Stanford's finding on rapid neuron evolution
Stanford researchers have identified something remarkable about human brain evolution: the most common neurons in our neocortex evolved exceptionally fast compared to other apes, breaking a fundamental rule of neuronal evolution in the process.
The finding, published in Molecular Biology and Evolution, suggests that rapid genetic changes in these neurons simultaneously lowered the activity of hundreds of autism-linked genes, pushing humans closer to a threshold where disruptions trigger autism. Alexander Starr and Hunter Fraser analysed gene expression patterns across over one million neurons from six mammal species.
The neurons in question — layer 2/3 intratelencephalic (IT) neurons — enable communication between different brain regions. They’re the most abundant neuronal type in the human neocortex. And according to evolutionary principles, that abundance should have made them evolve slowly and conservatively.
But, they didn’t. They evolved rapidly. And in doing so, they appear to have pushed humans closer to a neurological tipping point.
The abundance principle gets broken
Starr and Fraser first established what they call the “abundance principle” of neuronal evolution: more abundant brain cell types evolve more slowly than rare ones. The logic is straightforward — changes to common neurons affect more of the brain, making harmful mutations more costly. Natural selection therefore constrains their evolution.
This pattern held consistently across all mammalian comparisons. Except one.
Layer 2/3 IT neurons, despite being the most abundant cortical neuron type, showed unexpectedly rapid evolution specifically on the human lineage. When the researchers examined which genes changed most, they found 233 genes strongly linked to autism showing reduced activity in humans compared to chimpanzees.
The reduction wasn’t subtle. One gene helping brain cells communicate had 2.5 times lower activity in humans than chimpanzees.
Losing just one working copy of this gene causes autism in humans.
With baseline activity already dramatically lower than in chimpanzees, humans may be operating much closer to a breaking point.
233 genes turned down, vulnerability turned up
Think of it like a volume dial. Chimpanzees have it at 10. Losing one speaker (one gene copy) brings them to 5 — still functional. Humans already have the volume at 4, so losing one speaker drops us to 2 – where the signal becomes too weak (for the gene) to function properly.
By lowering this baseline in activity of autism-linked genes, human evolution brought us closer to the threshold where disruptions trigger autism.
The researchers used lab-grown brain tissue containing both human and chimpanzee DNA to determine whether this reduction happened through natural selection or random chance. The results strongly favoured selection: among autism-linked genes that differed between species, 27 out of 32 showed lower activity from the human version. The probability of this occurring randomly was less than 1%.
Several findings ruled out alternative explanations. Autism-linked genes showed no increase in harmful mutations in humans. Activity of these genes was actually less variable among individual humans than among chimpanzees, suggesting stronger evolutionary pressure in our species, not weaker.
What we don't know about why
Here’s where the research gets speculative and the “intelligence trade-off” framing falls apart.
The researchers propose that reducing activity of these genes provided some evolutionary advantage despite increasing autism vulnerability. But they don’t know what that advantage was. They suggest possibilities: slower brain development allowing extended learning periods, enhanced language capacity, or compensatory changes maintaining brain function amid expansion.
None of these possibilities are the same as “intelligence,” and the study provides no evidence connecting these gene changes to any specific cognitive enhancement. The research doesn’t measure intelligence, define it, or demonstrate that autism vulnerability is the “cost” of being smarter.
The trade-off language makes compelling headlines but misrepresents the science. What the study actually shows is that human brain evolution pushed certain gene expression patterns into a range where they’re more sensitive to disruption — and those disruptions are associated with autism. That’s not the same as proving we exchanged increased cognitive advancement for increased risk of autism.
Autism involves hundreds of genetic and environmental factors, not just low activity of these specific genes. And framing autism purely as “vulnerability” ignores the complex cognitive profiles many autistic people have, which include both challenges and genuine strengths.
What this means for understanding autism
The research provides a mechanism for why autism may be more common in humans than other primates. The evolutionary forces that made layer 2/3 IT neurons abundant and functionally important also pushed their gene expression into a range where small additional changes have outsized effects.
The research provides a mechanism for why autism may be more common in humans than other primates. As I confess, I’ve never met a ’tistic chimp. The evolutionary forces that made layer 2/3 IT neurons abundant and functionally important also pushed their gene expression into a range where small additional changes have outsized effects.
Most autism cases don’t result from a single catastrophic mutation but from accumulating many small genetic and environmental factors. By reducing baseline activity of autism-linked genes, human evolution may have lowered the threshold at which accumulated factors trigger autistic traits. Which makes much more sense.
This abundance-evolution relationship holds consistently across the mammalian brain. Analysing three independent datasets covering different brain regions, the team found more abundant cell types consistently showed greater conservation of gene activity between species.
The pattern held across comparisons spanning tens of millions of years — humans, chimpanzees, gorillas, macaques, marmosets, and mice. Only comparisons between humans and non-human great apes showed weaker correlations, likely because of the aforementioned rapid evolution in layer 2/3 IT neurons.
Understanding why these changes occurred — if they provided any advantage at all, or simply happened alongside other beneficial changes — remains unanswered. The same gene expression shifts that made our brains more vulnerable to autism might have supported other developments, or they might have been evolutionarily neutral side effects of changes happening elsewhere.
Evolution optimized for something. We’re still figuring out what that something was, whether it had anything to do with cognition, and whether the increased autism vulnerability was an unavoidable consequence or just an unfortunate coincidence.
