Daily Times

Arboreal primates such as chimpanzees exhibit pronounced curvature in their hand and foot phalanges, which is assumed to develop throughout life in response to mechanical loads produced by grasping and hanging from branches.

A trio of researchers from the University of New Mexico, Harvard University and the University of Southern California has found evidence that suggests the curved phalange in apes is an inherited trait, not one that comes about from climbing.

In their paper published in Proceedings of the National Academy of Sciences, Ian Wallace, Loring Burgess and Biren Patel describe their study of the skeletal remains of Suzy, a chimpanzee that was raised to behave like a human being back in the 1930s, and what they learned about phalangeal curvature in chimpanzees.

Primates that live in trees have curved bones in their hands and feet for better grip. The same features can be seen in fossils of human ancestors. However, scientists have debated whether these curves are genetic or a response to climbing from an early age, which would influence how we interpret the presence of similar features in fossils from our ancestors.

To answer the question, however, we either need to have humans spend their life climbing trees from childhood or deprive members of tree-dwelling species of the chance to climb. Even if there were scientists cruel enough to propose such an experiment today, they’d be unlikely to get it past an ethics committee. However, there is no problem with examining the bones of an animal mistreated before our standards improved.

Evidence that phalangeal curvature is determined mainly by genetic factors does not refute the hypothesis that the locomotion of extinct hominins included frequent arboreality. Such evidence only indicates that phalangeal curvature alone does not prove the importance of arboreal activities within the locomotor repertoire of a hominin species.

Assessing the behavioral significance of any single primitive retention in a fossil species is inevitably challenging, since the trait may have been actively maintained by stabilizing selection because it was still functionally important, or the trait may have become functionless and been retained simply because it was not selected against. If arboreality was common among extinct hominins, then phalangeal curvature would have been useful for minimizing diaphyseal bending strains produced by grasping and hanging from branches.

Yet, if phalangeal curvature was no longer functionally important to a hominin species, it might not have compromised bipedal locomotion or manual dexterity enough to have been under strong negative selection. It is thus impossible to resolve whether phalangeal curvature was functionally important or unimportant to ancient hominins without taking into account additional anatomical traits relevant to locomotor performance.