Could Autism Be a Result of Human Brain Evolution?
Approximately 3.2% of American children are diagnosed with Autism Spectrum Disorder, or ASD. Globally, the World Health Organization estimates that one in 100 children are affected by autism. Scientists have long speculated that conditions like autism and schizophrenia might be unique to the human species. This belief arises from the observation that traits linked to these disorders, such as speech production and comprehension, are primarily found in humans, or more developed in humans than in other species.
The Role of RNA-Sequencing
Thanks to advancements in technology, particularly single cell RNA-sequencing, scientists can now identify specific cell types throughout the brain. As more data is collected and analyzed, it becomes evident that the human brain contains a diverse range of neuronal cell types. Moreover, large-scale sequencing studies have discovered extensive genetic changes in the human brain that are not found in other mammals. These unique genomic elements have remained relatively unchanged throughout mammalian evolution, but have developed rapidly in humans.
Comparing Brain Evolution Across Species
While some cell types have remained more consistent throughout evolution than others, it is still unclear what factors contribute to these differences in evolutionary rate. In a recent study, scientists explored single-nucleus RNA sequencing datasets from three distinct regions of the mammalian brain. They discovered that L2/3 IT neurons, the most common type of neurons found in the outer layer of the brain, evolved much more rapidly in humans than in other apes. Interestingly, this rapid evolution was paired with significant changes in genes associated with autism, suggesting that natural selection specific to the human lineage might have played a role.
The Link Between Autism and Brain Evolution
While the research strongly suggests natural selection favored the evolution of ASD-associated genes, it is not clear why these genes would have offered an advantage to our ancestors. Answering this question is challenging, as we do not fully understand what unique features of cognition, brain anatomy, and neuronal wiring provided a competitive edge to early humans. However, it is speculated that these genes might be linked to developmental delay, thereby contributing to the slower postnatal brain development observed in humans as compared to chimpanzees. This slower development could have been beneficial, as it might have led to more complex thinking.
The Potential Advantage of Autism-Linked Genes
Another possibility is that the rapid evolution of autism-linked genes might have enhanced language capabilities, a trait that is uniquely advanced in humans and often affected by autism and schizophrenia. The extended brain development time in early childhood, potentially caused by these genes, could have been beneficial to human evolution by fostering more complex cognitive abilities. The study's lead researcher stated, "Our results suggest that some of the same genetic changes that make the human brain unique also made humans more neurodiverse."