How did the human brain evolve? A proposal based on new evidence from in vivo brain imaging during attention and...

It is proposed that brain evolution in nonhuman primates and humans was facilitated by heritable differences in neuroplasticity and in the number of neurons and synapses available during childhood and adolescence, therefore, in differences in modifiability and elaboration of neuronal networks in brains of immature primate genotypes. These differences were exploited when a primate population was forced to adapt to a new cognitively or behaviorally demanding milieu, to select more cognitively and brain competent adults who could best compete and reproduce in this new milieu, extending their genes within the population. Two recently solidified concepts suggest a mechanism for this evolutionary process: (1) "Association" neocortex can be activated by attention and ideation in the absence of sensory or motor contributions, as demonstrated by in vivo imaging and direct brain recording. (2) Activation of the immature brain can promote and stabilize neuronal networks that would disappear or otherwise lose their function by adulthood. Taking these two ideas together, it is proposed that the "thought" processes of attention and ideation, when used by immature primates to adapt to new cognitive or behavioral stresses, led by the repeated selection of genotype to more cognitively able, larger-brained species with more extensive "association" cortex and related regions.