behaviours changed at different times, rather than an entire skeleton switching at once from one way of life to another.
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
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. 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.
Professor Kristian J. Carlson, PhD, lead author
Division of Integrative Anatomical Sciences
Keck School of Medicine
University of Southern California
Los Angeles, CA, USA.
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.
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 Australopithecus was 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.
Professor Kristian J. Carlson.
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.
Publication:
These fossils add another dimension to the evidence for human evolution. Their internal structure preserves clues to how our ancient relatives used their bodies: walking upright while still making substantial use of the trees, followed by a greater commitment to life on the ground. The picture is one of changing combinations of anatomy and behaviour, with no foresight and no requirement for every characteristic to change at the same time.
For creationists, this presents two connected problems. These individuals lived millions of years before the supposed biblical “Creation Week”, and their combinations of characteristics undermine the insistence that every fossil must belong in one of two rigid categories: “just an ape” or “fully human”. Humans are themselves apes, and our evolutionary history involved changes within that ancestry, not a magical leap across an imaginary biological boundary.
There is still room for scientific debate about the extent of climbing, differences between species and the timing of particular changes. Seven individuals cannot supply a complete account of millions of years of evolution. But uncertainty over those details provides no evidence for special creation. It identifies questions that further discoveries and better analyses can address.
That is how science builds an account of our origins: by examining evidence, testing interpretations and revising conclusions where necessary. Here, even the architecture inside ancient bones contributes to that account. The result is an increasingly detailed history of how we became human — and an increasingly difficult history to reconcile with a literal reading of Genesis.
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