Planum Temporale Asymmetries in Primates: A Comparative Study in Great Apes and Monkeys

Most human brains exhibit left hemisphere asymmetry for planum temporale (PT) surface area and gray matter volume, which is interpreted as cerebral lateralization for language. Once considered a uniquely human feature, PT asymmetries have now been documented in chimpanzees and olive baboons. The goal of the current study was to further investigate the evolution of PT asymmetries in nonhuman primates.

We measured PT surface area in chimpanzees (Pan troglodytes, n = 90), bonobos (Pan paniscus, n = 21), gorillas (Gorilla gorilla, n = 34), orangutans (Pongo spp., n = 33), olive baboons (Papio anubis, n = 105), rhesus macaques (Macaca mulatta, n = 144), and tufted capuchins (Sapajus apella, n = 29) from magnetic resonance imaging scans.

Our findings reveal significant leftward biases in PT surface area among chimpanzees, gorillas, olive baboons, rhesus macaques, and capuchins. We did not find significant population-level asymmetries among orangutans and bonobos, which could be due, in part, to small sample sizes. We also detected significant age effects for rhesus macaques only, and no significant sex effects for any species.

The observation of a population-level leftward bias for PT surface area among not only hominids (chimpanzees and gorillas), but also two cercopithecoids (olive baboons and rhesus macaques) and one platyrrhine (tufted capuchins) suggests that PT lateralization was likely present in some early anthropoid primate ancestors and relatives. This provides further evidence that human brains have since undergone changes to the size and connectivity of the PT in response to selection for the cognitive processes needed to support the evolution of language and speech.