The Evolution of the Frontal Lobe in Humans

Abstract

In 1912, Korbinian Brodmann suggested that the “regio frontalis” (i.e., the prefrontal cortex) of the human brain was exceptionally large in comparison to other primates. His observations sparked over a century of neuroscientific inquiry into the frontal lobe and the prefrontal cortex in particular. Later work describing the role of the prefrontal cortex in human intelligence drove anthropologists and evolutionary neuroscientists to study its evolution as a means of revealing the evolutionary history of unique cognitive capacities of humans. Here we discuss the results of investigations into the frontal cortex from the perspectives of multiple disciplines: paleoneurology, comparative neuroanatomy, and phylogenetic comparative neuroanatomy. We will describe the different pieces of the puzzle that each of these disciplines contributes to forming a detailed picture of the evolution of the human frontal lobe. We then hone in on phylogenetic comparative approaches in order to investigate changes in frontal lobe scaling across anthropoids. We find that human frontal lobe enlargement is driven specifically by an expansion of the prefrontal cortex, not the frontal motor areas. These results are confirmed by comparisons of regions within the frontal lobe that indicate the human prefrontal cortex has expanded drastically in comparison to frontal motor areas. Furthermore, evolutionary rate analyses reveal that the rate of evolution of the prefrontal cortex size is higher than for the relative sizes of the frontal lobe or the frontal motor cortex. Overall, phylogenetic comparative analyses converge on the observation that different areas of the frontal lobe evolved at different rates of evolution, favoring exceptional enlargement of the prefrontal cortex, but not necessarily the frontal lobe as a whole. These perspectives thus confirm that the human brain is more than a scaled-up version of the monkey brain and that the putative unique expansion of the “regio frontalis” is indeed an important feature that may support human’s unique cognitive abilities.