The functional origins of speech-related hand gestures

Many theories of language posit its recent evolution, perhaps contemporaneous with the evolution of Homo sapiens. The embodied language theory, however, in proposing that language includes gestures, provides an avenue for tracing language origins to phylogenetically earlier ancestral species. Here, evidence is presented that the structure of functional hand movements (e.g., reaching for food, climbing a ladder, or crawling), in rats and humans is similar. The structure of these functional hand movements is then compared to speech-related hand gestures in humans. The sequence of language-related gestures are also found to be characteristic of functional hand movements. It is suggested that these findings show that the arm and hand gestures that accompany human speech are derived from the same neural substrates that produce functional movements. Additionally, evidence is reviewed that supports the idea that speech-related gestures resemble the movements elicited by long-train stimulation of the primate motor cortex. Together, this evidence suggests that speech-related hand gestures have their evolutionary origins in functional hand movements of ancestral non-primate and primate species and may be constrained by the neural substrate for those movements. These findings are further discussed in relation to the idea that speech-related gestures reflect forelimb motor cortex contributions to embodied language.