- A recent study suggests that the pace of human brain evolution may help explain the prevalence of autism in our species.
- The researchers noted that certain genes tied to autism show reduced expression in humans compared to other animals.
- They propose that autism may be an unintended consequence of the swift development of cognitive abilities in humans.
We often consider ourselves the apex of evolution, but that really doesn’t do the rest of nature justice. After all, we can’t create webs, soar through the skies, breathe underwater, or even swing like many primates.
What we do possess, though, is a remarkably complex brain. This organ allows for intricate language, advanced planning, deep empathy, and rich cultural expressions, among other things.
These advanced neurological traits granted our ancestors significant evolutionary advantages, enabling them to thrive and adapt in various environments across the globe.
However, according to the study’s authors, the intricate wiring of our brains—and how quickly some of that evolved—might explain why autism is relatively common among humans.
Using single-cell RNA sequencing, researchers have identified at least 49 distinct cell types in the mouse brain.
Interestingly, humans share the same types of brain cells as mice, indicating that we don’t possess unique cell types.
This implies, as the authors assert, that the significant differences between human minds and those of other species stem not from unique cells but from the connections between them and how genes are expressed in those cells.
Researchers have long observed that certain proteins evolve at different rates. For example, while some proteins in mice closely resemble human proteins, others diverge significantly.
Investigations into this phenomenon indicate that proteins expressed in larger quantities tend to evolve more slowly. Major changes to those proteins could disrupt essential functions in the body.
In contrast, proteins that are less prevalent in the body have more leeway for change, making them more adaptable during evolution.
The recent study raises the question: could it be that rare brain cell types have more room to evolve, possibly leading to greater cognitive abilities in humans? And might this also shed light on the brain changes associated with autism?
Supporting the authors’ theory, previous research has shown that specific genes related to autism risk are often located in human-accelerated regions (HARs) of the genome.
HARs are areas conserved in other mammals but which evolved rapidly in humans, suggesting their involvement in traits that distinguish us.
This may imply that somewhere in our evolutionary journey since our last common ancestor with chimpanzees, we developed unique neuronal characteristics that elevated our cognitive abilities while also increasing autism risk.
To explore this, the scientists focused on gene expression within cell types, noting that while cells may appear similar across species, their activities can be quite different.
Confirming their initial thoughts, researchers found that more abundant cell types exhibited similar gene expression patterns across several species. Conversely, rarer cell types displayed considerable variations.
They also found that Layer 2/3 intratelencephalic (L2/3 IT) neurons have evolved at an unexpected pace in humans compared to other apes, along with a noticeable down-regulation of autism-associated genes.
A neurologist, who wasn’t involved in the study, highlighted the significance of these neurons, stating they play a critical role in higher-order brain processing. These neurons connect different areas of the cortex and support complex cognitive functions, including social comprehension and language.
The research also points out that issues in these pathways could greatly affect how the brain integrates information, potentially linking to autism.
The study authors believe the rapid evolution of the human brain led to changes that made autism more likely. The neurologist described this idea as intriguing, suggesting that while it’s speculative, it aligns with long-standing theories in neuroscience. It suggests that the unique aspects of human cognition, such as enhanced neural connectivity, might also come with vulnerabilities.
However, he cautioned against equating correlation with causation, noting that this research is still largely theoretical. But he expressed optimism that by studying how these specialized neurons develop and interact, we might unveil new mechanisms related to autism.
This could ultimately lead to targeted interventions, be they medical or behavioral, aimed at enhancing connectivity within the brain.
Overall, the study contributes to the evolving view of autism, suggesting it may be a “neurodevelopmental variation” linked to the very neural systems that support human cognitive abilities.
Another expert in genetics, who did not participate in the study, pointed out that while the findings are interesting, they primarily reiterate existing data from various genome-wide association studies. He critiqued the lack of a clear hypothesis regarding why these evolutionary changes occurred.
The expert emphasized the importance of energy production, as the brain, though only 2% of body weight, consumes about 20% of our energy. He believes that the rapid evolution of the brain heightened energy demands, leading to a possible mismatch during development that could influence conditions like autism.
In his view, disturbances—minor as they may be—in energy production during childhood might tip the scales toward developmental disorders.
This topic is certainly intricate, and ongoing research will likely delve deeper into these questions. It’s a complex challenge to project scientific inquiry into our evolutionary background.
