The evolution of the complex sensory and motor systems of the human brain

Abstract

Inferences about how the complex sensory and motor systems of the human brain evolved are based on the results of comparative studies of brain organization across a range of mammalian species, and evidence from the endocasts of fossil skulls of key extinct species. The endocasts of the skulls of early mammals indicate that they had small brains with little neocortex. Evidence from comparative studies of cortical organization from small-brained mammals of the six major branches of mammalian evolution supports the conclusion that the small neocortex of early mammals was divided into roughly 20-25 cortical areas, including primary and secondary sensory fields. In early primates, vision was the dominant sense, and cortical areas associated with vision in temporal and occipital cortex underwent a significant expansion. Comparative studies indicate that early primates had 10 or more visual areas, and somatosensory areas with expanded representations of the forepaw. Posterior parietal cortex was also expanded, with a caudal half dominated by visual inputs, and a rostral half dominated by somatosensory inputs with outputs to an array of seven or more motor and visuomotor areas of the frontal lobe. Somatosensory areas and posterior parietal cortex became further differentiated in early anthropoid primates. As larger brains evolved in early apes and in our hominin ancestors, the number of cortical areas increased to reach an estimated 200 or so in present day humans, and hemispheric specializations emerged. The large human brain grew primarily by increasing neuron number rather than increasing average neuron size.

Similar patterns of cortical expansion during human development and evolution

“…Directly studying cortical expansion in human evolution would entail comparisons to evolutionary ancestors using the limited information in the fossil record. Important inferences can nonetheless be made through comparative studies with extant nonhuman primates (13,21). Human and macaque cortex differ ≈10-fold in total surface area.…”

“…Since the evolutionary divergence between humans and macaques ∼25 million years ago (11), cortical expansion has been far greater in human lineage than in the macaque lineage. Compared with the macaque cortex, the human cortex has ∼10-fold larger surface area and many more cortical areas distinguishable by various methods (12)(13)(14). Because this evolutionary expansion in cortical surface area is highly nonuniform (15, 16; translation in ref.…”

Comparative Anatomy of Marmoset and Mouse Cortex from Genomic Expression

“…During primate evolution, the cerebral cortex dramatically increased in the size and number of functional areas underlying the acquisition of higher cognitive functions including language, episodic memory, and executive control (Kaas, 2008). However, the genetic mechanism by which the cortex developed new areas remains a question of fundamental importance toward understanding higher brain functions and diseases.…”

Noncoding origins of anthropoid traits and a new null model of transposon functionalization

“…Finally, ASC enrichment near motor coordination and microcephaly genes (UBE3A, MECP2, CDKL5, and others) suggests that gain of new regulatory elements may have contributed to primate motor adaptation and enlargement of the primate motor cortex (Kaas 2008) and also potentially to primate-specific aspects of neurodevelopmental diseases. Overall, ASCs show a strong and consistent trend of enrichment in gene loci with clear links to known anthropoid-specific phenotypes.…”