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In several species of hominid primates with different types of locomotor behavior, we quantitatively studied the insertion sites of the brachialis and triceps brachii on the proximal epiphysis of the ulna. Our main objective was to evaluate the possibility of using the anatomical features of these insertion sites to infer the locomotor behavior of different species of fossil hominins.

The relative values for the brachialis insertion were highest in orangutans, followed by bonobos, chimpanzees, gorillas, and humans. Fossil Australopithecus and Paranthropus had values similar to those of bonobos, while fossil Homo had values similar to those of Homo sapiens. The observed similarity in ulnar attachment sites between Australopithecus and Paranthropus and extant bonobos suggest that these hominins used arboreal locomotion to complement their bipedalism. These adaptations to arboreal locomotion were not observed in Homo.

Summary

• There is a close relationship between the types of locomotion used by hominoid primates and the relative size of the brachialis and triceps brachii muscles.

• The relative mass of the brachialis and the triceps brachii muscles is related to the relative size of their insertion sites in the proximal epiphysis of the ulna.

• The morphology of the proximal epiphysis of the ulna can provide information on the locomotor behavior of fossil hominins.