Evolution of the hominin brain: A review of variations in size and shape

If we aim to understand the acquisition of human cognitive organization during hominin evolution, two fundamental and complementary lines of evidence must be taken into account: the first is paleoneurological, based on the analyses of brain "endocasts" (endocranial casts) of our evolutionary ancestors' fossils, namely casts made of the neurocranium interior of hominin skulls; the second is neurological, based on comparative neurological data on living primates, assuming that changes occurred from an ape-like forerunner, whose neuroanatomical relationships were somewhat similar to those of extant chimpanzees and gorillas, the closest living evolutionary relatives of present-day humans. In this article, we summarize how advances in the study of hominin endocasts, together with comparative neurological data presently available on the cerebral neocortex and cerebellum of modern humans and extant apes, impact on our understanding of the nature and timing of hominin brain evolution. The major external phenotypic effect that can be observed from the hominin fossil record is an increase in brain size, but the evolutionary development of the hominin brain must have actually been much more complex. The paleoneurological and comparative neuroanatomical evidence suggests that brain size and neurological reorganization were both important variables in the course of hominin brain evolution. During the last three million years, there have occurred allometric and non-allometric brain size increases, interspersed (or interdigitated) with critical reorganizational changes.