5 Minutes
Key findings
A new study published in Molecular Biology and Evolution links the relatively high prevalence of autism spectrum disorders to evolutionary changes in the human brain. Using cross-species single-nucleus RNA sequencing datasets, researchers identified rapid genetic and cellular changes in the outer layers of the human cortex. The most striking shifts occurred in a common population of excitatory neurons known as L2/3 intratelencephalic (L2/3 IT) neurons, and many of the genes that changed rapidly in humans are also associated with autism. The authors propose that natural selection on these genes may have produced a trade-off: modifications that supported advanced language and cognition also increased neurodiversity, including susceptibility to autism.
Autism-linked genes evolved rapidly in humans, shaping unique brain development and language abilities. This genetic trade-off may have fueled both neurodiversity and the rise of complex human thought.
Scientific background and methods
Advances in single-cell and single-nucleus RNA sequencing now allow scientists to identify and compare discrete neuronal cell types across species with unprecedented resolution. By analyzing published datasets from three mammalian brain regions, the research team mapped gene-expression differences across cell classes in humans and other apes. They focused on evolutionary rates for specific cell types and on genes previously implicated in autism spectrum disorder (ASD).
L2/3 IT neurons are a predominant excitatory cell type in the outer cortical layers involved in intracortical communication and long-range cortical networks that support language and higher cognition. The investigators found that L2/3 IT neurons show exceptional molecular divergence in the human lineage compared with other apes. Concurrently, ASD-associated genes were disproportionately affected by signals of accelerated evolution and human-lineage natural selection.
Key discoveries and evolutionary interpretation
The study presents three interlinked observations: first, the molecular profile of L2/3 IT neurons changed rapidly in humans; second, genes tied to autism show signatures of accelerated evolution in our lineage; third, those genetic shifts bear marks consistent with positive natural selection. Together, these results suggest that some genetic changes that made the human cortex unique also increased variation in neurodevelopmental outcomes.
The authors caution that direct causal links remain unresolved. It is not yet clear which specific cognitive or anatomical features underwent selection, or precisely how altered gene function produced fitness advantages. However, the paper notes that many autism-associated genes are involved in developmental timing. Human infants and children exhibit extended postnatal brain maturation relative to chimpanzees, and prolonged brain development may have provided a longer window for environmental shaping of neural circuits important for language and abstract thought.
Implications for language, cognition, and neurodiversity
Speech production and comprehension are characteristically human capacities that depend on complex cortical networks. Because autism and related psychiatric conditions frequently affect communication and social cognition, the study raises the possibility that selection for advanced language and flexible cognition inadvertently increased risk for atypical neural development.
These findings do not stigmatize autism as merely a biological cost. Instead, they frame autism as part of normal human variation connected to evolutionary innovations. Clinically, understanding which cell types and molecular pathways changed most may help prioritize targets for research into developmental mechanisms, early diagnosis, and tailored interventions.
Future research and technologies
Follow-up work will need to integrate comparative neuroanatomy, developmental timing studies, functional genomics, and behavioral data. Technologies such as spatial transcriptomics, organoid models, and CRISPR-based functional assays can test how human-specific gene variants alter neuronal development and circuit function. Ethical and social considerations should guide how these insights are communicated and applied, particularly when linking evolutionary hypotheses to contemporary clinical conditions.
Expert Insight
Alexander L. Starr, lead author of the study, summarized the perspective driving the analysis, saying that these genetic shifts could have both enabled advanced language networks and expanded the range of neurodevelopmental trajectories in modern humans. In other words, the same molecular innovations that powered uniquely human cognition may also underlie increased neurodiversity, including autism spectrum conditions.
Dr. Mira Patel, a fictional evolutionary neuroscientist, adds: 'When we examine the cortex at single-cell resolution across species, we start to see how small genetic changes compound into major shifts in circuitry. That provides a plausible route for both the emergence of complex language and the persistence of neurodivergent phenotypes.'
Conclusion
This study supports the idea that autism-linked genes and specific cortical neurons experienced accelerated evolution in the human lineage. The findings are best interpreted as evidence for an evolutionary trade-off: genetic changes that helped shape modern human cognition and language may have simultaneously increased variability in neurodevelopment. Future multidisciplinary work is needed to trace causal mechanisms, evaluate functional consequences, and translate evolutionary insights into ethically informed biomedical research.
Source: sciencedaily
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