New insights into how and when bipedal locomotion evolved in human ancestors

One of the most fascinating periods in the evolution of the human lineage is the appearance of the first ancestors capable of bipedalism. Knowing the type of locomotion used by 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 questions in the study of the process of hominization. Now, a paper published in the American Journal of Primatology provides new insights into how and when bipedal locomotion appeared during human evolution.

Professor Josep M. Potau, from the Human Anatomy and Embryology Unit of the Faculty of Medicine and Health Sciences and the Institute of Archaeology of the University of Barcelona (IAUB), leads this study. Neus Ciurana, from the Gimbernat University School, is the first author of the article, which includes the participation of teams of the University of Valladolid.

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

The conclusions corroborate that specimens of Australopithecus and Paranthropus combined bipedal locomotion with an arboreal locomotion similar to that of current bonobos (Pan paniscus), the species most phylogenetically related to humans that combines ground and arboreal locomotion, as well as occasional bipedalism episodes. The results of the study also state the presence of arboreal locomotion in Australopithecus sediba — a still little-known species — and in Paranthropus boisei, a fossil hominin that has aroused a certain scientific controversy regarding the way it moved around.

3D technology to study key areas in brachiating primates

The team has used a new methodology that involves making 3D scans of the ulna bone of present-day humans, present-day hominoid primates and fossil hominins. This technique makes it possible to identify and compare with greater precision the insertion sites of the brachialis and triceps brachii muscles in the proximal epiphysis of the ulna, an anatomical area that is decisive in arboreal locomotion (brachiation).

Josep M. Potau, member of the UB's AEPPRI research group(Anatomia Evolutiva i Patològica dels Humans i altres Primats), notes that "the elbow is a joint formed by three bones — humerus, ulna and radius — and its flexion-extension and pronation-supination mechanisms play a key role in different types of arboreal locomotion widely used by primates. This is due to the functional importance of the muscles that act on the joint, especially the brachialis, which participates in elbow flexion, and the triceps brachii, which participates in elbow extension".

The paper confirms that there is a good correlation between the forms of locomotion of hominoid primates, the relative mass of the brachialis and triceps brachii muscles and the relative surface area of their insertion zones.

"Primates that use arboreal locomotion more frequently — such as orangutans or bonobos — develop more elbow flexor muscles, such as the brachialis. In contrast, more terrestrial primates, such as chimpanzees and gorillas, have more developed elbow extensor muscles such as the triceps brachii", says the researcher. "These two muscles have well-defined insertion zones in the ulna, whose surface area can be measured", continues the professor, who admits that "more arboreal primates will thus have a greater relative surface area of the brachialis muscle insertion zone, while more terrestrial specimens will have a greater relative surface area of the triceps brachii muscle insertion zone".

Fossil species of the Homo genus

The results obtained through the study of bones support those of the muscle proportions obtained from anatomical dissections in present-day humans and primates. Thus, it has been possible to confirm that the existing differences in the proportion of the insertion areas in humans and hominoid primates — due to the different types of locomotion — can also be related to the degree of development of the muscles inserted in these areas.

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

However, most species of the Homo genus, to which the human species belongs, "have no anatomical adaptations to arboreal locomotion. This is reflected in the study, in which we have observed that representatives of three fossil species of the Homo genus — H. ergaster, H. neanderthalensis and archaic H. sapiens - have a proportion of the insertion zones of the brachialis and triceps brachii muscles similar to that observed in present-day humans".

The paper opens up a new scenario in the study of the human evolutionary lineage to obtain information on the type of locomotion typical of a given fossil species that retains these muscle insertion zones.

This approach could also be extended to other anatomical areas with well-defined muscle insertion zones, if the muscle characteristics are studied exhaustively in present-day species beforehand." 

Professor Josep M. Potau, Human Anatomy and Embryology Unit, Faculty of Medicine and Health Sciences and the Institute of Archaeology, University of Barcelona