A natural history of the human mind: tracing evolutionary changes in brain and cognition

Abstract

Since the last common ancestor shared by modern humans, chimpanzees and bonobos, the lineage leading to Homo sapiens has undergone a substantial change in brain size and organization. As a result, modern humans display striking differences from the living apes in the realm of cognition and linguistic expression. In this article, we review the evolutionary changes that occurred in the descent of Homo sapiens by reconstructing the neural and cognitive traits that would have characterized the last common ancestor and comparing these with the modern human condition. The last common ancestor can be reconstructed to have had a brain of approximately 300-400 g that displayed several unique phylogenetic specializations of development, anatomical organization, and biochemical function. These neuroanatomical substrates contributed to the enhancement of behavioral flexibility and social cognition. With this evolutionary history as precursor, the modern human mind may be conceived as a mosaic of traits inherited from a common ancestry with our close relatives, along with the addition of evolutionary specializations within particular domains. These modern human-specific cognitive and linguistic adaptations appear to be correlated with enlargement of the neocortex and related structures. Accompanying this general neocortical expansion, certain higher-order unimodal and multimodal cortical areas have grown disproportionately relative to primary cortical areas. Anatomical and molecular changes have also been identified that might relate to the greater metabolic demand and enhanced synaptic plasticity of modern human brain's. Finally, the unique brain growth trajectory of modern humans has made a significant contribution to our species' cognitive and linguistic abilities.

Fig. 1

A summary of several neuroanatomical features that are distinctive in the LCA (a) and in modern humans (b). Von Economo neurons are shown as black spindle-shaped cells among pyramidal neurons, represented as triangles. Modified in part from Nimchinsky et al. (1999).

Fig. 2

Allometric scaling plot of brain mass versus body mass in 86 species of primates based on Holloway (1996), with bonobo data from Rilling & Insel (1999), showing the least squares regression line fit to the non-human data (a). Modern humans have brains that are approximately three times larger than would be predicted for a primate of the same body mass. Brain growth trajectories in modern humans and chimpanzees in the first 20 years of life, modified from data in Leigh (2004) (b). The dashed line indicates the end of the first postnatal year.

Fig. 3

A model for the evolution of species-typical, domain-specific skills through interactions among genetically programmed biases in neural pathways. This figure shows one possible set of relationships among perception, sensation, and executive control for illustration; however, other domain-general capacities, such as attention, motivation or arousal, might also be modified in a similar manner. Selection acts on the performance of the organism, manifest as a repertoire of domain-specific skills, via modification of heritable traits encoded in the genome. Such traits might include, for example, adjustment to the graded expression of growth factors, cell adhesion, or regulation of cell cycle dynamics in neural precursors. These genetically determined traits establish biases within networks of connections that guide perception and sensation throughout ontogeny and, therefore, powerfully shape the organism's behavioral development by directing exploration and learning. We envision that multiple tiers of domain-specific skill sets might emerge from shifting such biases early in development.