Evolution of the modern human brain.

When compared to the brains of our closest living relatives, chimpanzees and bonobos, the brains of modern humans are larger and differently shaped. This chapter reviews what we know about the evolutionary history of these differences. We can make an educated guess about the size and shape of the brains of the hypothetical common ancestor of modern humans and chimpanzees/bonobos, but between ca. 8 million years ago and the present day evidence about the size and shape of the brain comes from either natural endocasts, which are literally brain-shaped rocks, or from individuals for which enough of the brain case is preserved to provide estimates of endocranial volume and/or the relative proportions of the different regions of the cerebral hemispheres and the cerebellum. The tempo and mode of brain size increase in the hominin clade has been the subject of spirited debate, but we suggest that some of this controversy is the combination of an overreliance on frequentist statistical tests and researchers addressing these issues at different taxonomic scales. The existence and significance of shape changes are also controversial topics, made more so by the dearth of reliable evidence.