The Evolution of the Human Brain in Regions, Cells, and Genes

Many of the earliest and most persistent questions in biological anthropology regard the evolutionary mechanisms responsible for the massive size increase of the human brain. Humans have the largest brain relative to body size of any vertebrate. The most pronounced changes to the human brain occurred in the cerebral cortex, which accounts for the majority of the volume of our brains. Regional size comparisons of the cerebral cortex between humans and extant primates reveal an interesting story about how our brains have changed. Cortical regions that process basic sensory andmotor information from the thalamus (e.g. primary visual cortex and primary motor cortex) are often smaller than expected inhumans based on our total brain size. However, regions of our brains that participate in human-specific behaviors such as language and complex social interactions are significantly expanded relative to other primates. These regions, collectively known as association cortex, integrate information from multiple brain regions and are crucial ingenerating the massive degree of behavioral diversity present across the human world. Recent advances in the fields of genetics and developmental biology provide a window into the biological mechanisms responsible for producing these changes to the human brain. In some cases, we have been able to link specific genetic changes to alterations inthe developmental processes in the generation of the human brain. These processes account for anatomical changes that vary in scale from the overall size of the brain, to the connections between cortical regions, and even to the structure of individual neurons.