Why did magic mushrooms evolve? Maybe we'll finally get an answer

Many types of mushrooms produce the psychoactive compound psilocybin

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Magic mushrooms have provided people with mind-altering experiences for thousands of years, but the real reason mushrooms developed these hallucinogenic chemicals may be as a biological weapon against the insects that feed on them.

Psilocybin is an active ingredient in many types of magic mushrooms that are found on every continent except Antarctica and have a long history of use by shamans in traditional cultures. More recently, researchers have explored psilocybin as a possible treatment for a range of mental disorders from depression to post-traumatic stress disorder.

The drug exerts its psychedelic effects primarily by binding to serotonin receptors in the human brain. But it’s not clear why many species of fungi have evolved to synthesize compounds that resemble animal neurotransmitters, he says Jon Ellis at the University of Plymouth in Great Britain. “There have been suggestions that psilocybin might have a defensive role against invertebrate mushroom-eaters, but these hypotheses have never been tested,” he says.

To investigate the effects of psilocybin on insects, Ellis and his colleagues mixed dried, powdered magic mushrooms (Psilocybe cubensis) to food served to fruit flies (Drosophila melanogaster) larvae. They followed the young larvae through their life cycle to see how many survived, how quickly they developed, and whether the adults were smaller than average or showed signs of developmental differences.

They also prepared liquid mushroom extracts, added some sucrose, and exposed the larvae to these extracts for an hour before filming them moving. It was “a bit like a bath in sweet magic mushroom soup,” says a team member Kirsty Matthews Nicholassalso at the University of Plymouth.

“By measuring how fast they crawled, how far they traveled and how coordinated their movements were, we were able to quantify the short-term effects on the insect’s nervous system,” says Nicholass.

Larvae raised on food containing magic mushrooms survived at a much lower rate than larvae fed normal food. Survival to adulthood was more than halved at lower doses, and only about a quarter of larvae survived at higher doses.

“Even among those that went through development, the effects were clear: adult flies were smaller, with shorter bodies and left-right wing asymmetries, a classic sign of developmental stress,” says Nicholass. “They crawled shorter distances, spent less time moving overall, and showed more erratic turning behavior. In practice, this means the insects were slower and less coordinated.”

But insects are unlikely to have the psychedelic experience that humans do, he says. “Our results suggest that compounds like psilocybin interfere with the basic physiology and behavior of insects in ways that are likely to be harmful rather than mind-altering.”

The team also collected seven species of sponges from Dartmoor in the UK and analyzed the DNA of the invertebrates present in the samples. This revealed that the collected psilocybin-producing mushrooms hosted a different group of insects than most of the other mushrooms sampled, suggesting that the psychedelic compounds may play a role in shaping which insects can live in or feed on them, the researchers say.

However, there were some unexpected results that suggest the role of psilocybin is more complex than the results suggest. For example, fruit flies with reduced levels of the serotonin receptor, which psilocybin normally interferes with, had worse effects.

The researchers say other hypotheses about the development of psychedelic mushrooms should be tested, such as the idea that psilocybin repels slugs and snails, or that the mushrooms are manipulated by invertebrates to help them disperse disputes.

Fabrizio Alberti at the University of Warwick in the UK says the experiment shows that even mushrooms that don’t produce psilocybin can produce other metabolites that interfere with insect pupation rates and survival.

“Further studies using pure psilocybin on insects will be needed to determine the ecological role of psilocybin and to investigate whether this hallucinogenic compound could have evolved as a defense against insects,” says Alberti.

The study highlights major challenges in investigating the evolutionary role of psilocybin-producing mushrooms, he says Bernhard Rupp at the University of Innsbruck, Austria.

“There are many ways that fungi producing psilocybin and other exotic compounds could gain an evolutionary benefit, such as by deterring consumption by insects or slugs,” he says.