As the twilight fell In the Finnish town of Lahti, on a still cool day in May 2016, a team of workers broke into the courtyard of an empty daycare center. Under swings and jungle gyms, they set up squares of forest floor – scruffy shrubs, tall berry bushes, wispy prairie grasses, and velvety mounds of moss – collected from the woods somewhere in a game. less developed country. On the edges they put soft green grass. In the morning, when the kids arrived, they found their playground – once a drab patchwork of asphalt, gravel and sand – transformed overnight into a micro-oasis of wilderness.

This scenario unfolded three more times that month in daycares in Lahti, and 500 miles west, in the city of Tampere. This was not the work of some nature-loving guerrillas, but the start of an ambitious scientific experiment to find out whether the lack of microbes in cobbled urban environments could turn people’s immune systems against them. “There is this”biodiversity hypothesis“That in the absence of a diverse environmental microbiota, people are more likely to contract immune-mediated diseases,” says Aki Sinkonnen, evolutionary ecologist at the University of Helsinki. “But no one had really tested this with children.”

You are probably more familiar with the “hygiene hypothesis”. First described by a British epidemiologist named David Strachan in the early 1990s, he postulates that the climb in chronic disorders related to an over-reactive immune system – such as asthma, diabetes and allergies – is driven by children growing up in increasingly sterile bubbles. Immune systems are, at their most basic, object classifiers. Their job is to recognize what is self and what is other. Germs encountered early in life are the primary guardians of this process – helping the developing immune system decipher what is dangerous and what is not. More families have used antibacterial soaps and gels, locked themselves in high-rise apartments and drove cars through concrete jungles, the less habitat there was for bacteria, protozoa, fungi and viruses to thrive – the less likely it was that children’s developing immune systems meet them. And less exposure means fewer training opportunities. A poorly trained immune system can fail when it comes to distinguishing between a body’s own cells and food allergens, gut microbes, or pollen in the air.

Laboratory experiments on rodents in the early 2000s supported this idea: Wild rats had immune systems well tuned to fight dangerous pathogens, but not minor irritants, while their lab-raised counterparts overworked at the smallest stimulus. Human epidemiological studies also provided circumstantial evidence – allergy and asthma rates tend to be higher in more industrialized areas than in rural areas. To counter these supposed negative effects of urban and modern lifestyles, dozens of companies have sprung up to sell immune-boosting probiotics – pills, drinks and creams filled with cocktails of live bacterial cultures. In the Covid-19 era, thousands of posts tagged #immuneboost promoting these and other home remedies appear on Instagram every week. So far, there is little evidence that this has worked.

That’s why, in recent years, scientists like Sinkkonen have taken this idea even further. People increasingly live in microbiodiversity deserts, they observed, failing to expose themselves to a variety of harmless insects. “The immune system doesn’t recognize microbes by species, but by type,” says Sinkonnen. “Probiotics usually only contain one or two types of bacteria, so they’re unlikely to activate the entire immune system. We wanted to see what would happen if we introduced a totally diverse microbial environment. Therefore, forest soils in playgrounds – the first randomized controlled trial to test the hypothesis of biodiversity in children. Biohacking, but make it cute.

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