Language, Paleoneurology, and the Fronto-Parietal System

Homo sapiens, Chimpanzees and the Enigma of Language

OBJECTIVES

The present study explores the hypothesis that the anatomical bone structures of the oral cavity have probably evolved under the influence of language function. The possible changes have been evaluated by comparing two close species essentially differentiated from each other by spoken language.

MATERIALS AND METHODS

Twenty dry skulls and 20 mandibles of modern Caucasians were compared with 12 dry skulls and 12 mandibles of chimpanzees, with the analysis of 37 variables and the definition of new anatomical parameters.

RESULTS

A number of highly significant differences were found between humans and chimpanzees. The human temporomandibular joint is comparatively less flat and has a more limited excursive movement range, with structural elements that seem to be lighter. A significant difference is noted in mandibular alveolar vergency and in the internal slope of the mandibular symphysis where the oral cavity's morphology is modified, thereby increasing the free space for tongue movements in humans. The chin, which is unique to the human species, is quantified through the external slope of the mandibular symphysis with a lesser angle in humans.

DISCUSSION

It is obvious that there are differences between humans and chimpanzees in the bone morphology of the oral cavity structures. This has been confirmed with the analysis of new variables. Together with other factors (bipedalism, habits, and genetics) speech in humans must have played an important role in the aforementioned differences between humans and chimpanzees. The number of mandibular movements involved in speech is far greater than those used in chewing, which must have conditioned the evolution of the oral structures implicated in the development of language. On average, humans weigh 70 kg and chimpanzees 44 kg. However, the majority of the variables studied in skulls and mandibles are greater in chimpanzees, which suggests that the evolution of the oral zone in humans has suffered a reduction in size with changes in shape. The refinement of the supralaryngeal vocal tract in the human species must have co-evolved with speech fairly recently. The human skull has temporomandibular joints that are comparatively less flat with a more limited movement. There is a greater lingual space and there is also a chin that suggests a muscular stimulant. This leads to the conclusion that, at least in part, speech is behind all these changes, although it is difficult to establish a cause-effect relationship.

A Brain for Speech. Evolutionary Continuity in Primate and Human Auditory-Vocal Processing

In this review article, I propose a continuous evolution from the auditory-vocal apparatus and its mechanisms of neural control in non-human primates, to the peripheral organs and the neural control of human speech. Although there is an overall conservatism both in peripheral systems and in central neural circuits, a few changes were critical for the expansion of vocal plasticity and the elaboration of proto-speech in early humans. Two of the most relevant changes were the acquisition of direct cortical control of the vocal fold musculature and the consolidation of an auditory-vocal articulatory circuit, encompassing auditory areas in the temporoparietal junction and prefrontal and motor areas in the frontal cortex. This articulatory loop, also referred to as the phonological loop, enhanced vocal working memory capacity, enabling early humans to learn increasingly complex utterances. The auditory-vocal circuit became progressively coupled to multimodal systems conveying information about objects and events, which gradually led to the acquisition of modern speech. Gestural communication accompanies the development of vocal communication since very early in human evolution, and although both systems co-evolved tightly in the beginning, at some point speech became the main channel of communication.