Primate Brain Evolution in Phylogenetic Context

The order Primates is the group of mammals that includes the hominoids (apes and humans), Old World monkeys, New World monkeys, tarsiers, lemurs, lorises, and bush babies. Enormous progress has been made over the past three decades in understanding the relationships of primates to other mammals, the relationships among primate groups, and the adaptive origins of primates and of primate subgroups. Several lines of evidence indicate that primates belong to a higher-order grouping of mammals, the Archonta, that includes at least tree shrews and flying lemurs. Primates probably originated as a group of small, nocturnally active animals, evolving grasping extremities and close-set, forward-facing eyes to locomote and feed in the fine terminal branches of tree. The origin of anthropoid primates (New World monkeys, Old World monkeys, and hominoids) was marked by a shift to diurnality and the evolution of a retinal fovea for enhanced visual acuity, followed by increased body size and the advent of more complex forms of social organization Primate brain evolution was marked by changes at many levels of structural organization. Brain size increased early in primate evolution, with expansion of the neocortex and the addition of numerous new cortical sensory areas and new systems of interconnections between areas. Also, primates evolved new areas in higher-order cortical regions such as posterior parietal, superior temporal sulcal, and dorsolateral prefrontal cortex, and a new thalamic nucleus, the dorsal (medial) pulvinar, which has extensive connections with the higher-order cortex. The evolution of anthropoid primates was accompanied by further increases in brain size, and the appearance of new areas, especially in higher-order and limbic regions, although it is not clear that the addition of new areas accounts for the increased encephalization of anthropoids. Evolutionary changes in primate brain organization were by no means limited to changes in the complement of areas and extrinsic connectivity: numerous changes in the internal laminar and modular organization of cortical areas have been documented, and there is increasing evidence of changes in the morphological and biochemical phenotypes of brain cells.