The differences between the brains of primates — humans, apes and monkeys — are small but significant

While the physiological differences between humans and non-human primates are very pronounced, a new study suggests that their brains may be very similar
.
However, the smallest changes can make a huge difference
in developmental and psychiatric disorders.

Understanding the molecular differences that set the human brain apart could help researchers study disturbances
during its development.
A new study recently published in the journal Science by Andre Sousa, a professor of neuroscience at the University of Wisconsin-Madison, looked at similarities and differences
between prefrontal cortex cells in humans and non-human primates such as chimpanzees, rhesus monkeys and velvet monkeys.
The prefrontal cortex is the frontmost region of the brain and plays a central role
in higher cognitive function.

Cellular differences between these species may shed light on the steps in which they evolved, and how these differences are related to
diseases such as autism and intellectual disability that appear in humans.
Sousa, who studies the developmental biology of the brain at the Waisman Center at the University of Wisconsin-Madison, decided to collaborate with the Yale lab, starting with studying and classifying cells in the prefrontal cortex, where he worked as a postdoctoral researcher
.

"We are analyzing the dorsolateral side of the prefrontal cortex because it is particularly interesting
.
This cortical area is only found in
primates.
It doesn't exist in other species," Sousa said
.
"It is associated with several related functions in high cognition, such as working memory
.
It has also been linked to
several neuropsychiatric disorders.
So, we decided to do this study to understand what makes humans unique in
this brain region.
”

Sousa and his lab collected genetic information
on more than 600,000 prefrontal cortical cells from tissue samples from humans, chimpanzees, macaques and velvet monkeys.
They analyzed the data, classified the cells, and identified differences
in similar cells in different species.
Unsurprisingly, the vast majority of cells are fairly similar
.

"Most cells are actually very similar because these species are relatively close in evolution," Sousa said
.

Sousa and his collaborators found five cell types in the prefrontal cortex that none of the four species had
.
They also found differences
in the abundance of certain cell types in different species and the diversity of similar cell populations.
When comparing chimpanzees to humans, the differences seem enormous
, from their appearance to the capabilities of their brains.
But at the cellular and genetic level, at least in the prefrontal cortex, there are many similarities and few
differences.

"Our lab really wants to know what
makes the human brain unique.
Obviously, from this study and our previous work, at least in primates, most of it is actually the same," Sousa said
.

The subtle differences discovered by the researchers could be the beginning of identifying some of these unique factors that could lead to the revelation of development and developmental disorders
at the molecular level.

"We wanted to know what happened after the evolutionary split of humans and other primates," Sousa said
.
"The idea is that you have a mutation in one gene or several genes that now have slightly different functions
.
But if these genes are related to brain development, for example, how many specific cells are produced, or how the cells connect to other cells, how does it affect neural circuits and their physiological characteristics? We wanted to understand how these differences lead to differences in the brain, which in turn lead to the differences
we observe in adults.
”

The observations for this study were made
after the completion of adult brain development.
This means that these differences can occur during
brain development.
So the researchers' next step is to study developing brain samples and expand their investigation beyond the prefrontal cortex to potentially discover when and where
these differences occur.
It is hoped that this information will provide a more solid basis
for research on developmental disorders.

"We can do extraordinary things, right?" We are studying life itself, the universe, and more
.
When you look around, it's really unique," Sousa said
.
"If we have these unique abilities, it must be something in the brain, right? There's something in the brain that allows us to do all these things, and we really want to know what it is
.
”