Human brain evolution is often considered synonymous with cortical expansion, in particular of the prefrontal cortex, a cortical region required for our remarkable cognitive abilities such as personality expression, planning, and decision making. In this study, we show that the expansion of numbers of cortical neurons in human and nonhuman primate evolution occurred in a similar manner across the cortex, without an increase in the relative number of neurons in the prefrontal region, and without a relative increase in the number of cells in the prefrontal white matter. One thing that distinguishes the human brain from other primate brains is thus not the relative size of its prefrontal cortex but its absolute number of neurons.

Abstract

Human evolution is widely thought to have involved a particular expansion of prefrontal cortex. This popular notion has recently been challenged, although controversies remain. Here we show that the prefrontal region of both human and nonhuman primates holds about 8% of cortical neurons, with no clear difference across humans and other primates in the distribution of cortical neurons or white matter cells along the anteroposterior axis. Further, we find that the volumes of human prefrontal gray and white matter match the expected volumes for the number of neurons in the gray matter and for the number of other cells in the white matter compared with other primate species. These results indicate that prefrontal cortical expansion in human evolution happened along the same allometric trajectory as for other primate species, without modification of the distribution of neurons across its surface or of the volume of the underlying white matter. We thus propose that the most distinctive feature of the human prefrontal cortex is its absolute number of neurons, not its relative volume.