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24 October 2025
The Brain’s Mathematics We Didn’t Know
Researchers at the American university analyzed over a million neurons from three regions of the cerebral cortex in humans, primates, and rodents. This allowed them to discover the pace at which nerve cells evolve. “We hypothesized that a change in gene expression in a more numerous cell type might have stronger negative effects on performance than the same change in a rarer type,” the authors explain. In other words: if a gene mutates in a cell that occurs millions of times in the brain, it’s a problem. If it happens in a sporadically occurring cell, evolution lets it slide. At least, that’s what scientists assumed, but the truth turned out to be different.
Among all the neuron types studied, one stood out in particular: Layer 2/3 Intratelencephalic (L2/3 IT) neurons. This is the most common cell type in our cerebral cortex, responsible for communication between brain areas and crucial for cognitive processing. Due to their large number, they should evolve slowly, yet Starr and Fraser’s research indicates that they underwent a rapid change in humans. “We observed a significantly weaker negative correlation in the human lineage,” the scientists note. L2/3 IT neurons defied previous evolutionary patterns.
Is Autism Susceptibility an Evolutionary Trade-Off?
Even more intriguing was what changed. There are genes whose reduced activity is linked to increased autism susceptibility. The Stanford researchers analyzed 233 such genes collected in the SFARI database. Their activity turned out to be significantly lower in humans than in chimpanzees. To be sure, the researchers also checked DNA segments related to other traits and diseases, but the correlation did not appear there. The more rapid evolution of human brain cells increased our species’ likelihood of autism.
The genes from the SFARI database had reduced activity four times more frequently than the expected results. Further tests conducted on hybrid cells cultured in laboratories ruled out coincidence. The phenomenon was a result of positive selection. Evolution actively favored the downregulation of autism-related genes. Starr and Fraser published the results of their breakthrough research in the journal Molecular Biology and Evolution.
The Price of Human Cognitive Abilities?
Why would evolution push us in a direction that increases the risk of autism? The scientists speak of two scenarios. First: the downregulation of these genes might have slowed brain development after birth, a unique human trait. Second: it might have been compensation for other changes, such as brain enlargement.
“We suggest that the downregulation of autism-related genes in the human lineage increased the probability of autism susceptibility so that small developmental disturbances are sufficient to cause autism traits in humans, but not in chimpanzees,” the researchers conclude.
What would be mere genetic noise in other species becomes a significant risk in humans—but also a potential source of unique abilities. The Stanford scientists’ discoveries shed new light on autism, viewing it not only as a neurodevelopmental disorder but also as an unintended cost of the rapid evolution that made us who we are. It is both unsettling and fascinating: our greatest strength—the brain’s plasticity and complexity—may go hand-in-hand with greater vulnerability to its disorders. Was this the price evolution was willing to pay for the human mind?
Read this article in Polish: Autyzm ceną za ewolucję? Naukowcy mają mocne dowody
