Endocranial shape asymmetries in Hylobates lar

Asymmetries in brain shape, or petalias, is the extension of one cerebral hemisphere beyond the other. In humans, anatomical asymmetry of the brain has been linked to handedness and cognitive functions. Previous research has demonstrated brain shape asymmetries in hominids with a pattern of fluctuating asymmetry. However, non-human great apes show lower variation and a lower degree of fluctuating asymmetry compared to modern humans. This study describes and quantifies the positions of the frontal and occipital petalias of a sample of Hylobates lar using 3d models of the cranium and endocranial cavity. Cranial landmarks were used to create a reference system onto which endocast landmarks were projected and the difference between the right and left projections produced the petalial components. This study shows the existence of frontal and occipital petalia in gibbons, and comparison to published literature on the brain shape asymmetry in hominids shows that gibbons have an antisymmetric endocast shape as opposed to the fluctuating asymmetry in hominids. This study also shows that the Broca’s area homologue in gibbons does not have the same right asymmetry that is observed in humans and to a lesser extent non-human great apes. This has evolutionary implications indicating that the Yakovlevian anticlockwise torque is unique to great apes, and that the gibbons do not have the same language area functions as humans and other great apes do.