Human, Ape and Monkey Brain Differences

While the physical differences between humans and non-human primates are pretty distinct, a new study reveals their brains may be remarkably similar. And yet, the most minor changes may make significant differences in developmental and psychiatric disorders.

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Understanding the molecular differences that make the human brain distinct can help researchers study disruptions in its development. A new study, published recently in the journal Science by a team including University of Wisconsin-Madison neuroscience professor Andre Sousa, investigates the differences and similarities of cells in the prefrontal cortex - the frontmost region of the brain, an area that plays a central role in higher cognitive functions - between humans and non-human primates such as chimpanzees, Rhesus macaques, and marmosets.

The cellular differences between these species may illuminate steps in their evolution and how those differences can be implicated in disorders, such as autism and intellectual disabilities, seen in humans. Sousa, who studies the developmental biology of the brain at UW-Madison's Waisman Center, decided to start by studying and categorizing the cells in the prefrontal cortex in partnership with the Yale University lab, where he worked as a postdoctoral researcher.

"We are profiling the dorsolateral prefrontal cortex because it is exciting. This cortical area only exists in primates. It doesn't exist in other species," Sousa says. "It has been associated with several relevant functions in terms of high cognition, like working memory. It has also been implicated in several neuropsychiatric disorders. So, we decided to study what is unique about humans in this brain region."

Sousa and his lab collected genetic information from more than 600,000 prefrontal cortex cells from human tissue samples, chimpanzees, macaques, and marmosets. They analyzed that data to categorize the cells into types and determine the differences in similar cells across species. Unsurprisingly, the vast majority of the cells were fairly comparable.

"Most of the cells are very similar because these species are relatively close evolutionarily," Sousa says.

Sousa and his collaborators found five cell types in the prefrontal cortex that were not present in all four species. They also found differences in the abundances of certain cell types and diversity among similar cell populations across species. When comparing a chimpanzee to a human, the differences seem huge - from their physical appearances down to the capabilities of their brains. But at the cellular and genetic level, at least in the prefrontal cortex, the similarities are many, and the dissimilarities are sparing.

"Our lab wants to know what is unique about the human brain. From this study and our previous work, most of it is the same, at least among primates," Sousa says.

The slight differences the researchers found may be the beginning of determining some of those unique factors, and that information could lead to revelations about development and developmental disorders at a molecular level.

"We want to know what happened after the evolutionary split between humans and other primates," Sousa says. "The idea is you have a mutation in a gene or several genes, and those genes now have slightly different functions. But if these genes are relevant for brain development, for example, how many of a certain cell is produced, or how cells connect to other cells, how does it affect the neuronal circuitry and their physiological properties? We want to understand how these differences lead to differences in the brain and then to differences we can observe in adults."

The study's observations were made in the brains of adults after much of the development was complete. This means that the differences may occur during the brain's development. So, the researchers' next step is to study samples from developing brains and extend their area of investigation past the prefrontal cortex to find where and when these differences potentially originate. The hope is that this information will lead to a more robust foundation to lay developmental disorder research on top of.

"We can do extraordinary things, right? We are studying life itself, the universe, and so much more. And this is unique when you look around," says Sousa, whose team included graduate students Ryan Risgaards and Zachary Gomez-Sanchez, research intern Danielle Schmidt, and undergraduate students Ashwin Debnath and Cade Hottman. "If we have these unique abilities, it has to be something in the brain, right? There is something in the brain that allows us to do all that, and we are really interested in knowing what it is."

Attribution/Source(s): This peer reviewed publication was selected for publishing by the editors of Disabled World (DW) due to its relevance to the disability community. Originally authored by University of Wisconsin-Madison and published on 2022/11/08, this content may have been edited for style, clarity, or brevity.