Temporal lobe evolution in extant and extinct Cercopithecoidea

Abstract

Changes to the environmental landscapes from the Eocene to Holocene have influenced the evolution of Cercopithecoidea from arboreal origins in wet, forested regions in the Early Oligocene Fayum to semi-terrestrial lifestyles in drier Neogene landscapes and social systems of larger group living. These eco-behavioural transitions likely accompanied changes in behaviour, brain function, and associated skull morphology. The temporal lobe of the brain, an association cortex, is in close proximity to the middle cranial fossa (MCF) allowing prediction of temporal lobe volume (TLV) and investigation of cerebral reorganisation. We used micro-computed tomography (µCT) cranial scans (n = 135) generated into 3D virtual crania with seven MCF metrics predicting TLV from a multiple regression of 11 extant anthropoid taxa. We studied eight extinct taxa Proteopithecus sylviae and Catopithecus browni from the Late Eocene, Early Oligocene Apidium phiomense, Parapithecus grangeri and Aegyptopithecus zeuxis, Middle Miocene Victoriapithecus macinnesi, Pliocene Dinopithecus ingens, Pleistocene Papio angusticeps, and extant cercopithecines Cercocebus atys, Macaca mulatta and Papio anubis. PGLS regressions examined relative TLV to brain size between extinct and extant taxa. We tested differences in slopes and intercepts between extinct and extant cercopithecoids with statistically significant differences in slopes but not the intercepts, with stem-anthropoids having relative smaller TLV for brain size compared to extant cercopithecoids. Potential drivers for temporal lobe evolution include paleoenvironmental shifts from Eocene tropical rainforests to Plio-Pleistocene savannas. Socio-behavioural implications include change from arboreal to semi-terrestrial lifestyles, higher visual acuity, larger group sizes and greater cognitive complexity.