Brain evolution and language: A comparative 3D analysis of Wernicke's area in extant and fossil hominids.

Abstract

The spoken word does not fossilize. Despite this, scientists have long sought to unearth the origins of language within the human lineage. One of the lines of evidence they have pursued is functional brain areas, such as Broca's and Wernicke's areas, which are associated with speech production and comprehension, respectively. Sulcal layout of Broca's area clearly differs between humans and our closest living relatives, the chimpanzees, enabling its homolog in fossil hominins to be deemed more chimpanzee-like (i.e., closer to the ancestral form) or more human-like (i.e., derived form) with relative ease. Yet, no such differences have been found for Wernicke's area. This study compares sulcal and gyral organization of Wernicke's area across extant human brains (n=4), extant chimpanzee brains (n=5) and fossil hominin endocasts (n=4). Some chimpanzee brains had indications of leftward Wernicke's area asymmetry in the form of a shorter Sylvian fissure and/or caudal superior temporal gyral bulging in the left hemisphere. Overlap between the superior and middle temporal sulci in human but not chimpanzee brains may be due to a relatively larger Wernicke's area in humans. Fragmentation of the main body of the superior temporal sulcus exclusively in human left hemispheres was ascribed to a leftward Wernicke's area asymmetry in this species. Endocast examination found that, while Paranthropus robustus exhibit human-like overlap between the superior and middle temporal sulci, Australopithecus africanus do not, although they do exhibit chimpanzee-like caudal superior temporal gyral bulging. Such findings signal, albeit loosely, a more human-like Wernicke's area in Paranthropus than Australopithecus.