Search for: All records

ABSTRACT ObjectivesMost 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. Materials and MethodsWe measured PT surface area in chimpanzees (Pan troglodytes,n = 90), bonobos (Pan paniscus,n = 21), gorillas (Gorilla gorilla,n = 34), orangutans (Pongospp.,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. ResultsOur 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. DiscussionThe 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. 

Abstract Astrocytes are the main homeostatic cell of the brain involved in many processes related to cognition, immune response, and energy expenditure. It has been suggested that the distribution of astrocytes is associated with brain size, and that they are specialized in humans. To evaluate these, we quantified astrocyte density, soma volume, and total glia density in layer I and white matter in Brodmann's area 9 of humans, chimpanzees, baboons, and macaques. We found that layer I astrocyte density, soma volume, and ratio of astrocytes to total glia cells were highest in humans and increased with brain size. Overall glia density in layer I and white matter were relatively invariant across brain sizes, potentially due to their important metabolic functions on a per volume basis. We also quantified two transporters involved in metabolism through the astrocyte‐neuron lactate shuttle, excitatory amino acid transporter 2 (EAAT2) and glucose transporter 1 (GLUT1). We expected these transporters would be increased in human brains due to their high rate of metabolic consumption and associated gene activity. While humans have higher EAAT2 cell density, GLUT1 vessel volume, and GLUT1 area fraction compared to baboons and chimpanzees, they did not differ from macaques. Therefore, EAAT2 and GLUT1 are not related to increased energetic demands of the human brain. Taken together, these data provide evidence that astrocytes play a unique role in both brain expansion and evolution among primates, with an emphasis on layer I astrocytes having a potentially significant role in human‐specific metabolic processing and cognition.