A detailed comparative analysis of adult human, chimpanzee, and macaque brains shows that all regions of the human brain have molecular signatures very similar to those of our primate relatives, yet some regions contain distinctly human patterns of gene activity that mark the brain’s evolution and may contribute to our cognitive abilities.

“Our brains are three times larger, have many more cells and therefore more processing power than chimpanzee or monkey. Yet there are also distinct small differences between the species in how individual cells function and form connections,” said co-lead author Dr. Andre Sousa, a postdoctoral researcher at the Yale School of Medicine.

To pinpoint differences among primate brains, Dr. Sousa and co-authors evaluated brain tissue samples from six humans, five chimpanzees, and five macaques.

They generated transcriptional profiles of 247 tissue samples in total, representing several different brain regions (hippocampus, amygdala, striatum, mediodorsal nucleus of thalamus, cerebellar cortex, and neocortex).

“We found striking similarities between primate species of gene expression in 16 regions of the brain — even in the prefrontal cortex, the seat of higher order learning that most distinguishes humans from other apes,” they researchers said.

“However, our study showed the one area of the brain with the most human-specific gene expression is the striatum, a region most commonly associated with movement.”

“Distinct differences were also found within regions of the brain, even in the cerebellum, one of the evolutionarily most ancient regions of the brain, and therefore most likely to share similarities across species.”

The team found one gene, ZP2 (zona pellucida glycoprotein 2), was active in only human cerebellum — a surprise, because the same gene had been linked to sperm selection by human ova.

“We have no idea what it is doing there,” said co-lead author Dr. Ying Zhu, also from the Yale School of Medicine.

The authors also focused on one gene, TH (tyrosine hydroxylase), which is involved in the production of dopamine, a neurotransmitter that regulates motor behavior, motivation, pleasure and emotional arousal.

They found that the TH gene was highly expressed in human neocortex and striatum but absent from the neocortex of chimpanzees.

“The neocortical expression of this gene was most likely lost in a common ancestor and reappeared in the human lineage,” Dr. Sousa said.

The team also found higher levels of expression of the gene MET, which is linked to autism spectrum disorder, in the human prefrontal cortex compared to the other primates tested.

The findings appear in today’s issue of the journal Science.

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André M.M. Sousa et al. 2017. Molecular and cellular reorganization of neural circuits in the human lineage. Science 358 (6366): 1027-1032; doi: 10.1126/science.aan3456