Spanish researchers examined how early human ancestors shifted toward walking on two legs, tracing the long road from climbing to bipedality. The study looked at fossil primates—the broader group that includes humans and their close relatives—and used advanced 3D modeling to compare the architecture of forearm muscles. By focusing on the relationships between bone structure and muscle placement, the team aimed to illuminate the functional changes that accompanied the move from life spent largely in the trees to a life dominated by upright steps on the ground. These insights help frame how early hominins balanced body weight, maintained stability, and adapted their limbs for new kinds of movement.
Employing high-resolution scans and digital reconstructions, the researchers mapped the insertions of the brachialis and triceps brachii at the proximal ulna, a region essential for forelimb control during arboreal activity and rapid changes in grip. The modeling allowed precise quantification of attachment areas and their leverage on elbow performance, giving a window into how forelimb function shifted as walking emerged.
The results indicate that Australopithecus and Paranthropus specimens combined upright walking with tree climbing. In modern primates, bonobo monkeys exhibit a similar blend of behaviors, suggesting that early hominins retained both ground-based and arboreal movements as part of their everyday repertoire.
The analysis also highlights arboreal locomotion in Australopithecus sediba and Paranthropus boisei, a topic that has fueled debate among paleoanthropologists. While some scholars argued for predominantly terrestrial behavior in these groups, the muscle attachment patterns point to an enduring capacity to move among branches.
Notably, differences in the proportions of muscle-attachment areas between humans and great apes appear tied to the kinds of movements each lineage favored. The data suggest that the development of muscles linked to these sites reflects the spectrum of locomotor strategies, not a single trajectory.
For instance, most species within the genus Homo, which includes modern humans, show anatomical characteristics that are not specialized for tree-based movement. Yet their upright gait became a defining feature of the lineage, indicating a complex balance between terrestrial efficiency and residual arboreal capability.
The paper opens a new scenario in the interpretation of human evolution by emphasizing the role of muscle attachment sites in reconstructing space-use movements preserved in fossils. By connecting anatomy to potential behaviors, it provides a framework for inferring how ancient bodies navigated their environments.
Earlier scientists proposed a straightforward account of how upright walking arose, but the new findings add nuance. They show that the evolution of bipedality did not erase climbing potentials but coexisted with it in several lineages, providing a richer picture of locomotor evolution. This nuance reshapes the idea of a single-path ascent to gait.
Taken together, the findings enrich our understanding of human evolution by linking forearm anatomy to a versatile locomotor repertoire. Rather than a single path to walking upright, the fossil record appears to preserve a mosaic of movement strategies that allowed early hominins to explore both ground and tree canopies.