Researchers have found that schizophrenia, which while not found in any other species, has existed right from the times of early man. So why would an ailment so debilitating and disadvantageous to an individual’s chances of survival or reproductive success persist throughout human evolutionary history?
According to a new study, this is because schizophrenia isn’t just about human intelligence. It is connected with the very thing that makes us human. The study, conducted by researchers from Mount Sinai in the US and published in Molecular Biology and Evolution, found that schizophrenia is directly linked with the exact genes that make us human.
The researchers studied human accelerated regions (HARs) in human DNA. HARs are signposts in the genome structure that have been conserved over the entirety of mammalian evolution. They exist in all mammals, but have experienced faster mutation rates and are thus different in humans. In a way, they are what make our species different from other mammals.
Utilising a recently completed study on schizophrenia, which included 36,989 schizophrenia cases and 113,075 others as a control group, the researchers tried to identify patterns between the location of HARs and the recently-identified schizophrenia gene loci. They found that the schizophrenic loci were most strongly associated in regions near HARs. These HAR-associated schizophrenic loci were found to be under stronger evolutionary pressure when compared with other schizophrenic loci. It was also discovered that the areas of the greatest correlation between the two were involved in important functions, like controlling the expression of the neurotransmitter GABA, brain development, synaptic formations, adhesion and signalling molecules.
According to the researchers, this means that the genes that are crucial for human existence, intelligence and survival abilities are closely linked to those found causing schizophrenia. They argue that schizophrenia continues to exist in humans because natural selection is choosing genes that are crucial for our existence.