Human brains are amazing, capable of inventing everything from the wheel to New York City. At some point in our evolution, our brains developed our signature sophisticated cognition, and we never looked back. At the same time, not all brains are alike.
Some people are built a bit different, like in the case of autism. Turns out those two facts might be the same fact. There's some evidence to suggest that far from being a recent or purely environmental phenomenon, autism might be so hardcoded into our brains, it's responsible for us becoming smart in the first place.
So, let's dive into what autism has to do with the very existence of the human brain. Real quick disclaimer that autistic people like me are human and don't have to like prove their worth by doing some special brain thing to be deserving of basic dignity. But if autism really did make all of us the way we are, that's pretty interesting.
Like certain neurosychiatric disorders, including schizophrenia, some research suggests that true autism only occurs in humans. There are animal models of autism that are useful in research. For instance, mice are highly social, so scientists can compare so-called deficits in a mouse social skills with those in humans.
But autism is uniquely human in many ways. Like, our social patterns are much more complex and evolve a lot more abstraction than what mice do. Non-human primates show behaviors more akin to human autism than rodents do, but still not the same.
So, only humans seem to experience autism as we define it. But there might be even more to it than that. A 2025 study published in the journal Molecular Biology and Evolution presents some of the most compelling evidence to support the idea that autism is related to the evolution of the human brain.
Essentially, something about the brain changed in the evolutionary time between our last common ancestor with our closest living primate relatives. These changes carved out humanity's cognitive edge. But the study also linked those changes directly to autism.
To understand how, we need to talk about what the researchers looked at and why. It involves a decades old hypothesis that variations in gene expression drove the evolution of the human brain. Each cell in your body has the same genome blueprint in its nucleus.
But different types of cells, like liver cells and brain cells, have different jobs. Each cell only reads certain pieces of the complete genome blueprint that are relevant to its job and carries out those specific instructions. That reading and execution of different specific instructions is what's referred to as gene expression.
And organs aren't homogeneous masses of identical cells. They have different types of tissues that work together to do their jobs. Tissues within organs also vary all the way down to the single cell.
There are differences in gene expression all the way down. The 2025 study honed in on these cellular differences within the brain. The researchers focused on the cerebral neoortex or the outermost portion of the mamalian brain with its distinctive folds.
This folded neoortex is composed of six layers of tissue. These layers are similar from mammal to mammal, but the second and third layers from the outside called L23IT neurons for short have changed a lot when it comes to humans. So what does the complexity of these layers have to do with autism and human thinking?
That is where gene expression comes in. The researchers used data created through a technique called single cell RNA sequencing which separates cells from each other to see exactly what each one is doing. Looking at those cell level differences both within humans and compared with other species provides evolutionary clues.
The researchers compared human L23IT neurons to those of other primates. It turns out that in evolutionary time, enhancements in those layers happened relatively rapidly in the human lineage while they stayed mostly the same in other primates. They also found a simultaneous dramatic change in the expression of several genes that had previously been linked to autism.
In other words, with the rapid changes to these layers in humans came slight genetic variations within our species that have been connected to autistic traits in other research. So, as genes related to autism were seeing changes in expression, at the same time, our brains were evolving their advanced cognitive abilities. But did these changes in gene expression cause those abilities?
And if so, what was the evolutionary benefit? That's still hazy. But before we get into the possibilities, all science needs funding.
So, here's a quick break. This Sihow video is supported by our presidents of science. Thank you to Beor the Drnk Dwarf, Charlie Stanley, and Harry Plumbley for your support of everything we're making here at SciHow.
Because of that support, we can make videos about spice relief, octopus cities, autism, and pretty much any science we think is cool and surprising. And there's a lot of cool and surprising science. If you've heard about science stuff that makes you say, "Wo," tell us about it in the comments.
And if you'd like to join our presidents of science in supporting those stories, you can do that at patreon. com/sihow. Thank you.
It's plausible that a community would benefit from a mix of neurotypes. Suppose some people are charismatic and great in social situations while others have an ability to spot complex patterns or pay extreme attention to certain details. This could have looked like the group leaders taking on the negotiating and trade with strangers, while others gravitate toward roles that suit them, like keeping a close watch on predators or keeping track of the seasons and daylight.
Not that those things are mutually exclusive or unique to autism, of course. It's just that having a mix of tendencies benefits the population, creating a theoretical selective pressure in favor of multiple behavioral patterns. It's as if human cognition is governed by an infinitely complex switchboard, far more intricate and varied than other species switches.
In this model, some switches govern social cognition and others non-social cognition. Differences in the configuration of those switches from person to person look like what we now call neurodeiversity. So, the autism spectrum refers to the kaleidoscope of human neurology.
Sort of like how people can have very similar fingerprints but not identical ones. Basically in social species neurode divergence helped groups overall survival whether or not that neurode divergence looks like human autism per se. It's sort of an evolutionary trade-off.
In this way neurodeiversity could have helped keep groups safe. Again all humans are inherently valuable whether or not the way their brains are led to our unique cognition. Also none of this is to say that autistic people don't face challenges in modern-day society.
This research only establishes a correlation between the emergence of autism and the evolution of the human brain. Future research will dig deeper. If they are causally linked, there could have been additional factors at play, the authors of the study point out.
But in a nutshell, there's a pretty good chance that a fundamental cause of autism may be humanity itself. How's that for the human brain figuring itself out?
