Schizophrenia seems to be a condition unique to humans. It's plagued people for as long as recorded history (and presumably longer, it's just we don't have the records to prove it).
Schizophrenia's long life appears to be the result of the fact genes play a role in it; allowing it to be passed from one generation to the next.
But if it's genetic natural selection can influence it. So why hasn't evolution purged this potentially harmful condition from the gene pool?
Evolution likes schizophrenia
Over the past few years many researchers have begun speculating that evolution might actually have helped schizophrenia stick around.
They argue that the persistence of schizophrenia might be because it's a side effect of something we can't live without. Some aspect of our cognition we desperately need to survive. If language produced a negative side-effect in 1% of people it would still persist because it's so damn useful to the other 99%. Thus, evolution favours this crucial trait, accidentally keeping schizophrenia around.
As long as the advantages outweight the negatives it will evolve. Like how we walk upright, even though it gives some of us back problems.
This association between positive and negative traits is often seen between the sexes. For example, some male lizards are more attractive if they're brightly coloured. It helps display dominance and health. This leads brighter lizards to evolve. Yet this also makes females brighter, which doesn't help them. In fact, it actually makes it easier for predators to spot them. But as long as the trait is a net benefit (giving more benefit to males than harm to females) it will persist.
Evolution is kind of a dick like that. It only cares about the net result: passing genes on.
Evidence evolution likes it
So, scientists have a handy hypothesis for why schizophrenia sticks around. But is there any evidence that it's true?
Well, yes. Scientists examined the genetic locations associated with an increased risk of schizophrenia. These were then compared to the Neanderthal genome. This comparison revealed that the loci associated with schizophrenia were those least similar to the Neanderthals. In other words they'd evolved a lot in modern humans; hence why they were so dissimilar to Neanderthals.
The fact that these locations had undergone such a significant amount of evolution strongly implies that they are responsible for something important. Something that gives us a big advantage. That's why natural selection went to town on them, rapidly altering these parts of the genome. This would seem to confirm the hypothesis that schizophrenia is a bi-product of (or maybe piggybacked on) a more important part of the genome.
Unfortunately our genome is a big, messy thing. So we don't know what it all does; including many of the bits this study uncovered (although many do seem to be linked to brain development). Thus we can't say exactly what traits schizophrenia was "accidentally" produced by. But hopefully with this knowledge we can start examining these regions in more detail, ultimately uncovering the "why" behind the disorder. And once you have the "why", the "how" (including the how to stop) are hopefully not that far behind.
Whoever said evolution only helped us understand the past?
References
Allen, J. S., & Sarich, V. M. (1988). Schizophrenia in an evolutionary perspective. Perspectives in biology and medicine, 32(1), 132-153.
J ESTE , D.; D EL C ARMEN , R.; L OHR , J.; and W YATT , R. Did schizophrenia exist before the 18th century? Compr. Psychiatry26:493-503, 1985
Plesnar Bielak, A., Skrzynecka, A. M., Miler, K., & Radwan, J. (2014). Selection for alternative male reproductive tactics alters intralocus sexual conflict. Evolution, 68(7), 2137-2144.
Srinivasan, S., Bettella, F., Mattingsdal, M., Wang, Y., Witoelar, A., Schork, A. J., ... & International Headache Genetics Consortium. (2015). Genetic markers of human evolution are enriched in schizophrenia. Biological Psychiatry.
