Schizophrenia has existed throughout recorded human history and persists despite its severe effects on thought and behavior, and its reduced rates of producing offspring. Dr. Andreassen and colleagues looked at the genome of Neanderthals to pinpoint specific regions of the genome that could provide insight on the origin of the mental disorder in evolutionary history. They analyzed genetic data from recent genome-wide association studies of people with schizophrenia for overlap with Neanderthal genomic information.
"Some scientists think that schizophrenia could be a ‘side effect’ of advantageous gene variants related to the acquisition of human traits, like language and complex cognitive skills, that might have increased our propensity to developing psychoses," Dr. Andreassen said.
"We analyzed recent large genome-wide association studies of schizophrenia and a range of other human phenotypes (anthropometric measures, cardiovascular disease risk factors, immune-mediated diseases) using a statistical framework that draws on polygenic architecture and ancillary information on genetic variants," they explained.
"We used information from the evolutionary proxy measure called the Neanderthal selective sweep score."
Parts of the human genome associated with schizophrenia, so called "risk loci", were more likely to be found in regions that diverge from the Neanderthal genome. An additional analysis to pinpoint loci associated with evolutionary markers suggests that several gene variants that have undergone positive selection are related to cognitive processes. Other such gene loci are known to be associated with schizophrenia and have previously been considered for a causal role in the disorder.
"Our findings suggest that schizophrenia vulnerability rose after the divergence of modern humans from Neanderthals and thus support the hypothesis that schizophrenia is a by-product of the complex evolution of the human brain," Dr. Andreassen said.
"This study suggests that schizophrenia is a modern development, one that emerged after humans diverged from Neanderthals. It suggests that early hominids did not have this disorder," said Dr. John H. Krystal, Editor of Biological Psychiatry.