Hand grasping is a behaviour that has been suggested to play a key role in the evolutionary success of tetrapods (frogs, lizards and mammals) because of its implication in vital behaviours such as feeding, mating, moving or social exchanges. However, even if the ability to grab food, to hold it with hand, and to manipulate it with fingers has been observed in a variety of species including frogs and lizards, it is thought to be essentially a primate trait. Moreover, fine prehensile abilities are thought to be uniquely associated with the evolution of human hands and with tool-making and tool-use in the earliest humans. However, given the single evolutionary origin of primates and the unique anatomical features they possess, quantitative tests of evolutionary scenarios pertaining to the origin of grasping behavior remain difficult. This is why this project aimed to explore the origin of this complex behavior among tetrapods more generally.
Two main hypotheses have been suggested to explain the origin of manual grasping: 1) grasping may be derived from arboreal locomotion on thin branches. As the importance of the forelimb in body weight support is largely diminished in arboreal animals, the forelimbs are functionally decoupled and can become specialized for novel behaviours. 2) grasping may have originated in the context of the capture of mobile prey which initiates forelimb grasping movements. Both hypotheses are not mutually exclusive and could confer the selective environment driving the evolution of grasping ability and fine manipulation skills.
This project aimed to understand the origin and evolution of manual grasping in tetrapods by studying the morphology of the forelimb in relation to its function and manual grasping behaviour. We tested these two-main hypotheses in three groups of tetrapods showing grasping abilities by studying the morphology, behavior and locomotion in species that are either arboreal or terrestrial and species with different manual grasping abilities. Our results show significant diversity in forelimb morphology related to and arboreal lifestyle in all clades examined. Moreover, behavioral trial suggests that moving objects elicited more manual grasping compared to stationary objects. Overall our results support both the 'arboreal origins' and 'predation' hypotheses and suggest that manual grasping may have originated in different ecological and functional contexts.
