An evolutionary perspective on habenular asymmetry in humans
The habenula (Hb) of vertebrates is a dorsal and bilateral diencephalic nuclear complex that works as an anatomical hub integrating cognitive, emotional and sensory networks to regulate mood, motivation and value based decision making, among other functions. Across vertebrates, the Hb organises into two conserved separate components (medial and lateral in mammals equivalent to dorsal and ventral in more basal vertebrate species) which are thought to subserve different functions based on a partial independence of their connectivity systems. As a complex, the Hb shows morphological, molecular and connectivity differences between the left and right sides in a wide range of vertebrate species, which in some cases extend to the functional and behavioural levels. Habenular asymmetries are particularly prominent in basal vertebrate species but become less evident in amniotes and particular mammals. In humans, recent evidence reveals that, under an overall symmetry morphology, the Hb shows lateral differences in volume, activation, metabolism and susceptibility to damage that suggest an asymmetric condition of this nuclear complex. Here, we review the evidence supporting this view and discuss the possible origin of this asymmetric trait in humans from an evolutionary developmental perspective.