Why do humans seem uniquely prone to autism? A groundbreaking study from Stanford University suggests that the answer lies deep in the wiring of our brains and in the peculiar evolutionary bargains that have made us distinctly human.
Researchers analyzed brain cells across six mammal species and uncovered a striking pattern: the more common a type of neuron, the slower it evolves. This is because altering abundant cells typically harms survival. However, in humans, one of the brain’s most common neurons defies this rule.
The Fast-Evolving Neurons That Define Humanity
These neurons, known as layer 2/3 intratelencephalic neurons, are crucial for the brain’s outer layer. They connect various 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 strikingly, many genes associated with autism spectrum disorder (ASD) showed altered expression. These genes are among those most strongly linked to autism risk.
This phenomenon is not about a single rogue mutation but rather dozens of small changes acting together, subtly altering the baseline wiring of human brains. The researchers argue that natural selection actively favored these changes, despite their trade-offs.
Evolution’s Role in Autism-Linked Genes
This raises a compelling question: why would evolution favor such a path?
One possibility involves timing. Many autism-linked genes are also crucial in brain development. Their reduced activity might have slowed down childhood brain growth in humans. This delay is not necessarily detrimental; it may have provided our species with more time for neurons to wire up complex skills like speech.
The authors of the study 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 shifts that enabled human intelligence might also have made our brains more prone to anomalies, such as neurodivergent conditions like ASD.
Another possibility is the balance of brain signals. Brains rely on a finely tuned ratio of excitatory and inhibitory signals. If evolution disrupted this balance during the expansion of the human cortex, reducing autism-linked gene activity might have helped stabilize the system.
Yet, scientists remain uncertain about the exact advantages these changes conferred. As Starr remarked,
“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 prevalent in humans than in other primates. Both autism and schizophrenia affect the same cortical neurons and exhibit genetic changes clustered in rapidly evolving genomic regions in our lineage.
Globally, autism affects about 1 in 100 children, according to the World Health Organization. In the United States, the rate is closer to 1 in 31, or 3.2%. This is significantly higher than what researchers observe in primates, reinforcing the idea that autism is intricately linked with human-specific brain evolution.
This new research reframes autism not as a defect but as a byproduct of the extraordinary rewiring that made humans what we are. The same cellular shifts that paved the way for language, theory of mind, and creativity also facilitated the emergence of neurodiversity.
While the exact trade-offs faced by our ancestors may remain elusive, the evidence suggests that autism’s prevalence is not an evolutionary accident. Instead, it is embedded in the very process that shaped the human mind.
