The evolution of mental health disorders

Written by Abigail Sawyer

Mental health disorders can be debilitating and agonising, and they are becoming ever more prominent factors of human health. Scientists are now just beginning to unravel how these disorders could have been the trade-off for rapid and impressive human evolution.

Mental health disorders in the human brain

In recent years, scientists have been looking to the genetic underpinnings of mental health disorders in order to discover more about their origins and development. As mental health disorders can involve potentially thousands of different genes and DNA mutations, the advent of human genome databases is the driving factor enabling more of this kind of research.

Among this research are some key areas of insight: scientists have been looking for indications of the environmental conditions that could push forward the emergence of mental health disorders; they have begun comparing Neanderthal and human genomes and are now looking into how increased cognitive abilities in humans could have led to mental health disorders as a side effect.

Selection for schizophrenia

One study looked to detect human adaptation over the past 2000 years utilizing a statistical method to identify patterns of selection. They discovered that evolution seemed to select for DNA variants thought to protect against schizophrenia but there were no signs of selection in genetic regions associated with other mental illnesses.

The symptoms of schizophrenia – including auditory hallucinations – involve brain regions that are associated with speech. According to Bernard Crespi, an evolutionary biologist at Simon Fraser University (BC, Canada), the ability to speak probably outweighed the risk that the associated genes may malfunction and result in schizophrenia.

Evolution from environment

It is also thought that genes coding for mental illnesses could have inadvertently been carried along in natural selection because they are located close to genes that selected for traits that enabled people to cope better in their external environment.

For example, a team of scientists at Yale University (CT, USA) discovered that there was a link between schizophrenia prevalence and those who live in regions of Europe with relatively lower winter temperatures. Therefore, the genes that were selected to help people living in colder environments could be located close to those promoting schizophrenia and were therefore carried along in evolution.

The team, led by Renato Polimanti, analyzed 2455 DNA samples from individuals from 23 different locations across Europe and calculated each person’s risk of developing mental health disorders. They plan to continue by examining these links in different regions of the world.

Evolution from Neanderthals

Other groups of scientists are looking to the Neanderthal genome for answers. Their hypothesis is that if they can identify the same genetic markers in Neanderthals, they will be able to assess how human brains have evolved in comparison to Neanderthal brains and whether they suffered from the same mental health problems human do now.

The team, led by Tony Capra of Vanderbilt University (TN, USA), discovered that genes associated with neurological development are regulated differently in human and Neanderthal brains. The FOXP2 gene has the same DNA sequence in Neanderthals and humans but there could be differences in its regulation.

As the FOXP2 gene is associated with language, this could be overexpressed in humans in comparison to Neanderthals – which accounts for the increased language capabilities of humans. Capra plans to continue looking for differences in how Neanderthal and human genes are expressed, hoping to account for the emergence of mental health disorders.

Mental illness as a side effect of evolution

A team from Stanford University (CA, USA) has discovered repeated DNA sequences within the CACNA1C gene, which are unique to humans, may be linked to the development of schizophrenia and bipolar disorder. The mutations exist within a stretch of DNA that has been previously linked to these disorders; however, it is difficult to recognise the repetitions with traditional sequencing methods, so hadn’t been identified until now.

The findings suggest that the rapid evolution of the human brain may have predisposed humans to conditions that are not seen in other animals.

“The human genome reference sequence shows only 10 repeats of this 30-nucleotide sequence, but we’ve found that individuals actually have from 100 to 1,000 repeats, and that the sequence itself can vary,” explained David Kingsley, senior author of the study and professor of developmental biology at Stanford University.

“In contrast, chimpanzees and other primates have just one repeat of the sequence, indicating that the region has greatly expanded during human evolution. Some of the sequence variants now found in people are also closely associated with the development of schizophrenia and bipolar disorder.”

This research is all still in the early stages, however, the use of human genome databases has propelled this kind of study forward. The future may now bring some insight as to how and why mental health disorders developed in the modern human brain.