Autism is broadly considered to be a multi-factorial condition that results from genetic as well as non-genetic risk factors, with their majority being genetic. Considering the social and emotional implications of autism for the affected individual and their environment, a question inevitably comes to mind: what is the origin and evolutionary importance of autism?
Genetics
In a recent large study on population data from 5 countries that involved more than 2 million individuals, Dan Bai et al show us that the heritability of autism reaches approximately 80% of the cases. It was still not clear after their study, though, how exactly autism genes and environmental factors including high levels of fetal testosterone affect the risk of autism.
There have been approximately thirty genes identified that have been so far linked to autism. For example, a recent gene-set analysis revealed a significant association of ‘rigidity’ as a personality trait with known ‘autism’ genes. These genes are encoding proteins that interact within a molecular network and eventually regulate the function of neurons. This specific study provided evidence of a direct biological link between ASD and a specific autistic trait.
Imaging findings
Imaging studies of autistic individuals have found a few differences in the development of several regions of the brain, including the hippocampus, the cerebellum and the cortex. Resting‐state functional magnetic resonance imaging scans were used to compare the neuron connectivity in autistic individuals’ brains to the respective neurotypicals’. The findings revealed increased indirect functional connectivity in the autistic brain that was indicative of reduced functional network integration. That consisted mostly of under-connectivity between distant brain regions and over- connectivity between adjacent regions compared to neurotypical brains.
Molecular mechanisms
The molecular mechanisms underlying autism have been studied multiple times but unfortunately they are still not well understood. On a biochemical basis, it was observed that several brain neurotransmitters, e.g., dopamine, serotonin , GABA, acetylcholine and histamine participate in the onset and progression of ASD. Lately, evidence for a possible cause-effect relationship between ASD and gut microbiota has also come to light, although it is not yet well established.
Important mutations
The increased heritability of autism in combination with some of its traits that often pose a social challenge to autistic individuals, makes one wonder what the evolutionary advantage of autism was in the first place and why it hasn’t been eliminated by natural selection. What makes this notion even more interesting is the latest discovery that small regions of the human DNA that mark where humans have diverged from chimpanzees, actually contain a variety of mutations implicated in autism and other neurodevelopmental disorders. These mutations can give us information about the sorts of changes that led humans having a different brain than other primates, but also pose a question on how these changes are linked to neurodevelopmental disorders.
Origin of autism theories
Despite the latest scientific findings, there is still a lot of information missing regarding the origins and evolutionary pathways of autism. There are however a few theories that have so far been used in order to explain the evolutionary meaning of autism. These include the following:
1.The extreme male brain theory by Simon Baron-Cohen according to which the male brain is better developed to systemise than empathise, while the female brain is defined as the opposite cognitive profile. According to Cohen this leads him to conclude that autism can be the result of an extreme male brain profile.
2.The extremely imprinted brain theory by Badcock et al. According to this theory autism is the result of imbalances in brain development caused by the expression of imprinted genes (genes that violate the usual rule of inheritance since their expression is determined by the parent that contributed them). In particular, autism is thought to be caused by imbalances that involve increased genetic, neurological and behavioural effects of the paternal brain in relation to the maternal brain.
3.The reptile brain theory or Polyvagal theory according to which the different variations of vagus (10th cranial) nerve, by regulating the automomous nervous system , affect emotions and social behavior in different ways. The three stages of autonomic nervous system phylogeny reflect the emergence of three distinct parts each with a different behavioral function. In the first evolutionary stage, the unmyelinated vagus nerve appeared. This would regulate immobilisation for faking death and passive avoidance. These are typical responses to danger in reptiles, but atypical in mammals like humans. In the second stage, the sympathic- adrenal system emerged, which is characterised by mobilisation as a response to danger. In the third stage, the myelinated vagus emerged, which is involved in social communication, self-soothing and calming, behaviours observed in mammals. This theory suggests that autistic individuals present a limited expression of the mammalian response. Rather, they exhibit fight or flight responses that include mobilisation and immobilisation, while their nervous system is in a constant state of hypervigilance or shutdown.
4.The low fitness extreme theory suggests that autism is a sort of low-fitness extreme indicator of mental abilities. The ability of very young children to charm their parents evolved as a parentally selected fitness indicator and autistic children fail to do so as an expression of parent low fitness for reproduction.
Many genes, few answers
None of the above theories have yet been proved or disproved. There are suggestions, though, that a more holistic approach involving all theories might be closer to the truth. Autism is caused by 30 or more interacting genes. An evolutionary theory of autism should take into account the developmental effects of both its polygenic nature and of interactions among the genes. Although the altered social and emotional behaviour of autistic individuals suggest a lower reproductive success, autistic genes are still present if not more prevalent than in the past in our gene pool.
One explanation according to Annemie Ploeger and Frietson Galis is that the genes underlying autism are also involved in traits that are beneficial for the autistic individual. For example, it is possible that genes involved in the development of autism are also involved in the development of intelligence and exceptional skills. As these are traits positively correlated with reproductive success, this 30 genes that are involved in autism could potentially have spread in the population thanks to this link.
There is still so much to find out about the causes of autism, the genes that are involved in it and the importance of its role in human evolution. Current and future studies involving new technologies are hopefully going to shade some more light on this intriguing neurological presentation. After all, autism, among other neurodevelopmental variations, has a lot to teach us about who we are, where we came from and why we humans ended up displaying and integrating such complex social behaviours and structures.
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