Urban living linked to chronic stress epidemic in modern humans

Chronic stress is on the rise—the result of an evolutionary mismatch that our bodies and brains, adapted over hundreds of thousands of years to hunter-gatherer conditions, are experiencing in industrialized, urbanized environments, argues evolutionary anthropologist Colin Shaw. Is there a remedy?

It’s the end of July and, uncharacteristically, it’s pouring down in Zurich. Sitting on a foldable chair underneath the canopy of old-growth beech trees, and protected by an umbrella, however, I feel sheltered and at peace. I take deep breaths as I listen to the birdsong breaking through the steady sound of raindrops splashing on the forest floor. The water flows steadily down the wide tree trunks, having accumulated from the branches high above. A knotty maze of roots on the slope ahead seems to form a natural enclosure. “How was it?” asks a voice.

Colin Shaw walks over from where he had been standing barefoot, in his trekking sandals in the rain for the past few minutes. The evolutionary anthropologist and head of the Human Evolutionary EcoPhysiology (HEEP) research group at the University of Zurich had given me an assignment when we arrived at this clearing on the edge of Zürichberg forest: choose your favorite spot. Take in the environment. Focus on each sense, step by step. What sounds can you hear? What can you smell? What movements can you observe?

Rolling in the mud for science

With these instructions, we aim to recreate part of an experiment that Shaw and his research group—comprised of specialists from the fields of ecology, immunology, microbiology, cognitive psychology and exercise science—conducted last summer, when 160 people spent three hours walking and sitting in one of three different environments: Sihlwald, a coniferous forest just outside Zurich; Mont Tendre, a deciduous forest outside Lausanne; and Zurich’s Hardbrücke area, an urban setting.

“In the forest,” Shaw laughs, “we got people down and dirty and hugging trees and playing with the soil microbiome and everything else.”

Before and after being exposed to the natural and urban environments, the researchers measured a whole range of biomarkers in the participants’ blood, saliva as well as their cognitive capacity. In the woods, people exhibited significantly lower blood pressure, a better immune response and an improved psychological state—in the urban environment, however, they presented with higher blood pressure and strong physiological and psychological stress reactions.

Although we aren’t taking any measurements in today’s torrential rain, I can feel the effect of the experiment. In the woodland environment—which, as Shaw points out, is “closer to our ancestral condition than a city environment”—I feel calm. My pulse is steady, and my stress level dissipates. Shaw jokingly adds that the rain may have even improved the dispersal of phytoncides. These volatile organic compounds, released by trees, offer immune-boosting benefits to humans—as demonstrated by the forest bathing (shinrin-yoku) movement in Japan.

Relaxed? Stressed? Click on the markers to see how the images and sounds from our reporter’s walk with Colin Shaw affect you.

Lions everywhere

The next experimental setting for today will be a busy intersection. As we walk along a small path and climb over fallen branches to rejoin the main forest road, Shaw gives me a summary of his main research hypothesis. From an evolutionary standpoint, he says, the industrialized, urbanized environments we’ve built place a chronic stress load on our bodies, taking a toll on both physical and mental health.

“Whereas physicians would talk about this as ‘ill health,” we try to determine the evolutionary context to understand whether our surroundings are making us sick—and which environment will help us recover,” he says.

In a recent research paper co-authored with Daniel Longman, a longtime collaborator and fellow Cambridge alumnus from Loughborough University, Shaw argues that the extensive environmental shifts of the Anthropocene have undermined human evolutionary fitness.

Evolutionary success of a species amounts to survival and reproduction, and, according to the authors, both factors have been severely compromised in the last 300 years since the beginning of the Industrial Revolution. They support their theory with evidence of declining global fertility rates and increase in chronic inflammatory conditions such as autoimmune diseases. They also cite impaired cognitive function in urban environments. Chronic stress plays a key role as the cause of many of these conditions.

“In our ancestral state, we were well-adapted to deal with acute stress to evade or confront predators. Fight or flight. The lion would come around occasionally, and you had to be ready to defend yourself—or run,” Shaw explains. “The key is that the lion goes away again. Such an all-out effort guaranteed survival, but it was very costly and required lengthy recovery.”

