Evolution of Frontal Cortex and Thalamus in Primates

Abstract The frontal cortex and thalamus changed during primate evolution, both in size and composition. New, primate-specific areas include the granular prefrontal cortex (PFC) and several premotor areas, and an emphasis on vision increased the size and complexity of cortical visual areas, which directly influenced the frontal cortex. When they first evolved, visual inputs to the PFC guided foraging, while those to premotor areas improved reaching and leaping. A key development occurred during the Miocene, when the frontal lobe enlarged independently in New World monkeys, Old World monkeys, and hominoids (the ape-human lineage). Parallel developments occurred in thalamic nuclei connected to the PFC. The magnocellular division of the mediodorsal nucleus (MDmc) functions like sensory and cerebellar relay nuclei, but—unlike its homolog in other mammals—some of its outputs go to new, primate-specific orbitofrontal areas. These thalamocortical projections relay information from the amygdala about the current desirability of hidden food items, which improves foraging choices. In early primates, these specializations enhanced fitness as they foraged among the terminal branches of rainforest trees in dim light. Like the granular PFC, the medial pulvinar (PulM) and most of the lateral mediodorsal nucleus (MD) are primate specializations. These nuclei, which include the parvocellular division of MD (MDpc), integrate inputs from several cortical areas, including many primate-specific ones. MDpc and PulM expanded during anthropoid evolution, in parallel with the granular PFC; and, during human evolution, MD and the pulvinar became the largest thalamic nuclei as the granular PFC came to dominate the frontal lobe.