A groundbreaking study published in the journal Molecular Biology and Evolution has shed light on the relatively high rate of autism spectrum disorders (ASD) observed in humans. Titled A General Principle of Neuronal Evolution Reveals a Human Accelerated Neuron Type Potentially Underlying the High Prevalence of Autism in Humans, the paper presents compelling evidence suggesting that our evolutionary past significantly influences the prevalence of autism in contemporary populations.
According to recent data, approximately one in 31 (3.2%) children in the United States has been diagnosed with autism spectrum disorder. Globally, the World Health Organization estimates that around one in 100 children are affected by autism. From an evolutionary perspective, many researchers argue that both autism and schizophrenia are conditions that may be unique to humans, with very few documented instances of similar behaviors in non-human primates.
Behaviors linked to autism and schizophrenia typically involve cognitive abilities such as speech production and comprehension, which are either unique to humans or significantly more advanced compared to other species. The advent of single-cell RNA sequencing has revolutionized our understanding of brain structure, allowing scientists to identify specific cell types across various brain regions. This research has revealed a diverse range of neuronal cell types present in the mammalian brain.
Large-scale sequencing studies have also uncovered extensive genetic changes that are unique to Homo sapiens. These genetic components did not change significantly over the course of mammalian evolution, yet they evolved rapidly in humans. While earlier research indicated that some neuronal cell types have remained stable throughout evolution, the factors influencing these variations in evolutionary rates were previously unknown.
In this recent study, researchers analyzed cross-species single-nucleus RNA sequencing datasets from three distinct regions of the mammalian brain. Their findings revealed that the outer-layer brain neurons known as L2/3 IT neurons evolved at an extraordinary pace in the human lineage compared to other apes. This rapid evolution coincided with significant changes in genes associated with autism, likely driven by natural selection specific to human ancestors.
Although the study strongly suggests that natural selection favored genes linked to autism spectrum disorders, the reasons behind these fitness advantages remain elusive. Researchers face challenges in determining which unique aspects of human cognition, brain anatomy, and neuronal wiring provided evolutionary benefits to our ancestors. Speculatively, many of these autism-associated genes are linked to developmental delays, suggesting their evolution may have contributed to the slower postnatal brain development observed in humans compared to chimpanzees.
Moreover, the capacity for speech production and comprehension, which is distinctive to humans, is often impacted by autism and schizophrenia. The accelerated evolution of autism-related genes may have conferred a fitness advantage by extending the duration of brain development during early childhood. This longer developmental period could have facilitated complex thinking and social interaction, both crucial for human evolution.
As lead author Alexander L. Starr noted, the findings suggest that some genetic changes that contribute to the uniqueness of the human brain also enhance our neurodiversity. This research opens new avenues for understanding the intricate relationships between human evolution and neurological conditions, emphasizing the complex interplay of genetics, environment, and cognitive development.
