The ASPM gene controls the size of the cerebral cortex, and a consequence of inheriting only one variant of this gene is a small brain and mental disability. Differences in this gene between humans and chimps indicates it evolved rapidly at some point in our past, but not in that of chimps. 

Villanova University

Humans and chimpanzees, who share nearly 99% of DNA, still differ in a plethora of physical, behavioral and cognitive ways- this may be because we differ in the specific sequences of our DNA, as well as in how genes express themselves in us. 

Gene Expression

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Differences in DNA Sequences 

Although humans and chimpanzees are remarkably similar in gene expression when comparing samples from our blood cells and livers, even more so than chimps are to other monkeys, when comparing samples from our brains, chimpanzees have much more in common with other monkeys than with us. This suggests that, at some point in our past, gene expression in human brains rapidly evolved, perhaps due to selection pressures. 

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