A study carried out by two American specialists in biology brings a new hypothesis in the case of the origin of autism in the human species. They suspect that autism spectrum disorders are a consequence of the accelerated evolution of human brain cells compared to other species.
Autism is a disorder that affects more and more children PHOTO Shutterstock
Autism spectrum disorders are a neurobiological condition that affects brain development, influencing communication, social interaction and behavior.
The spectrum is wide and symptoms can range from mild to severe. There is no cure, but early intervention with therapy can significantly improve quality of life and have very good outcomes. The incidence of autism has increased considerably in recent years. According to the US Centers for Disease Control and Prevention, the incidence of autism has increased from 1 in 150 children in 2000 to 1 in 36 today. That is precisely why the interest of scientists is increasing in discovering the causes of autism in human beings. A new study conducted by American biologists Hunter B Fraser and Alexander L Starr, from Stanford University, shows that it is possible that autism is a consequence of the extremely accelerated evolution of the human brain, especially the brain cells.
Similar brain structures, but a much higher rate of evolution
The cognitive capacity of the human brain is unique in the animal kingdom. Although we are anatomically and cell type-wise very similar to the rest of mammals, our neurological capacity has experienced accelerated evolution and managed to surpass in complexity that of other animals. This is how we came to have a unique, powerful and complex brain.
This led to an evolved mode of expression and language, detailed planning, deep empathy and vibrant culture. Our neurological capacity undoubtedly provided evolutionary benefits to our ancient ancestors. It has allowed us to spread around the world and adapt to all environments. Perhaps surprisingly, the human brain does not have brain cell types unique to us. We use the same collection of cell types as a rodent. This, the authors of the new study infer, means that the incredible difference between the human mind and other minds may not be due to specialized cells, but rather to the ways in which they are connected and the levels of gene expression in each cell. Scientists have noticed over time that some proteins in cells (they are the internal structural elements of cells) evolve and change much faster than others.
Specialists have carried out complex studies to understand what factors influence the conservation of one protein over millennia or the modification of others as species evolve. The ones that evolve the hardest, that is, are more conservative, are those proteins that are found in abundance in the human body, so as not to disturb the body’s functions or metabolic reactions. During all this time, the rare proteins have more freedom to change, to evolve quickly, because the impact of their changes is smaller on the whole organism.
The two biologists from Stanford wondered in their study, entitled “A General Principle of Neuronal Evolution Reveals a Human-Accelerated Neuron Type Potentially Underlying the High Prevalence of Autism in Humans”, whether the same behavior or the same rule can be applied to the different types of cells in the body. That is, if it is possible that the rarest types of brain cells have the greatest freedom to evolve and, in the case of our species, this evolutionary freedom has led to exaggerated cognitive power.
Autism, a price of the accelerated evolution of the human brain?
Specialists issued a research hypothesis. Is this exacerbated, rapid development related to brain changes related to autism spectrum disorders? Previous research has shown that certain genes involved in susceptibility to autism are often found in so-called accelerated regions of the human genome (Human Accelerated Regions-HAR). These accelerated regions in humans are sections of the genome that are well conserved in other mammals, meaning they evolved slowly, but which in humans evolved very quickly, accelerated. For example, this is what set us apart from other primates. In chimpanzees the rhythm was slower, conservative, in humans it was fast, with neural changes specific to our cognitive ability, but which could also increase the probability of autism. Should we be the victims of our own evolutionary speed, including in recent centuries?
The hypothesis was tested by the group of researchers on six categories of mammals and was confirmed. The more abundant a cell type was, the more genetically similar it was among mammalian species. In contrast, rare cells showed large differences, at the genetic level, between the six mammalian species compared. “L2/3 IT neurons evolved unexpectedly rapidly in the human lineage compared to other monkeys”. The researchers also observed a disproportionate downregulation of genes associated with autism.
Other specialists, who did not participate in the study but have read its results, argue that these neurons that the Stanford biologists are talking about, IT L2/3, are very important in abstract reasoning, how we interact with others and in language. So the problems that may arise in their way of functioning would affect our cognitive ability. “It could have significant effects on how the brain coordinates information, which could be linked to autism”says neurologist Luke Barr for Medical News Today. In conclusion, the authors of the said study believe that as the human brain evolved rapidly, changes occurred that made autism more possible in humans.
