Language evolution from a perspective of Broca’s area

Abstract

Broca’s area is one of language-related brain areas that typically locates in the posterior part of the inferior frontal region of the left hemisphere. Broca’s area plays an important role in the perception and production of sequential/hierarchical structures of language. A contribution of two major frontotemporal fiber bundles connected to Broca’s area also supports our language faculty: the dorsal and ventral pathways. The dorsal pathway supports syntactics, while the ventral pathway supports semantic processes and local phrase structure. Homologues of these brain structures have been identified in nonhuman primates. However, Broca’s area of humans exhibits disproportionate expansion compared to that of chimpanzees. In addition, the dorsal pathway connections have undergone considerable modification as evidenced by differences compared to both chimpanzees and macaques, while the ventral pathway in nonhuman primate is well developed as in human. These structural differences indicate that nonhuman primates can recognize meaning of individual calls or tones but cannot unify constituents to represent temporally structured sequences. Such assumption is consistent with the well-documented evidence from auditory memory task. Differences in the languagerelated brain structures between human and nonhuman primate species indicate that language faculty emerged in recent human evolution, since our lineage branched from a common ancestor with chimpanzees. Difference between human and nonhuman primates is firstly found in vocal production. Monkey’s call production is mediated by limbic and brain stem regions, not by Broca’s area. Thus, their calls largely represent involuntary symptoms of specific emotional and arousal states like laugh and cry of humans. But this system may be sufficient for the species-specific vocalizations that does not typically require rapid articulation. Secondly, difference between human and nonhuman primates is also found in the sequential processing of vocal production and perception. In the vocalization of monkeys, monosyllabic calls are typically produced, or the same calls are repeated. Different types of calls are occasionally combined but in a stereotyped order. Thus, combined calls unlikely generate new meaning. Both laboratory and field studies of nonhuman primate call/tone perception indicate that they are sensitive to some aspects of call/tone streams. However, they may perceive call/tone sequences based on local acoustic feature or perceive them as holistic chunks. Recently, the involvement of the inferior frontal region along the ventral stream in the processing tonal sequential rule was found in both humans and monkeys. Thus, it is reasonable to assume that modification of pre-existing brain structures, i.e., expansion of Broca’s area and extension of the frontotemporal connections to this area, allowed humans language faculties. Consequently, humans can intentionally control speech production, create new words by combining several different syllables, and can learn new language. However, Broca’s area may not be a structure designed exclusively for language processing. Studies have revealed that Broca’s area contributes in the domains that require sequential/hierarchical processing, such as music and action domains along with language domain. Language is one of unique features of human. From a perspective of the function of Broca’s area, however, language may be a byproduct of the evolution of sequential/hierarchical processing.