Bones are not simply scaffolding. They are archives—quiet records of how a body moved through the world.
That elegant idea sits at the center of a new Science Advances paper led by my long-time friend and colleague Kristian J. “Kris” Carlson, PhD, professor of clinical medical education at the Keck School of Medicine of USC. By comparing the relative strength of fossilized arm and leg bones, Kris and an international team have added a wonderfully tangible clue to one of the biggest questions in human evolution: when did our ancestors move from a life shared between the trees and the ground to one committed primarily to walking?
Two locomotor worlds in one skeleton
The team analyzed CT-based measurements from seven fossil individuals dating from approximately 1.5 million to 3.7 million years ago. The premise is beautifully direct. Bone remodels in response to repeated loading, so the internal structure of a limb bone can preserve a signal of how that limb was used during life.
In Australopithecus, the investigators found a striking mosaic. The arms were relatively strong compared with the thighs—more like the pattern seen in living apes and consistent with substantial climbing or other weight-bearing use of the upper limbs. Yet within the legs, the relationship between thigh and shin strength looked more human-like, consistent with habitual upright walking on the ground.
Early Homo showed a different balance: relatively stronger thighs and weaker arms, closer to the pattern in modern humans. Kris and colleagues describe this proportional shift as a possible “threshold trait”—a marker of a fundamental change in behavior, with the upper limb no longer carrying the same locomotor burden.
The Kris Carlson I know
Kris and I share more than a Keck address. We are long-time friends, and each of us has had the privilege of serving as president of the Keck School Faculty Council. I have seen in his faculty leadership the same qualities evident in this work: curiosity, rigor, generosity, and a gift for bringing people together around a consequential question.
This study also reflects what I have long admired about Kris as a scientist. He can take anatomy that has been silent for millions of years and ask it a deceptively simple question: What did you do all day? The answer is not a cartoon of evolution as a single march forward. It is a more interesting story of mixed strategies, changing environments, and bodies adapting over time.
A clue, not the final word
The evidence should be read with the care the authors bring to it. Fossil samples are necessarily small and incomplete, and biomechanical signals are inferences—not direct observations of behavior. The team also notes that the shift toward more terrestrial walking occurred around the broad period when brain size began to increase in our lineage. Their suggestion that new locomotor demands might have contributed to both changes is intriguing, but it remains a hypothesis to test rather than a causal conclusion.
Even with those limits, the central insight is powerful: evolution can be read not only in the shape of bones, but in the relative work those bones once performed. Congratulations to Kris and the entire team for giving us a fresh way to see that transition—and for reminding us that sometimes the longest journeys really do begin with a change in how we carry ourselves.
References
- Carlson KJ, Ruff CB, Burgess ML, et al. Proportional limb strengths signal an adaptive shift in arboreality in early human evolution. Science Advances. 2026;12(38):aeh1752. doi:10.1126/sciadv.aeh1752.
- Abrams Z. Study of bone strength reveals new clues about human evolution. Keck School of Medicine of USC. September 16, 2026.
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