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Study reveals that genetic changes linked to autism and schizophrenia may have been favored by natural selection.
The same factors driving these changes may have contributed to the complexity of the human brain.
The conditions themselves don't offer evolutionary advantages but highlight trade-offs in brain evolution.
Certain genetic changes linked to autism spectrum disorders and schizophrenia may have been favoured by natural selection during human evolution, according to a new study. The findings offer fresh insight into how the human brain became so complex.
Published in the journal Molecular Biology and Evolution, the research showed that the evolutionary processes that shaped advanced human cognition may also have increased vulnerability to certain neurodevelopmental conditions.
The findings, however, do not imply that autism or schizophrenia themselves provided an evolutionary advantage, the scientists clarified. Instead, the study points to trade-offs that emerged as the human brain evolved. “Whatever it is that led to the evolution of powerful, complex human brains may have led as well to autism and other neurological disorders,” said Hunter Fraser, senior author of the paper and professor of biology at Stanford University.
Fraser and his graduate student, Alex Starr, originally set out to understand why some brain cells evolved more rapidly than others. They drew inspiration from protein evolution patterns known: Abundant cell types evolve slower than rare ones to protect organisms from catastrophic mutations overall.
To test this, the researchers focused on the neocortex, the outer layer of the mammalian brain responsible for higher functions such as language, perception and reasoning. Using existing datasets, they compared gene expression patterns in individual neocortical cells across species, including mice, chimpanzees and humans.
Their results largely supported the hypothesis: Common cell types showed less variation across species, suggesting slower evolution. But there was a striking exception unique to humans. A type of neuron known as layer 2/3 intratelencephalic excitatory neurons, which are abundant and crucial for communication between different parts of the neocortex, showed unusually rapid changes in gene expression during human evolution.
Crucially, the researchers found that around 100 genes believed to protect against autism were expressed at lower levels in these neurons in humans compared with chimpanzees. Similar patterns were observed for genes linked to protection against schizophrenia.
Further genetic and statistical analyses, including experiments on human and chimpanzee brain organoids, supported the idea that these changes were driven by evolutionary pressures.
Why these changes proved advantageous remain unclear to the scientists. One possibility, Starr said, is that they are linked to the slower development of the human brain, which is thought to allow for greater flexibility and computational power. The same genetic shifts may also have increased the risk of neurodevelopmental disorders.
“Neurodiversity could be an essential part of being human,” Starr said, suggesting that variation in brain development may be inseparable from the evolutionary path that shaped modern humans.