Evolution of the frontal aslant tract and implications for primate vocalization and human speech.

Monkeys, chimpanzees and humans share dorsomedial and ventrolateral frontal regions for vocalization, yet only humans have speech. These regions are interconnected by the frontal aslant tract (FAT), a pathway increasingly recognized as central to speech production. Using spherical deconvolution tractography, we compared the FAT across primate species. In humans, the FAT is left-lateralized and enlarged, predominantly in its anterior parts connecting prefrontal areas. This expansion parallels the enlargement of the arcuate fasciculus, which conveys auditory input to the ventrolateral frontal region. To probe the functional significance of these anatomical modifications, we measured correlations between degeneration of FAT subsegments and language deficits in patients with primary progressive aphasia. Posterior FAT degeneration correlated with impaired verbal fluency whereas anterior FAT degeneration disrupted syntactic processing. Together, these findings support the view that an ancestral vocalization network was exapted in humans to sequence speech sounds, providing a neural substrate for the emergence of language.