Deciphering how the ancestors of the human species moved

One of the most fascinating periods in the evolution of the human lineage is the appearance of the first ancestors capable of bipedal movement. Knowing the type of locomotion of many fossil species (walking upright on the ground or climbing from branch to branch with the strength of their arms) has been one of the most classic big questions in the study of the hominization process. Now, a study provides new perspectives to facilitate the understanding of how and when bipedal locomotion appeared during human evolution.

The team that carried out the new study includes, among others, Professor Josep M. Potau, from the Human Anatomy and Embryology Unit of the Faculty of Medicine and Health Sciences at the University of Barcelona (UB) and the Institute of Archeology of the UB (IAUB), as well as Neus Ciurana, of the Gimbernat University School; Scientists from the University of Valladolid, in Spain, have also worked on the team.

The research helps to infer how individuals of some fossil hominin species moved through an innovative technique that analyzes and compares the muscle insertion sites characteristic of locomotor behavior in primates of the Hominidae family (chimpanzees, bonobos, gorillas, orangutans and humans).

The conclusions corroborate that australopithecines and paranthropes combined bipedal locomotion with arboreal locomotion similar to that of current bonobos (Pan paniscus), the species most phylogenetically related to humans that combines terrestrial and arboreal locomotion, in addition to episodes of bipedalism. The results also reaffirm the presence of arboreal-type locomotion in Australopithecus sediba (a still little-known species) and Paranthropus boisei, a fossil hominid that has aroused a certain degree of scientific controversy regarding its form of locomotion.

3D technology to study key areas in brachiatory primates

The team has used a new methodology that consists of making 3D scans of the ulna bone of current humans, current hominoid primates and fossil hominins. This technique allows the identification and comparison with greater precision of the insertion sites of the brachialis and triceps brachii muscles in the proximal epiphysis of the ulna, a decisive anatomical area in arboreal locomotion (brachiation).

Josep M. Potau, member of the research group on Evolutionary and Pathological Anatomy of Humans and other Primates (AEPPRI) at the University of Barcelona (UB), explains that “the elbow is a joint formed by three bones (humerus, ulna and radius) and its flexion-extension and pronation-supination mechanisms play an essential role in different types of arboreal locomotion widely used by primates. This is due, above all, to the functional importance of the muscles that act on the joint, especially the brachialis, which participates in the flexion of the elbow, and the triceps brachii, which does so in its extension.”

“Primates that use arboreal locomotion more frequently – such as orangutans or bonobos – develop more elbow flexor muscles such as the brachialis. On the other hand, more terrestrial primates, such as chimpanzees and gorillas, have more developed elbow extensor muscles such as the triceps brachii,” the researcher points out. “These two muscles have well-defined insertion zones in the ulna whose surface can be measured,” continues the professor, who admits that “the most arboreal primates, thus, will have a greater relative surface area of ​​the insertion zone of the brachialis muscle, while “The more terrestrial specimens will have a greater relative surface area of ​​the insertion zone of the triceps brachii muscle.”

Researchers Josep M. Potau and Neus Ciurana. (Photo: UB / Gimbernat University School)

Fossil species of the genus Homo

Within the framework of the research, the results that have been obtained through the study of the bones support those of the muscle proportions achieved from anatomical dissections made in current humans and primates. In this way, it has been confirmed that the existing differences in the proportion of the insertion zones in humans and hominoid primates (due to the different types of locomotion) can also be related to the degree of development of the muscles that are inserted. in these areas.

In the case of hominins of the genera Australopithecus and Paranthropus, endowed with anatomical adaptations associated with habitual bipedal locomotion and brachiation, Potau indicates that “we discovered that the proportion between the insertion zones of the brachialis and triceps brachii muscles that had been analyzed in four species of these two genera is similar to that observed in bonobos, which are African hominoid primates. (bonobos, chimpanzees and gorillas) that use arboreal locomotion most frequently.”

On the other hand, most species of the genus Homo, to which the human species belongs, “do not present anatomical adaptations to arboreal locomotion. This is reflected in the work, in which we have observed that the representatives of three fossil species of the genus Homo—Homo ergaster, Homo neanderthalensis and archaic Homo sapiens—present a proportion of the insertion zones of the brachialis and triceps brachii muscles similar to the one we observe in current humans.”

The research opens a new scenario in the study of human evolutionary lineage to obtain information about the type of locomotion typical of a given fossil species that preserves these muscle insertion areas. “This approach could also be extended to other anatomical areas that present well-defined muscle insertion zones, if the muscle characteristics in current species are previously exhaustively studied,” concludes Professor Potau.

The study is titled “Quantitative analysis of the Brachialis and Triceps Brachii Insertion Sites on the Proximal Epiphysis of the Ulna in Modern Hominid Primates and Fossil Hominins.” And it has been published in the academic journal American Journal of Primatology. (Source: UB)