Asymmetry in the Cytoarchitecture of the Area 44 Homolog of the Brain of the Chimpanzee Pan troglodytes

Abstract

The evolution of the brain in apes and man followed a joint pathway stemming from common ancestors 5-10 million years ago. However, although apparently sharing similar organization and neurochemical properties, association areas of the isocortex remain one of the cornerstones of what sets humans aside from other primates. Brodmann's area 44, the area of Broca, is known for its implication in speech, and thus indirectly is a key mark of human uniqueness. This latero-caudal part of the frontal lobe shows a marked functional asymmetry in humans, and takes part in other complex functions, including learning and imitation, tool use, music and contains the mirror neuron system (MNS). Since the main features in the cytoarchitecture of Broca's area remains relatively constant in hominids, including in our closest relative, the chimpanzee Pan troglodytes, investigations on the finer structure, cellular organization, connectivity and eventual asymmetry of area 44 have a direct bearing on the understanding of the neural mechanisms at the base of our language. The semi-automated image analysis technology that we employed in the current study showed that the structure of the cortical layers of the chimpanzee contains elements of asymmetry that are discussed in relation to the corresponding human areas and the putative resulting disparity of function.

The miRNome of Depression

“…BA44 in the human brain is located in the third frontal convolution of the frontal lobe [ 55 ]. BA44—especially that from the left hemisphere—is functionally engaged in different aspects of speech and higher cognitive functions, such as learning, imitation, creation of music [ 56 ], and expression of emotional information [ 57 ]. BA44 is also another well-studied cortical region in terms of changes in the levels of miRNA expression in depression and suicide.…”

How Asymmetries Evolved: Hearts, Brains, and Molecules

“…Cantalupo and Hopkins [60] report that the homolog of Broca's area in chimpanzees is also larger on the left. Graïc et al [61] report a structural asymmetry in area 44 of the chimpanzee characterised by smaller neurons, perhaps suggesting increased computational capacity. In this and other respects, the cyto-architectural structure of area 44 seems to resemble closely that in humans.…”

Broca's cerebral asymmetry reflects gestural communication's lateralisation in monkeys (Papio anubis)

“…Given such potential lateralisation links between gesture and language in humans, it is thus not excluded that the specific lateralisation's signature found for communicative gestures in nonhuman primates might reflect evolutionary continuities with frontal hemispheric specialisation for speech/gesture integration. This hypothesis might be relevant to investigate given brain studies in nonhuman primates have shown human-like gross left-hemispheric asymmetries of homologous language areas at a populational level: In particular Broca's homologue in great apes (Cantalupo & Hopkins, 2001;Graïc et al, 2020) as well as the Planum Temporale in great apes and even in baboons, an Old World monkey species, in both adult and newborns (Gannon et al, 1998;Marie et al, 2018;Becker et al, 2021b,c).…”