Are big brains to blame for schizophrenia?

Could mental illness be the price we pay for our bulging brains? The idea has been knocking around for some time now as an explanation for autism, depression and other diseases.

It’s an attractive thought. Over the past two million years, the human brain has nearly doubled in size, perhaps birthing to consciousness, language and religious belief. Mental illness could be an unfortunate side effect of our overactive, over-wired and over-stimulated brains, the theory goes.

But the melding of two highly contentious and often speculative lines of research – the biology of mental illness and human evolution – demands we take any argument with a heaping pinch of salt.

The latest, published in Genome Biology, connects a big brain’s energy demands to schizophrenia. It comes from Philipp Khaitovich, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues.

They looked at genes expressed in the brain that had been selected for in human evolution and compared them to genes that are turned excessively up or down in people with schizophrenia. Lo and behold, the researchers found some overlap, especially among genes involved in powering cells.

Next, Khaitovich’s team compared the levels of 21 metabolic chemicals found in post-mortem brains of humans (both with and without schizophrenia) and chimpanzees. Nine of the chemicals – which come from the break-down of sugars, neurotransmitters and fats – showed different levels in people that had schizophrenia and other people.

When the researchers compared the levels of these 21 chemicals in the brains of humans and primates via a mathematical analysis, those same nine chemicals stuck out. Their levels were more likely to vary between humans and chimps than the other 12. In other words, some of the significant metabolic chemical differences between human and chimp brains are disrupted in people with schizophrenia.

However, post-mortem brain research is notoriously unreliable. A person’s (or chimp’s) age, sex, cause of death and post-mortem interval – the time between death and when the brain is preserved – can all muck up chemicals in the brain.

And schizophrenia genetics is notoriously fickle, with few studies implicating the same genes. Just last week, two teams revealed a host of genetic differences that seem to predispose some people to schizophrenia, but these mutations only account for half of 1% of all cases.

On the human evolution side, no one doubts that genetic change drove the development of bigger brains, and it follows that more cells with more connections demand more energy.

But Khaitovich’s team notes that their analysis of the genetic changes that occurred during human evolution can only peer back 200,000 years, when some of the modern lineages of humans began to gel. Yet fossil records show that our ancestor’s brains started swelling a full two million years ago.

So while it’s interesting to ponder whether debilitating mental illnesses like schizophrenia and bipolar syndrome are a product of our big brains, the evidence just isn’t there yet.

The following two tabs change content below.