New methodology to decipher how human ancestors moved

When studying how fossil hominids moved, researchers usually analyse the morphology of bones — which is crucial for understanding the evolution of bipedalism — focusing mainly on muscle insertion sites. However, the potential information of studying other types of soft tissue, such as joint ligaments, is often overlooked. Now, an article published in the journal Scientific Reports emphasizes the importance of studying the anatomy of the insertion points of the wrist ligaments to reconstruct the locomotor behaviour and manipulative dexterity of fossil hominids throughout human evolution.

The study is led by professors Josep M. Potau and Aroa Casado, from the Human Anatomy and Embryology Unit of the Faculty of Medicine and Health Sciences and the Institute of Archaeology (IAUB) at the University of Barcelona.

With an innovative perspective, the study applies the technique of three-dimensional geometric morphometric analysis (3D GM) to examine the insertion sites of the ligaments at the end of the radius that contacts the wrist (distal epiphysis).

The results confirm the existence of diverse locomotor behaviours in hominids and show significant differences in the size, orientation, and shape of wrist ligament insertions in fossil hominids — Australopithecus afarensis, Australopithecus anamensis, Australopithecus sediba, Paranthropus robustus, Homo neanderthalensis and archaic Homo sapiens — and modern hominids: Homo sapiens, Pan troglodytes (chimpanzees), Gorilla gorilla (gorillas) and Pongo pygmaeus (orangutans).

The study expands the set of methodological tools for understanding the evolution of hominids in palaeoanthropology and proposes an innovative methodology that complements more traditional research based on bone morphology.

A new approach to reconstructing locomotion in hominids

Ligaments are a type of soft tissue that is rarely used in studies of hominid locomotion. They act as stabilizers for the joints and also enable the brain to determine the functional status of each joint. The different forms of locomotion in hominids involve anatomical differences in the ligaments of the wrist and, therefore, modifications in the areas where the ligaments insert into the distal epiphysis of the 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 team developed research lines specializing in the anatomical study of the ligaments of the upper limbs in primates, aiming to create anatomical models that can be compared with modern humans and estimating the types of locomotion of fossil primates by studying the areas of ligament insertion.

“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.”

The distal epiphysis of the radius, which is well represented in many species of fossil primates, “has no identifiable muscle insertion area that allows for locomotor inferences to be made, but it does have two well-defined areas where a large part of the wrist ligaments are inserted, which has allowed this type of inference to be made in the study," says Professor Aroa Casado.

“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.”

Unravelling the mysteries of bipedalism

As the study reveals, Australopithecus afarensis, Australopithecus anamensis, and Australopithecus sediba show anatomy very similar to that observed in chimpanzees and orangutans. This would support the hypothesis that these ancestors combined habitual bipedal locomotion with highly effective arboreal locomotion, a conclusion indicated in previous studies on muscle insertion sites.

Paranthropus robustus — a fossil primate that evolved in parallel with human phylogeny — is a more contradictory case. While some authors consider it a bipedal species with effective arboreal locomotion — similar to that of Australopithecus —, others postulate that it may have had other forms of terrestrial locomotion. “In our study, the Paranthropus robustus specimen has an anatomy more similar to that of gorillas, so the conclusions would reinforce the idea that, in addition to bipedal and arboreal locomotion, it could also have developed other forms of terrestrial locomotion,” the authors point out, since gorillas are the hominoid primates that most frequently use terrestrial-type locomotion.

Regarding the Homo neanderthalensis and archaic Homo sapiens — closer to the human species —, the anatomy is very similar to that of modern humans, but researchers detected “some characteristic features that would indicate a relatively different use of the hands compared to that of modern human populations.” These ancestors have a typical anatomy in the area where a particular ligament inserts, the short radiolunate ligament, which suggests greater robustness in the wrist area, probably related to a higher demand for manual load bearing associated with hunting or tool making.

The application of the 3D GM technique could be extended to other evolutionary studies of both fossil and extant species to infer locomotor patterns and manual precision. “Our results demonstrate the value of this technique in reconstructing locomotor behaviour and make it a valuable tool for future studies in the fields of comparative anatomy, biological anthropology, palaeoanthropology and human evolution,” the team concludes.