Scientists may have figured out why humans are so much smarter than other primates, after disproving one of the leading theories regarding the evolution of our intelligence. Until now, it had been widely believed that a brain region called the prefrontal cortex – which is associated with cognitive ability – had become enlarged in humans, occupying a greater proportion of the total brain than in primates. However, new research reveals that the prefrontal cortex is in fact proportionally equal in both human and primate brains.

As a result, the study authors claim that humanity’s superior intelligence is not the product of an evolutionary expansion of the prefrontal cortex. Instead, they suggest that the reason for our amazing smarts may be much simpler, and is probably explained by the fact that our brains are just bigger than those of other primates, meaning we have a greater absolute number of neurons in our prefrontal cortex, even though the relative number of neurons is not increased.

To reach this conclusion, the researchers used a technique called the isotropic fractionator to determine the number of neurons in sections of the prefrontal of cortex of human brains, comparing these to the brains of seven other primates, including capuchin monkeys, baboons, and macaques. Reporting their results in the Proceedings of the National Academy of Sciences, the study authors found that in all brains involved in the study, the prefrontal cortex contained 8 percent of the total number of cortical neurons.

As such, the researchers conclude that the relative allocations of neurons to cognitive functions is the same in humans and non-human primates.

^{The prefrontal cortex contains the same number of neurons relative to the rest of the brain in both humans and non-human primates. Alila Medical Media/Shutterstock}

The volume of grey matter, which contains the cell bodies of neurons, and white matter, which contains the connecting branches of neurons – known as axons – were also proportionally the same in the prefrontal cortex of all brains under investigation.

All of this points to the fact that human evolution did not involve a shift in the distribution of neurons towards the prefrontal cortex. As such, the study authors describe the human prefrontal cortex as “nonextraordinary” in comparison to that of other primates.

In fact, the only difference between the human and non-human primate prefrontal cortex is that, since human brains are larger overall, with a greater number of total neurons, this also applies to this particular region of the brain. In other words, humans are smarter than monkeys because we simply have more neurons than them, not because these neurons are distributed in any unique way.