Recent work has started to reveal the role of natural selection, offering a glimpse at how the genetic underpinning of mental illness has changed over time.
Many psychiatric disorders are polygenic, which means that they can involve hundreds or thousands of genes and DNA mutations, writes Nature. It can be difficult to track how so many genetic regions evolved, and such studies require large genome data sets. However, human genome databases are now enabling researchers to look for possible connections that might have driven their emergence and development. Others are looking to Neanderthal genetic sequences to help inform the picture of these disorders, as well as cognitive abilities, in humans.
One project in particular discovered that evolution selected for DNA variants thought to protect against schizophrenia. The study was led by Barbara Stranger of the University of Chicago in Illinois, who looked at hundreds of thousands of human genomes using a statistical method that identified signals of selection over the past 2,000 years. There were no signs of selection in genetic regions associated with any other mental illness.
Over the course of evolution, it has been suggested by Bernard Crespi, an evolutionary biologist at Simon Fraser University that genes involved in language could malfunction and result in schizophrenia in a small percentage of the population.
In addition, a team led by human geneticist, Renato Polimanti at Yale University in New Haven, Connecticut, is attempting to discover link between environmental factors, mental illnesses and behavioural traits.
Polimanti and his colleagues looked at 2,455 DNA samples from individuals at 23 sites across Europe and quantified each person’s overall genetic risk for mental disorders, such as autism, and personality traits, such as extraversion. They then calculated whether that risk was associatd with certain environmental factors, such as rainfall, winter temperatures or the prevalence of infectious disease, exploring the idea that these factors might have been involved in selection for the human traits.
Their results revealed that people who live in European regions with relatively lower winter temperatures, were slightly more genetically prone to schizophrenia. Polimanti suggests that if genes that helped people tolerate could have been inadvertently carried along during evolution as a “fellow traveller.”
Tony Capra, an evolutionary geneticist at Vanderbilt University in Nashville, Tennessee, Polimanti, explained, “This was a nice first attempt to put some environmental context.” He now plans to repeat the study in other parts of the world.
Trying to make sense of the roles of genetics and the environment can be a tough task, largely due to unknown environmental conditions in the past could have selected for traits that were advantageous then, but considered negative today. Moreover, other evolutionary factors could even contribute to the mental illness indirectly.
Sanger explained that an overactive immune system is thought to be involved in many psychiatric disorders, such as depression, but a stronger immune system would have made human ancestors more resistant to diseases.
Although the study of how mental illness evolved is still at an early stage, the ability to use massive human genome databases is an exciting step forward, concluded Capra. His team plans to task advantage of this by carrying out a survey of genetic areas that differ between Neanderthals and humans, searching for differences in how the genes are expressed.