Molecular and cellular reorganization of neural circuits in the human lineage

To better understand the molecular and cellular differences in brain organization between human and nonhuman primates, we performed transcriptome sequencing of 16 regions of adult human, chimpanzee, and macaque brains. Integration with human single-cell transcriptomic data revealed global, regional, and cell-type-specific species expression differences in genes representing distinct functional categories. We validated and further characterized the human specificity of genes enriched in distinct cell types through histological and functional analyses, including rare subpallial-derived interneurons expressing dopamine biosynthesis genes enriched in the human striatum and absent in the nonhuman African ape neocortex. Our integrated analysis of the generated data revealed diverse molecular and cellular features of the phylogenetic reorganization of the human brain across multiple levels, with relevance for brain function and disease.

A neurochemical hypothesis for the origin of hominids

It has always been difficult to account for the evolution of certain human characters such as language, empathy, and altruism via individual reproductive success. However, the striatum, a subcortical region originally thought to be exclusively motor, is now known to contribute to social behaviors and "personality styles" that may link such complexities with natural selection. We here report that the human striatum exhibits a unique neurochemical profile that differs dramatically from those of other primates. The human signature of elevated striatal dopamine, serotonin, and neuropeptide Y, coupled with lowered acetylcholine, systematically favors externally driven behavior and greatly amplifies sensitivity to social cues that promote social conformity, empathy, and altruism. We propose that selection induced an initial form of this profile in early hominids, which increased their affiliative behavior, and that this shift either preceded or accompanied the adoption of bipedality and elimination of the sectorial canine. We further hypothesize that these changes were critical for increased individual fitness and promoted the adoption of social monogamy, which progressively increased cooperation as well as a dependence on tradition-based cultural transmission. These eventually facilitated the acquisition of language by elevating the reproductive advantage afforded those most sensitive to social cues.

Human-specific increase of dopaminergic innervation in a striatal region associated with speech and language: A comparative analysis of the primate basal ganglia

The dopaminergic innervation of the striatum has been implicated in learning processes and in the development of human speech and language. Several lines of evidence suggest that evolutionary changes in dopaminergic afferents of the striatum may be associated with uniquely human cognitive and behavioral abilities, including the association of the human-specific sequence of the FOXP2 gene with decreased dopamine in the dorsomedial striatum of mice. To examine this possibility, we quantified the density of tyrosine hydroxylase-immunoreactive axons as a measure of dopaminergic innervation within five basal ganglia regions in humans, great apes, and New and Old World monkeys. Our results indicate that humans differ from nonhuman primate species in having a significant increase in dopaminergic innervation selectively localized to the medial caudate nucleus. This region of the striatum is highly interconnected, receiving afferents from multiple neocortical regions, and supports behavioral and cognitive flexibility. The medial caudate nucleus also shows hyperactivity in humans lacking a functional FOXP2 allele and exhibits altered dopamine concentrations in humanized Foxp2 mice. Additionally, striatal dopaminergic input was not altered in chimpanzees that used socially learned attention-getting sounds versus those that did not. This evidence indicates that the increase in dopamine innervation of the medial caudate nucleus in humans is a species-typical characteristic not associated with experience-dependent plasticity. The specificity of this increase may be related to the degree of convergence from cortical areas within this region of the striatum and may also be involved in human speech and language. J. Comp. Neurol. 524:2117-2129, 2016. © 2015 Wiley Periodicals, Inc.