Scientists attempt to study the role of evolution in psychiatric disorders
Genes are at least partly responsible for most psychiatric disorders. But if certain genes cause severe problems like schizophrenia, why hasn’t evolution eliminated them?
The evolution of psychiatric disorders has historically been difficult for researchers to study because most psychiatric disorders are polygenic (influenced by many genes, not just by a single allele). The number of genes causing a psychiatric disorder to be expressed can be in the hundreds or thousands. To learn more about the disorder’s evolution, researchers must sequence each of these genetic regions and figure out how they’ve changed over time. This requires access to huge amounts of data on the human genome, something hard to come by. According to biology writer Sara Reardon, “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.”
Biologists around the world are exploring these connections in unique ways. By looking for correlations in huge datasets, Yale geneticist Renato Polimanti is trying to uncover relations between environmental factors and the expression of mental illnesses. His team examined thousands of genetic samples and environmental data points from across Europe, and found that people in colder regions had a higher risk for schizophrenia. Reardon writes, “If genes that helped people tolerate cold were located close to variants that promote schizophrenia in the genome, then the latter could have been inadvertently carried along during evolution as a fellow traveler.” However, the correlation may not be so straightforward, Reardon continues: “unknown environmental conditions in the past could have selected for traits that were advantageous then, but considered negative today.”
Bernard Crespi, an evolutionary biologist at Simon Fraser University, explains that schizophrenia is tightly tied to parts of the brain that allow us to produce speech. If the genes for speech and the schizophrenia genes are closely related, it’s possible that the speech genes were prioritized in human evolution, bringing schizophrenia genes along for the ride in a mechanism similar to Polimanti’s ‘cold’ gene hypothesis.
In another approach, biologists are comparing the genetic processes of modern humans to Neanderthals to learn more about how genes evolved. Tony Capra and Laura Colbran, geneticists at Vanderbilt University, are studying how the same exact genes are expressed differently in the two species. For example, both modern humans and Neanderthals carry the gene FOXP2, associated with language, but our abilities to use language are greater.
One explanation is that differences in the way FOXP2 is processed allows modern humans to produce more of the protein encoded by FOXP2. Capra and Colbran’s team hope that by learning more about Neanderthal brains, they will be able to find out whether they experienced psychiatric disorders the same way humans do today.
Our understanding of the evolution of mental illness is still developing. However, as more genetic databases become available, existing research can be confirmed and expanded, and new research can be developed.