The research, led by Josep M. Potau and Aroa Casado of the Human Anatomy and Embryology Unit in the Faculty of Medicine and Health Sciences and the Institute of Archaeology at the University of Barcelona, applies three-dimensional geometric morphometric analysis to the ligament insertion sites on the distal epiphysis of the radius. The team used 3D GM to quantify the size, orientation, and shape of the insertion areas where wrist ligaments attach to the end of the radius that articulates with the wrist.
Their analysis shows diverse locomotor patterns among hominins and clear differences in wrist ligament insertion size, orientation, and shape between fossil hominins - Australopithecus afarensis, Australopithecus anamensis, Australopithecus sediba, Paranthropus robustus, Homo neanderthalensis and archaic Homo sapiens - and living hominins, including modern Homo sapiens, chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus). The work extends the methodological toolkit of paleoanthropology by adding a ligament-focused approach that complements more traditional bone-based studies.
Ligaments act as joint stabilizers and also provide sensory feedback that helps the brain monitor joint function, and differences in hominin locomotion are reflected in the configuration of wrist ligaments and the bony areas where they insert on the distal radius. "Humans, who are bipedal and use their hands primarily to manipulate objects, have anatomical characteristics in these insertion areas that are markedly different from those observed in other non-human hominoid primates (chimpanzees, gorillas and orangutans)", says Professor Josep Maria Potau, from the Department of Surgery and Surgical Specializations at the UB, "which use their hands differently, mainly due to their arboreal and terrestrial locomotion."
The group has developed research lines on upper limb ligament anatomy in primates to build comparative anatomical models with modern humans and to estimate the locomotion of fossil primates from ligament insertion patterns. "The study of ligament insertion sites in 3D models allows, in most cases, the corroboration or questioning of locomotor inferences based strictly on the study of muscle insertions," notes Josep Maria Potau. "Furthermore, it provides unique information that cannot be obtained from the study of muscle insertions."
In many fossil primates the distal radius is preserved, and this epiphysis does not carry identifiable muscle insertions for locomotor inference, but it does contain two distinct areas where most wrist ligaments attach. According to Professor Aroa Casado, these well-defined ligament insertion zones make it possible to infer locomotor behavior even when muscle-related features are absent. "If the results obtained with the ligaments coincide with those obtained in the muscular study - she continues - this reinforces the identification of a specific type of locomotion in a specific fossil species. However, if the results are contradictory, it may suggest other alternative forms of locomotion."
The study reports that Australopithecus afarensis, Australopithecus anamensis and Australopithecus sediba share wrist insertion anatomies that closely resemble those of chimpanzees and orangutans. This pattern supports the view that these taxa combined habitual bipedal walking with effective tree climbing, consistent with previous findings from muscle insertion analyses.
Paranthropus robustus, which evolved in parallel with the human lineage, presents a more complex case because earlier work has alternately described it as a biped with strong arboreal capabilities or as using different terrestrial locomotor strategies. In the new analysis, the Paranthropus robustus specimen shows a ligament insertion anatomy more similar to that of gorillas, supporting the idea that, in addition to bipedal and arboreal behaviors, it may have relied more on terrestrial locomotion, as gorillas commonly do.
For Homo neanderthalensis and archaic Homo sapiens, whose anatomy is closer to that of modern humans, the distal radius ligament insertion patterns largely match those of present-day Homo sapiens but also include distinctive traits. The researchers describe a characteristic configuration in the area where the short radiolunate ligament inserts, which indicates a more robust wrist region that may reflect greater manual load bearing linked to activities such as hunting or tool production.
The authors suggest that 3D GM analysis of ligament insertions can be extended to other fossil and living species to explore locomotor strategies and manual precision in a broader evolutionary framework. They conclude that their results demonstrate the value of this technique for reconstructing locomotor behavior and position it as a useful tool for future work in comparative anatomy, biological anthropology, paleoanthropology and studies of human evolution.
Research Report:Inferences about fossil hominin locomotion through 3D morphometric analysis of wrist ligament insertion sites
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