Why We Thrive In Nature – And Why Cities Make Us Sick

Hoping to answer these questions, a team of scientists from the University of Zurich and Loughborough University have launched the Human Evolutionary EcoPhysiology (HEEP) research group, which aims to measure how immersion in both natural environments and cities alters our biology. The team’s experiments are guided by the so-called Environmental Mismatch Hypothesis – which states that our species is poorly adapted to the modern world – and the researchers say their data could inform the creation of future cities that are more compatible with our evolutionary needs.

An evolutionary mismatch?

The relentless march of industrialization has been accompanied by a massive increase in the average human lifespan, allowing the global population to surge beyond the eight-billion mark – all of which gives the impression that our species is thriving in the artificial habitat we’ve created for ourselves. But look beneath the surface and there’s cause for alarm. 

If you look at my and your sperm count today in 2025, and if it’s around the average, then we’re 67 percent lower than our grandfathers in the 1950s.

Dr Colin Shaw

“If you look at my and your sperm count today in 2025, and if it’s around the average, then we’re 67 percent lower than our grandfathers in the 1950s,” says group co-leader Dr Colin Shaw. “So something’s changed, and it’s most likely to be environmental factors like pollution, plastic, and pesticides.”

In fact, a lot’s changed, and there’s a growing body of evidence to suggest that, despite our ever-increasing numbers, we may not be cut out for the modern world. To highlight that point, Shaw and project co-lead Dr Danny Longman have just published a review paper summarizing how the destruction of our ancestral habitats may be hindering our health.

For instance, the alarming decline in average sperm counts seems to be linked to air pollution and exposure to agricultural chemicals; loss of diversity within our microbiome appears to be driving a rise in autoimmune disorders and allergies; cognitive function is considerably worse in people living in urban centers than those who reside in greener areas; and athletes’ running times become demonstrably slower when air pollution levels rise by even tiny amounts.

Offering an explanation for these growing problems, Shaw and Longman write that “industrialisation has created a mismatch between our nature-adapted biology and the novel challenges imposed by contemporary industrialised environments.” In other words, ancestral humans spent hundreds of thousands of years adapting to specific cues in their natural environment, yet in just a few centuries, we’ve annihilated these habitats, and the absence of these vital signals has numerous negative impacts on our biology.

But which cues are we missing exactly, and what impact does each of these have on our health? The HEEP team is yet to publish its latest research on this topic, but group members have been speaking exclusively to IFLScience to reveal what they’ve discovered so far.

Nature boosts physical performance

Among the group’s many remarkable findings is that people from urban areas appear capable of biking harder and faster after spending an afternoon in a forest. Collaborating on that research was PhD student Matt Putland, who later conducted further experiments to measure the performance and physical exertion levels of joggers in both forests and cities. Y

It really seems to be the case that people feel better after they spend time in nature.

Ronja Schmoll

“We found that people ran further in nature and enjoyed it more and indicated that they were more likely to do it again in the future,” he says. “So it may be that the body is better adapted to perform in nature, maybe because there’s less urban pollution and more natural inputs.”

Regardless of the mechanisms behind this performance boost, this discovery could have immediate real-world applications. “We hope these results can encourage the development of nature-based public health initiatives that increase physical activity,” says Putland.

Yet it’s not just when we’re busting a gut that we experience these benefits. Separate experiments run by research assistant Zora van der Bie showed that heart rate, blood pressure, and salivary cortisol levels differ after spending three hours chilling in a forest as opposed to in a city. Overall, the data shows that stress markers are higher when we’re in an urban environment, even when we’re not doing anything stressful, while the entire organism seems to go into relaxation mode in the woods.

Once again, the exact environmental cues behind this effect will need to be revealed by future studies, but initial findings already point towards several things that could be done to help make our cities healthier. “We saw correlations with urban air and sound pollution, and that definitely gives indications to work on urban greening and traffic reduction,” says van der Bie.

Cognition and mental health

Perhaps unsurprisingly, the team also uncovered a link between this tendency to naturally decompress in the forest and an elevation in mood. Conversely, we default to a grumpier version of ourselves in the big smoke, even when there’s nothing in particular giving us the hump.

“It really seems to be the case that people feel better after they spend time in nature,” says PhD student Ronja Schmoll, whose experiments involved taking volunteers into deciduous forests, coniferous forests and urban environments before assessing aspects of their psychology and cognition. Fascinatingly, while both types of forest environment were associated with enhanced mood, the two types of woodland were not equal when it came to their influence on depressive symptoms.

