Abstract
Anatomical asymmetries of the human brain are a topic of major interest because of their link with handedness and cognitive functions. Their emergence and occurrence have been extensively explored in human fossil records to document the evolution of brain capacities and behaviour. We quantified for the first time antero-posterior endocranial shape asymmetries in large samples of great apes, modern humans and fossil hominins through analysis of “virtual” 3D models of skull and endocranial cavity and we statistically test for departures from symmetry. Once based on continuous variables, we show that the analysis of these brain asymmetries gives original results that build upon previous analysis based on discrete traits. In particular, it emerges that the degree of petalial asymmetries differs between great apes and hominins without modification of their pattern. We indeed demonstrate the presence of shape asymmetries in great apes, with a pattern similar to modern humans but with a lower variation and a lower degree of fluctuating asymmetry. More importantly, variations in the position of the frontal and occipital poles on the right and left hemispheres would be expected to show some degree of antisymmetry when population distribution is considered, but the observed pattern of variation among the samples is related to fluctuating asymmetry for most of the components of the petalias. Moreover, the presence of a common pattern of significant directional asymmetry for two components of the petalias in hominids implicates that the observed traits were probably inherited from the last common ancestor of extant African great apes and Homo sapiens.These results also have important implications for the possible relationships between endocranial shape asymmetries and functional capacities in hominins. It emphasizes the uncoupling between lateralized activities, some of them well probably distinctive to Homo, and large-scale cerebral lateralization itself, which is not unique to Homo.
Highlights
Human brain asymmetries have been documented since the time of Dax [1], [2] and Broca [3] and have been widely investigated for their functional, physiological or behavioural implications
Positive values corresponded to a right asymmetry and negative values indicated a left asymmetry
Signed asymmetry is the difference between the right and left side for each petalia in an individual (Rxi-Lxi)
Summary
Human brain asymmetries have been documented since the time of Dax [1], [2] and Broca [3] and have been widely investigated for their functional, physiological or behavioural implications. It emerges from several studies that the combination of right frontal and left occipital protrusions represents brain shape asymmetries that are characteristic of the hominin lineage. This combination is usually described as the ‘‘torque’’ pattern and represents the extension of one cerebral hemisphere beyond the other. It is currently accepted that this pattern of asymmetries appeared with early Homo [4]–[7] and is most common in human right-handed individuals [8]–[14]. These asymmetries were a topic in non human primate brain studies [6], [8], [10], [15]–[20] and raised a special interest in paleoanthropology [4]–[7], [21]–[22] because of their relationships with handedness and, more generally, with specific aspects of human cognition
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