Brains

Advances in noninvasive neuroimaging and comparative genomics over the past two decades have yielded a remarkable body of new knowledge about the differences in brain organization between humans, apes, and other primates. Although human specializations have been identified in many different brain systems, modifications of the classical association cortical regions—the prefrontal cortex, posterior parietal cortex, and the middle and inferior temporal cortex—are especially pertinent to the evolution of human cognitive capacities. Recent studies support the traditional view that the evolutionary enlargement of the human brain, which is more than 3X as large as those of African apes of similar body size, reflects primarily the expansion of association cortex. While it is currently unclear whether humans possess cortical areas that apes lack, it is clear that evolution repurposed certain areas shared by humans and nonhuman primates to serve new, human-specific functions. The best studied example of this is the inferior frontal gyrus (Broca’s area), modified for language in the left hemisphere and for complex, hierarchically organized action planning and execution (as for example in making tools) in the right hemisphere. These functional changes were accompanied by, and presumably supported by, changes in the connections between the inferior temporal and parietal association areas, as demonstrated by diffusion-weighted MRI studies.

Human association areas also appear to be hotspots for changes in gene expression. Comparative genomic research has identified non-coding DNA regions that regulate gene expression and underwent rapid changes in human evolution (“human accelerated regions,” or HARs). Genes associated with HARs show more expression changes in human association cortex than do the homologous genes of chimpanzees and macaque monkeys. Many of these genes regulate synaptic formation and could thus shape the intrinsic information-processing architecture of association areas, and mutations of HAR-associated genes are known to disrupt cognition and social behavior.

Like gray matter, the white matter, which carries fibers traveling between cortical areas, is known to undergo plastic modifications in response to learning and experience. Compared to the chimpanzee lineage, the human lineage underwent many more changes in gene expression by oligodendrocytes, the cells that make the myelin sheaths that invest fibers in white matter. What’s more, a subset of these expression-modified genes has alleles associated with schizophrenia and other neurocognitive disorders.

One of the truly exciting aspects of this new era of discovery is the accessibility of the data. MRI scans are publicly available for chimpanzees through the National Chimpanzee Brain Resource and for humans from a variety of sources. Gene-expression data for chimpanzees are available online through PsychENCODE and for humans from the Allen Human Brain Atlas. We can expect these datasets to provide new insights into human brain specializations and human disease. Go for it.