This finding is important because it shows how specific signals in different environments help to shape our mental health, offering clues as to how we might incorporate certain aspects of nature into urban design in order to combat the city blues. 

“For some of the variables that we collected, we really could say things like ‘it was this specific alpha pinene that is linked to how depressed you felt’,” explains Schmoll. “And if we know that, then we can plant more trees that distribute a lot of alpha pinene, which makes people feel better,” she says.

Yet the chemical profile of a forest is likely to be just one part of the equation. After all, environmental signals are delivered to the human body via our eyes, ears, noses, mouths, and skin, and part of HEEP’s mission is to figure out how our ancestors adapted to the sensory experience of nature. “We’re systematically isolating each sense, starting with sound,” says Longman.

A few weeks ago, the team published the first of these sensory studies, revealing how natural soundscapes impact mood and cognition. During these experiments, participants sat blindfolded and listened to recordings taken either in a temperate rainforest or a city center. Findings indicated that markers of mood and cognition were significantly better in those who listened to the forest soundscape than those who endured the urban racket.

“Lots of people live in built-up city environments and don’t have the means to access nature – for instance, if they’re disabled in some way,” says Longman. “But bringing certain aspects of nature into their home could have a very significant benefit. Even just listening to nature sounds, for example, can boost cognition.”

The team is now analyzing the data from their tactile study, which saw participants fondle either a series of items representative of modern life – such as door handles, mobile phones, and plastic objects – or natural features like pine cones and pieces of tree bark. According to Longman, the results are “subtly different” from those of the soundscapes study, underscoring how each of our senses is adapted to pick up specific signals from nature that enhance our biological function.

Forests, the microbiome, and immune function

The role of the microbiome in maintaining physical and mental health is something that is becoming increasingly recognized, yet until now, few scientists had ever considered how living in an industrialized world might mess up our microscopic helpers. To investigate, HEEP researcher Gina Sommacal investigated how the nasal and skin microbiomes change after a three-hour immersion in either a natural or an urban setting. Importantly, she also looked at the types of microbes present in each environment.

What happens if you live your whole life in a more biodiverse environment with microorganisms that your body evolutionarily knows?

Leandra Keusch

Overall, she found that forests contained a much greater diversity of microbes than cities, which is crucial since our health depends on us encountering the widest possible range of germs in our surroundings. 

“The so-called ‘biodiversity hypothesis’ states that we have adapted to very diverse natural environments with lots of different microbes, and therefore we would expect that we need to be exposed to this diversity in order to reach our full biological potential,” explains Sommacal. “And the fact that natural environments actually harbor greater diversity versus cities, suggests that in cities we lack key microbes or or microbial communities which are essential for certain biological processes.”

The good news, however, is that environmental microbes – whether from the forest or the city – do appear to transfer to the microbiome on the skin and inside the nose after a very short time. “So say you go to a city park for one hour, and your microbiome already gets an effect and maybe even a benefit. That’s a huge argument in favor of building our cities greener,” says Sommacal.

Yet the biodiversity hypothesis extends beyond the microbiome, and has major implications for immune function too. After all, our immune system co-evolved with the microorganisms present in our ancestral environments, and there’s a wealth of evidence to suggest that autoimmune diseases and allergies are way more common in urbanites than those who live closer to nature.

HEEP researcher Leandra Keusch therefore looked at whether changes in immune cells called peripheral blood mononuclear cells (PBMCs) could be detected following a short trip to either the forest or the city. “We asked whether just going to a different environment would be enough to see a difference – and yes, it actually was,” she says. Despite this not insignificant find, though, Keusch insists that, at this stage, her research is just “proof of concept”. 

“After three hours [in a forest or a city] we already see small changes. But what happens if you live your whole life in a more biodiverse environment with microorganisms that your body evolutionarily knows? And how harmful is it to live your whole life exposed to pollutants?” she ponders.

To start providing answers to these questions, Shaw and Longman are planning a series of longer-term studies, including one scheduled for next summer that will see participants spend an entire week in a forest. The ultimate goal, however, goes far beyond evolutionary science, and the HEEP team are now actively forging links with governmental and industry partners in order to shape public health policy and urban design, with the aim of creating future cities that are better adapted to human biology.

Because as Keusch points out, “We’re dependent on cities and we’re not going to go back to the forest. So the way we design cities really matters.”