Schizophrenia connected to evolution

Schizophrenia is more closely linked to human evolution than scientists previously believed, according to a study published in the journal Biological Psychiatry.

The study, entitled “Genetic Markers of Human Evolution Are Enriched in Schizophrenia,” determined that the persistence of the brain disorder is related to the period of time in which humans were split from Neanderthals, aka the period of time when the human species began to evolve differently from the Neanderthal subspecies.

This study suggests that schizophrenia is a modern development, one that emerged after humans diverged from Neanderthals,” said John Krystal, editor of Biological Psychiatry.

Researchers from the University of Oslo in Norway began by obtaining summary statistics for over two million single nucleotide polymorphisms—a common genetic variation among the human genome—from a genome-wide association study of schizophrenia, as well as other phenotypes, including body mass index, height, cardiovascular disease risk factors, celiac disease, Crohn’s disease, nervous system disorders, psychiatric disorders and a multitude of other phenotypes.

Using a “Neanderthal selective sweep score” developed through an alignment of human, primate and Neanderthal consensus sequences, they were able to create an index of positive selections in humans following their genetic divergence from Neanderthals, who went extinct 40,000 years ago in what is now considered to be Europe.

The NSS score measures the abundance of ancestral/non-ancestral alleles—alternative forms of a gene that occur due to mutation—in the human genetic line and the Neanderthal genetic line. A negative NSS score indicates a lack of alleles, while a positive NSS score indicates the opposite. Researchers used the scores to determine where the human genome underwent a positive selection—the adoption of a certain allele.

In order to better control their analyses for the affiliation to brain genes, the researchers utilized information from the National Center for Biotechnology Information, a database that allows users to search for catalogued genes. They queried the term “human brain” in order to find thousands of genes. A 2011 study entitled “Spatio-temporal transcriptome of the human brain,” was also used in order to find further types of brain genes.

“Our findings suggest that schizophrenia vulnerability rose after the divergence of modern humans from Neanderthals,” said the study’s senior author Ole Andreassen, “and thus support the hypothesis that schizophrenia is a by-product of the complex evolution of the human brain.”

This most recent study, published on Aug. 15, differs from previous studies in that it explores the evolutionary factors of schizophrenia using an enormous set of genes. The findings of the study are conducive with the idea of polygenic adaptation, as the disease was found to have likely manifested itself due to the adaptation of humans to certain pathogens in their environments. This also proves false the idea that natural selection is the only factor that had shaped human variation.

Besides schizophrenia, several other genes present in modern humans were found to have been a result of positive selection, such as those affecting cognitive function. This is unsurprising considering the fact that modern humans possess higher brain function than their Neanderthal cousins, despite having similarly-sized brains.

Prior studies suggest that it was during the last Ice Age when humans began to develop specialized tools, burial rituals and the development of language. It was the latter development that was thought to have helped shape the modern human brain, allowing them to think abstractly and creatively. However, this also left the brain prone to the development of chronic mental disorders like schizophrenia.

Schizophrenia affects approximately 1 percent of the world’s total population of 7.4 billion people and is a severe thought disorder that causes hallucinations and an overall disassociation with reality.