Researchers led by Bernard Crespi of Simon Fraser University in Burnaby, Canada, focused on a number of genes closely linked to the disease and found that they had been "positively selected" all through genetic evolution to go forward, despite the negative effects schizophrenia can have on health.
Schizophrenia is a severe mental disorder characterized by impairments in the perception or expression of reality, most commonly marked by auditory hallucinations, paranoid or bizarre delusions or disorganized speech and thinking. The precise cause of schizophrenia remains unknown, making it difficult to treat.
But the latest findings support one theory that the disease could be a consequence of rapid human evolution.
The research team examined 76 DNA sequences linked to schizophrenia. They compared these human sequences with one another and with those of primates such as chimps and macaques, as well as with some from mice, rats, cows and dogs.
The results showed that of the 76 genes studied, 28 displayed evidence of being favoured by natural selection. They showed less variation than other control sequences from elsewhere in the genome, and had less evidence of having been jumbled up by the random mixing of genes that occurs during sexual reproduction. These findings suggest that these schizophrenia-linked sequences may have conferred an evolutionary advantage.
"The world-wide presence of this disorder at an appreciable frequency, despite its impact on human health and reproductive fitness, is somewhat of a paradox. This may be explained by the existing theory that the condition represents in part a by-product of adaptive changes during human evolution," Nature quoted Crespi's colleague Steve Dorus of the University of Bath, UK, as saying.
However, the genetic data provide no clues regarding the kind of advantage that schizophrenia-linked genes might have offered. "That is the big question and we don't really have a good answer to that," admits Crespi's colleague Steve Dorus of the University of Bath, UK. Even so, the results might explain why schizophrenia has continued without being wiped out by evolutionary forces.
Psychiatric studies have suggested that people with schizophrenia could be more creative or imaginative than the general population, which raises the likelihood that schizophrenia genes helped carriers to solve survival problems or attract a mate.
However, Dorus says that it's too early to draw definite conclusions from this theory, adding, "From a strict genetic standpoint, links between schizophrenia and creativity are still tenuous."
He says that problem is compounded by the fact that so many genes are linked to the disease, and that they vary between populations around the world. Only four of the gene sequences identified by the researchers showed evidence of natural selection in more than one continent.
This suggests that different genes may have been favoured in the evolution of different populations, even though schizophrenia is found throughout the world. It also suggests that these mutations were subject to natural selection after humankind embarked on its spread throughout the world over the past 60,000 years.
The findings of the study are published in the Proceedings of the Royal Society B1.