The evolution of schizophrenia reveals the nature of contingency
Shaman from the Mbukushu of Namibia
J.B.S. Haldane famously quipped that, if there is a God, he’s inordinately fond of beetles. Others may choose to be somewhat less kind and argue that, with around 2 million species of the beastly little things, such a design could only be the product of a disorganized mind. Perhaps that’s the solution Intelligent Design proponents have been looking for: God is schizophrenic! However, as it turns out, schizophrenia is the perfect metaphor for how our evolutionary history is not a well ordered and implemented design, but is rather full of twists and turns and ill-adapted consequences that are best explained through the contingencies of natural selection.
Several years ago Robert Sapolsky suggested that genes promoting schizophrenia may have been selected for in human evolution because some of them conferred benefits that outweighed the 1% of people worldwide that were disabled by the disorder. Like the sickle-cell trait that confers resistance to malaria (so long as you don’t receive two recessive alleles and develop full fledged sickle cell anemia) a partial schizophrenia may be beneficial in some way. He observed that relatives of schizophrenics have a high likelihood of “schizotypal personalities,” or a mild form of the disorder that just makes these people a little strange and allows them to see the world in a unique way. What if, he wondered, schizophrenia maintained itself in human populations because of selection for schizotypal personalites? As luck would have it, for a hundred years anthropologists had observed such individuals thriving in nearly every society they encountered: shamans.
As Sapolsky stated in 2003 while accepting an award from the Freedom from Religion Foundation:
The critical thing with schizotypal shamanism is, it is not uncontrolled the way it is in the schizophrenic. This is not somebody babbling in tongues all the time in the middle of the hunt. This is someone babbling during the right ceremony. This is not somebody hearing voices all the time, this is somebody hearing voices only at the right point. It's a milder, more controlled version.
Shamans are not evolutionarily unfit. Shamans are not leaving fewer copies of their genes. These are some of the most powerful, honored members of society. This is where the selection is coming from.
Sapolsky hypothesized that the evolution of schizophrenia was ultimately a byproduct of selection for beneficial cognitive adaptations. In the early edition of Proceedings of the Royal Society: Biological Sciences Bernard Crespi, Kyle Summers and Steve Dorus have found that schizophrenia evolved in human populations in just this way. By analyzing specific selective sweeps (or the non-coding regions of DNA that get “pulled along for the ride” when a coding region undergoes a beneficial mutation) they’ve determined that 28 of 76 genes that have been linked to schizophrenia have undergone positive selection during human evolution. These 28 genes are all closely linked to cognitive abilities involved in complex thought.
As the authors summarized their findings:
[G]enetic liability to schizophrenia has evolved as a secondary consequence of selection for human cognitive traits. . . The selective forces underlying adaptive evolution of these genes remain largely unknown, but these findings provide convergent evidence consistent with the hypothesis that schizophrenia represents, in part, a maladaptive by-product of adaptive changes during human evolution.
In other words, the same genes that make us so smart and our species so successful can sometimes (specifically, about 1% of the time) result in a debilitating mental disorder. The other 99% of us are doing so well that these genes continue to perpetuate themselves. In the evolution of complex thought, schizophrenia was accepted as a devil’s bargain.
Whether sexy shamans are the ultimate source for this selection or not remains to be seen. However, what Crespi, Summers, Dorus and Sapolsky have emphasized is that evolution is a messy business and is rarely as straightforward as we might assume. There is no long-term view or plan in the evolutionary narrative. Organisms make do with the raw materials they’re born with and the occasional beneficial mutation simply adds additional supports to a jury rigged foundation. God, if such a being exists, must be inordinately fond of such haphazard construction, his “design” is chock full of them.
Bernard Crespi, Kyle Summers and Steve Dorus (2007). Adaptive evolution of genes underlying schizophrenia. Proceedings of the Royal Society: Biological Sciences doi:10.1098/rspb.2007.0876