Psychiatric disorders can be debilitating and often involve a genetic element, yet, evolution hasn’t weeded them out. Currently, recent work is beginning to show the role of natural selection — offering a peek at how the genetic underpinnings of mental illness has changed over time.
Many psychiatric disorders are polygenic: they can involve hundreds or thousands of genes and DNA mutations. It can be difficult to track how so many genetic regions evolved, and such studies require large genome datasets. But the advent of massive human genome databases is enabling researchers to look for possible connections between mental illnesses and the environmental and societal conditions 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. Several of these teams presented their findings at the American Society of Human Genetics (ASHG) meeting in Orlando, Florida, in late October.
One project found that evolution selected for DNA variants thought to protect against schizophrenia. The study, led by population geneticist Barbara Stranger of the University of Chicago in Illinois, looked at hundreds of thousands of human genomes using a statistical method that identified signals of selection over the past 2,000 years1. There were no signs of selection in genetic regions associated with any other mental illness.
Many of schizophrenia’s symptoms, such as auditory hallucinations and jumbling sentences, involve brain regions tied to speech, says Bernard Crespi, an evolutionary biologist at Simon Fraser University in Burnaby, Canada. Over the course of hominid evolution, he says, the ability to speak could have outweighed the small, but unavoidable risk that the genes involved in language could malfunction and result in schizophrenia in a small percentage of the population.