Why do humans seem uniquely prone to autism? A new study suggests the answer lies deep in the wiring of our brains, and in the peculiar bargains evolution struck to make us human.

Researchers from Stanford University analyzed brain cells across six mammal species and found a pattern: the more common a type of neuron, the slower it evolves. That’s because tinkering with abundant cells tends to hurt survival. But in humans, one of the brain’s most common neurons broke the rule.

The fast-evolving cells that set humans apart

These cells, called layer 2/3 intratelencephalic neurons, are workhorses of the brain’s outer layer. They connect different parts of the cortex and are essential for language and abstract thought.

According to lead author Alexander L. Starr, “Our results suggest that some of the same genetic changes that make the human brain unique also made humans more neurodiverse.”

The team discovered that these neurons evolved unusually fast in humans compared to chimpanzees and other primates. Even more striking: many genes tied to autism spectrum disorder (ASD) shifted in expression. The genes were among the most strongly linked to autism risk.

This isn’t about one rogue mutation. It’s about dozens of small changes acting together, nudging the baseline wiring of human brains. The researchers argue that natural selection actively favored these changes, even though they came with trade-offs.

Why Evolution Might Have Selected Autism-Linked Genes

That leads to the unsettling question: why would evolution push us down this path?

One possibility is timing. Many autism-linked genes are also involved in brain development. Their reduced activity might have slowed down childhood brain growth in humans. That’s not necessarily bad — in fact, it may have given our species more time for neurons to wire up complex skills like speech.

The authors note that “down-regulation of ASD-linked genes may have increased ASD prevalence by bringing humans closer to a hypothetical ‘ASD expression threshold’ below which ASD characteristics manifest.” In other words, the very shifts that enabled human intelligence may also have made our brains more prone to anomalies such as neurodivergent conditions like ASD.

Another possibility is balance. Brains rely on a fine-tuned ratio of excitatory and inhibitory signals. If evolution pushed that balance out of sync during the expansion of the human cortex, reducing autism-linked gene activity might have helped stabilize things.

Still, scientists can’t say for sure what exact advantage these changes conferred. As Starr put it, the mystery is that “the reason why this conferred fitness benefits to our ancestors is unclear.”

Autism, Schizophrenia, and the Human Condition

The study also hints at overlap with schizophrenia, another disorder far more common in humans than in other primates. Both autism and schizophrenia affect the same cortical neurons, and both show genetic changes clustered in regions of the genome that evolved rapidly in our lineage.

Globally, autism affects about 1 in 100 children, according to the World Health Organization. In the U.S., the rate is closer to 1 in 31, or 3.2%. That’s far higher than what researchers observe in primates, reinforcing the idea that autism is deeply entangled with human-specific brain evolution.

This new research reframes autism not as a bug, but as a byproduct of the extraordinary rewiring that made humans what we are. The same cellular shifts that opened the door to language, theory of mind, and creativity also made the door to neurodiversity easier to walk through.

And while we may never know the exact trade-offs faced by our ancestors, the evidence suggests that autism’s prevalence is not an evolutionary accident. It is, instead, written into the very process that shaped the human mind.

The new findings appeared in Molecular Biology and Evolution.