Brain asymmetries, particularly asymmetries within regions associated with language, have been suggested as a key difference between humans and our nearest ancestors. These regions include the planum temporale (PT) - the bank of tissue that lies posterior to Heschl's gyrus and encompasses Wernicke's area, an important brain region involved in language and speech in the human brain. In the human brain, both the surface area and the grey matter volume of the PT are larger in the left compared to right hemisphere, particularly among right-handed individuals. Here we compared the grey matter volume and asymmetry of the PT in chimpanzees and three other species of nonhuman primate in two Genera including vervet monkeys (Chlorocebus aethiops sabaeus), rhesus macaques (Macaca mulatta) and bonnet macaques (Macaca radiata). We show that the three monkey species do not show population-level asymmetries in this region whereas the chimpanzees do, suggesting that the evolutionary brain development that gave rise to PT asymmetry occurred after our split with the monkey species, but before our split with the chimpanzees.
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