The two main language areas of the human brain - Broca's Area and Wernicke's Area - are connected by a major bundle of fibres called the arcuate fasciculus, which also ramifies extensively in the temporal lobe and in areas adjacent to Broca's Area in the frontal lobe. From what areas of the primate brain did the brain structures necessary for language evolve? Michael Petrides and Deepak Pandya, in this PLoS Biology article, report detailed tracing of pathways in the macaque brain they think are those evolutionary precursors. The detailed forensic work is made possible by the injection of radiographic dyes into the white matter nerve fibres. The dye runs along the network allowing tracing to occur. This is their summary:
"Two distinct cortical areas in the frontal lobe of the human brain, known as Broca’s region, are involved with language production. This region has also been shown to exist in nonhuman primates. In this study, we explored the precise neural connectivity of Broca’s region in macaque monkeys using the autoradiographic method to achieve a level of detail impossible in the human brain. We identified two major streams of connections feeding into Broca’s area: a ventral stream from the temporal region, which includes areas processing auditory, multisensory, and visual information and a dorsal stream originating from the inferior parietal lobule and the adjacent superior temporal sulcus. Our detailed connectivity analysis illuminates the pathways via which posterior cortical areas can interact functionally with Broca’s region, in addition to contributing to an understanding of the evolution of language. We suggest that a fundamental function of Broca’s region is to retrieve information in a controlled strategic way from posterior cortical regions and to translate this information into action. This fundamental function was adapted during evolution of the left hemisphere of the human brain to serve language."
Specifically, what they have found is that one of the component parts of Broca's Area, area 45, and a related ventro-lateral area, is involved with the controlled retrieval and selection from non-verbal memory in the monkey and in the right - or non-dominant hemisphere of the human brain. In the human, this area on the left side of the brain has been co-opted for language. Specifically they say:
"During the evolution of the human brain, these high-level forms of programming (the basic elements of which are already present in the macaque monkey brain) came to include complex syntactical structure (e.g., hierarchical level of control) that is
necessary for language, and which has been argued to be a major contribution of Broca’s region. If we were to extrapolate these arguments on the basis of the present monkey anatomical study, our recording study in the monkey, and our functional neuroimaging studies of the human right hemisphere homologue of Broca’s region, we
could say that a common primate circuitry was adapted, during millions of years of evolution, in the human brain for the strategic retrieval and selection of information from verbal memory (including the mental lexicon) in posterior temporo-parietal
cortical regions by one component of Broca’s region, area 45, and the transformation of this selected conceptual information into gestural/speech acts by the other component of Broca’s region, area 44, via its connections with motor structures, such as the premotor cortex, the basal ganglia, and the rostral inferior parietal lobule. Our suggestion here is simply that an area that served higher control of action in the macaque monkey may have been adapted for the control of complex hierarchical sequences of gestural and vocal action with the evolution of communication leading to human speech."
This neatly links the complex hierarchical welding of phonemes into language with the complex and bewilderingly fast oro-facial movements necessary to articulate language in the larynx. When might this evolutionary process have occurred? The authors note that some degree of leftward asymmetry has been found in the planum temporale (associated with Wernicke's Area) in chimps as has left hemisphere dominance or the oro-facial movements involved in learned communicative vocalizations. Also, handedness has been noted for chimps connected to tool use, suggesting a left hemisphere specialization for control of complex actions involving the right hand. Remarkable asymmetry of Broca's cap has been discovered in endocasts of Homo erectus skulls, suggesting the various precursors were coming into place before manifesting themselves, much more recently, in a language faculty. The authors conclude:
"These findings are consistent with suggestions that specialization for the control of action and gesture may have preceded specialization for language. Note also that our close primate relatives, chimpanzees and bonobos, use arm/hand gestures more flexibly in their natural communication across contexts than facial expressions and vocalizations. The above facts suggest that the use of gestures for early forms of communication may have been an adaptation distinguishing the Hominoidea from other primates, and that the use of vocalization in the form of modern speech emerged much later with the evolution of language in the narrow sense, i.e., a uniquely human adaptation. It is interesting in this respect that the supralaryngeal vocal tract of humans differs significantly from those of other primates, making the human vocal apparatus unique in transmitting information at fast rates."
Thus language appears to be a combination of ancient, modern and unique evolutionary developments in the primate and hominin lineage.
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