Human Brains are Just Bigger But Not Significantly Different From Other Primates

The neural circuits responsible for traits that we consider uniquely human – such as the ability to plan, make complex decisions and speak – could have emerged simply as a natural consequence of the evolution of larger brains, researchers report.

Neuroscientists have shown that differences between primate brains, from tiny marmosets to humans, can be largely explained as consequences of the same genetic program.

In research published in the Journal of Neuroscience, scientists use computer modeling to demonstrate that the substantial enlargement of some areas of the human brain, vital to advanced cognition, reflect a consistent pattern that is seen across primate species of all sizes.

“We have known for a long time that certain areas of the human brain are much larger than one would expect based on how monkey brains are organized,” says Professor Marcello Rosa of Monash University’s School of Biomedical Sciences.

“What no one had realized is that this selective enlargement is part of a trend that has been present since the dawn of primates.”

Using publicly available brain maps, MRI imaging data, and modeling software, the neuroscientists compared the sizes of different brain areas in humans and three monkey species: marmosets, capuchins, and macaques. They found that two regions, the lateral prefrontal cortex and the temporal parietal junction, expand disproportionally to the rest of the brain.

The prefrontal cortex is related to long-term planning, personality expression, decision-making, and behavior modification. The temporal parietal junction is related to self-awareness and self-other distinction.

The findings show that those areas of the brain grew disproportionately in a predictable way, says lead author Tristan Chaplin, from the department of physiology. ”We found that the larger the brain is, the larger these areas get.

“When you go from a small to big monkey—the marmoset to macaque—the prefrontal cortex and temporal parietal junction get larger relative to the rest of the cortex, and we see the same thing again when you compare macaques to humans.”

“This trend argues against the view that specific human mutations gave us these larger areas and advanced cognition and behavior, but are a consequence of what happens in development when you grow a larger brain,” Chaplin says.

Rosa says the pattern holds for primate species that evolved completely separately.

“If you compare the capuchin of South America and the macaque of Asia, their brains are almost identical, although they developed on opposite sides of the world. They both reflect the genetic plan of how a primate brain grows,” says Rosa.

This is the first computational comparative study conducted across several primate species. Chaplin now hopes, in collaboration with zoos, to check if our closest primate relatives, chimpanzees and gorillas, also have brain areas organized as his theory predicts. -- Monash University