David Robson in BBC:
Once upon a time, your origins were easy to understand. Your dad met your mum, they had some fun, and from a tiny fertilised egg you emerged kicking and screaming into the world. You are half your mum, half your dad – and 100% yourself. Except, that simple tale has now become a lot more complicated. Besides your genes from parents, you are a mosaic of viruses, bacteria – and potentially, other humans. Indeed, if you are a twin, you are particularly likely to be carrying bits of your sibling within your body and brain. Stranger still, they may be influencing how you act. “Humans are not unitary individuals but superorganisms,” says Peter Kramer at the University of Padua. “A very large number of different human and non-human individuals are all incessantly struggling inside us for control.” Together with Paola Bressan, he recently wrote a paper in the journal Perspectives in Psychological Science, calling for psychologists and psychiatrists to appreciate the ways this may influence our behavior.
That may sound alarming, but it has long been known that our bodies are really a mishmash of many different organisms. Microbes in your gut can produce neurotransmitters that alter your mood; some scientists have even proposed that the microbes may sway your appetite, so that you crave their favourite food. An infection of a parasite called Toxoplasma gondii, meanwhile, might just lead you to your death. In nature, the microbe warps rats’ brains so that they are attracted to cats, which will then offer a cosy home for it to reproduce. But humans can be infected and subjected to the same kind of mind control too: the microbe seems to make someone risky, and increases the chance they will suffer from schizophrenia or suicidal depression. Currently, around a third of British meat carries this parasite, for instance – despite the fact an infection could contribute to these mental illnesses. “We should stop this,” says Kramer.