Fossil Limb Bones Reveal When Human Ancestors Finally Left the Trees

One of the most profound transformations in the history of our species did not happen in a single leap. Walking upright on two legs, the defining feature that separates humans from every other living ape, emerged gradually over millions of years, and the precise moment when our ancestors abandoned the trees for a life spent almost entirely on the ground has remained one of the most contested questions in human evolutionary science. Now, a new study published in the journal Science Advances offers a fresh and unusually intimate piece of evidence, drawn not from skulls or stone tools but from the internal architecture of fossilized arm and thigh bones.

An international research team led by the Keck School of Medicine of USC analyzed fossil material from seven human ancestors ranging in age from roughly 1.5 million to 3.7 million years old, spanning two pivotal groups: Australopithecus and its descendants, the early members of the genus Homo. Rather than focusing on the outward shapes of bones, the researchers measured their strength, a property that changes during an individual’s lifetime in response to the mechanical forces placed upon them. This quality makes bone strength a living record of behavior, capturing how an animal actually moved through its world rather than simply what its skeleton was capable of doing.

The biological logic behind the approach is elegantly simple. Bones remodel themselves under load, becoming thicker and more resistant in response to repeated stress. Animals that spend more time climbing and moving through trees, such as modern apes, tend to develop relatively strong arm bones, because their upper limbs bear and absorb substantial forces. Humans, who walk upright on the ground, concentrate mechanical loading in their legs and consequently build stronger thigh bones. By comparing the relative strength of upper and lower limb bones within the same individuals, the team could reconstruct how each ancestor divided its activity between the trees and the ground.

To estimate bone strength, the researchers collected computed tomography scans, which use X-rays to generate a series of detailed cross-sectional images of the bones and their internal structure. From these images they calculated the thickness of the bone shafts, quantified their internal architecture, and estimated how resistant each bone would have been to bending and twisting forces. Crucially, they compared the strength of the upper arm, thigh, and shin bones within each individual, allowing a direct, body-by-body assessment of how mechanical demands were distributed across the skeleton during life.

The results revealed that Australopithecus, an early ancestor that lived roughly two to four million years ago, occupied a genuinely intermediate position that has no exact parallel among living species. These individuals possessed relatively strong arms compared with their thighs, a pattern resembling modern apes and indicating that they spent substantial time in trees. Yet when the researchers looked specifically within the legs, comparing the strength of the thigh and shin bones, they found a distinctly human-like pattern. The implication is striking: Australopithecus was already using its lower limbs for upright walking on the ground while still relying on its upper limbs for movement in the trees.

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, a professor of clinical medical education at the Keck School of Medicine and the study’s lead author. This mosaic of traits complicates any simple narrative in which our ancestors descended from the trees in a single clean transition. Instead, the evidence points to a prolonged period in which climbing and bipedal walking coexisted within the same bodies, and likely within the same lifetimes.

The early Homo individuals in the sample, descendants of Australopithecus who lived about 1.8 to 2.3 million years ago, told a very different story. They had relatively strong leg bones and weaker arm bones, a pattern much more similar to that of modern humans. The findings suggest that the transition from Australopithecus to Homo included a major shift in how human ancestors moved and lived, one that appears to have been completed by roughly two million years ago. 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.

The concept of a threshold trait is significant because it frames the change not as a slow, imperceptible drift but as a fundamental behavioral boundary. Below the threshold, ancestors like Australopithecus maintained a mixed strategy, retaining the capacity and habit of climbing even as they walked upright. Above it, early Homo had committed to a predominantly terrestrial existence, with skeletons shaped by the demands of walking long distances on the ground rather than hauling the body through branches. The relative strength of the limbs, in this view, serves as a reliable signal of that behavioral divide.

The findings also speak directly to a longstanding debate over how much time Australopithecus actually spent on the ground. Some researchers have argued that these early ancestors were already spending the majority of their time walking upright terrestrially, treating tree climbing as an occasional refuge. The new evidence suggests otherwise, indicating that Australopithecus continued to spend substantial time in trees, Carlson said. In this interpretation, the safety and resources of the canopy remained an important part of daily life for these hominins long after their legs had adopted a human-like walking pattern.

Perhaps the most tantalizing dimension of the study is its suggested connection to another landmark event in human evolution: the dramatic expansion of brain size that began around the same period, roughly two million years ago. Scientists have proposed many explanations for that increase, including language, tool use, and changes in how human ancestors found food. Carlson and his colleagues speculate 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. 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. While the researchers are careful to present this as a hypothesis rather than a conclusion, the temporal coincidence is difficult to ignore: at the very moment the skeleton committed to the ground, the brain began its remarkable growth. The study, which included co-authors from institutions including the University of the Witwatersrand, Johns Hopkins University, Harvard University, Pennsylvania State University, and the Georgian National Museum, adds a distinctive kind of evidence to the record of bipedalism. Where earlier work documented how skeletal features changed over many generations, this research shows how individuals used their limbs during their own lifetimes, and in doing so it captures evolution in motion, one step at a time.

Subject of Research: Limb bone strength as evidence of the evolutionary transition from arboreal climbing to habitual bipedalism in Australopithecus and early Homo

Article Title: Study of bone strength reveals new clues about human evolution

Article References: Study of bone strength reveals new clues about human evolution. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: human evolution, bipedalism, Australopithecus, early Homo, bone strength, paleoanthropology, fossils, computed tomography, arboreality, limb bones, Science Advances, Keck School of Medicine of USC