The human brain: rewired and running hot

Abstract

The past two decades have witnessed tremendous advances in noninvasive and postmortem neuroscientific techniques, advances that have made it possible, for the first time, to compare in detail the organization of the human brain to that of other primates. Studies comparing humans to chimpanzees and other great apes reveal that human brain evolution was not merely a matter of enlargement, but involved changes at all levels of organization that have been examined. These include the cellular and laminar organization of cortical areas; the higher order organization of the cortex, as reflected in the expansion of association cortex (in absolute terms, as well as relative to primary areas); the distribution of long-distance cortical connections; and hemispheric asymmetry. Additionally, genetic differences between humans and other primates have proven to be more extensive than previously thought, raising the possibility that human brain evolution involved significant modifications of neurophysiology and cerebral energy metabolism.

Figure 1

Summary of results from a DTI study by Rilling et al. [34] comparing the organization of the arcuate fasciculus, a white-matter bundle conveying fibers between the frontal lobe and posterior cortex, in humans (Homo sapiens) and chimpanzees (Pan troglodytes). In both species, the arcuate fasciculus (AF) carries fibers between frontal language cortex, including areas 44, 45, and 47 (Broca’s area), and posterior language cortex, including area 22 (Wernicke’s area) and the inferior parietal lobule (areas 40 and 39). In humans, however, the AF carries fibers from middle temporal cortex (area 21) that represent word meanings. Fibers also pass between the temporal and frontal lobes via a ventral pathway (V), which is relatively prominent in chimpanzees and macaques (not shown).