Human brains teach us a surprising lesson

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Science  07 Oct 2016:
Vol. 354, Issue 6308, pp. 38-39
DOI: 10.1126/science.aai9379

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The unique cognitive abilities of humans have long captured the imagination of philosophers and neuroscientists alike. But which features of the human brain set us apart from other mammals? Most likely, our intellectual advantages result from a relative expansion of cortical regions responsible for associative and executive function (1). However, the question of how this cortical expansion is achieved during human development has remained unresolved. A major clue came from the elucidation of neurogenic events taking place during the later phases of embryonic human brain development. This began with the recognition that the human cortical subventricular zone is greatly expanded relative to that of lower mammals. This evolutionary innovation allowed for the marked expansion of associative cortex, especially the frontal lobes. Subsequently, a thorough investigation of the fetal human cortex revealed the existence of a number of distinct excitatory neuronal progenitor types (e.g., outer radial glia) that were identified as key to driving a remarkable burst of late neurogenesis (2). However, the cortex is able to function only when excitatory and inhibitory activities in the brain are balanced. On page 81 of this issue, Paredes et al. (3) identify a population of interneurons that migrate to the cortex during infancy to establish inhibitory circuits.