This acute stress response was ideal for mobilizing adrenaline and cortisol while fighting for survival in our hunter-gatherer past. However, it is mismatched for today’s steady stream of challenges.

“Our body reacts as though all these stressors were lions,” he continues. “Whether it’s a difficult discussion with your partner or your boss, or traffic noise, your stress response system is still pretty much the same as if you were facing lion after lion after lion. As a result, you have this very powerful response from your nervous system, but no comedown.”

The hidden costs of progress

The water gushes down the gutter as we continue our walk down Letzistrasse into the city, and the traffic noise, amplified by the rain, swells. “Essentially, there’s a paradox where, on the one hand, over the last three hundred years we’ve created this tremendous wealth and comfort and healthcare for a lot of people on the planet.” Shaw speaks louder to be heard over the roar of a large construction vehicle passing by on Winterthurerstrasse.

“But on the other hand, some of these industrial achievements are having quite detrimental effects on our immune, cognitive, physical and reproductive functions. For example, since the 1950s sperm count and motility rates have dropped dramatically in men, which is tied to pesticides and herbicides in food, but also to microplastics,” Shaw says.

As we arrive at the intersection with Irchelstrasse, I get to choose where to set up my foldable chair again. Instinctively, I opt for a corner where I can at least feel the greenery from Irchel Campus behind me. For the next 15 minutes, I observe the heavy traffic approaching from all sides, my eyes darting around. The deafening noise—a mix of roaring engines, water spraying from the wheels plus jackhammers from roadworks—drowns out any other thought in my brain. My breathing becomes shallower, the entire body tenses up. I’m relieved when Colin Shaw tells me we can now move on to friendlier surroundings, and we head into Irchel Park.

“There was no real danger, yet my jaw is clenched,” he states. “It’s the constant stimulation. We didn’t evolve to be constantly stimulated.”

Of course, compared to megacities with tens of millions of inhabitants, such as Tokyo, Delhi and Shanghai, “Zurich is barely a city,” Shaw concedes. “It’s surrounded by forests; there’s a lake and a river. It also has a comfortable public transit system.”

However, research by the HEEP group clearly indicates that even in a city that is frequently ranked among the most livable, urban exposure is physiologically and psychologically stressful and impairs immune function.

Today, an estimated 4.5 billion people—more than half the world’s population—live in urban agglomerations. By 2050, that figure is projected to rise to 6.5 billion, or more than two-thirds of humanity. Recognizing industrialization and urbanization as health risks will be crucial for safeguarding public health—or, in evolutionary terms: the fitness of our species.

We can’t adapt our way out of this

It’s hard to believe that our brains have grown accustomed to juggling ever-new digital innovations—yet remain rooted in a prehistoric past when it comes to regulating our nervous systems. Why haven’t we adapted to the living conditions that our species has created?

“You could argue that the stress responses we’re seeing today are a form of adaptation. However, biological adaptation is very slow. Longer-term genetic adaptations are multigenerational. So that’s tens to hundreds of thousands of years,” Shaw points out.

“From an evolutionary perspective, if people are dying from chronic stress or stress-related diseases, you could say that this is natural selection taking place. If you let that go on for hundreds of generations, people would probably become better able to deal with chronic stress,” he says. Clearly, that’s not a feasible solution to our current predicament—a physiological conundrum with no quick evolutionary fix.

So, if there’s no way our current physiology will buffer chronic stress, how can we redress this mismatch? According to Colin Shaw, one solution is to fundamentally rethink our relationship with nature—treating it as a key health factor and protecting or regenerating spaces that resemble those from our hunter-gatherer past. Another is to design healthier, more resilient cities.

“I’m not an engineer or an architect,” he says, “but our research can identify which stimuli most affect blood pressure or heart rate and pass that knowledge on to decisionmakers.”

And both avenues are deeply interconnected, he argues: “We need to get our cities right—and at the same time regenerate, value and spend more time in natural spaces.”

We have returned to his office with a standing desk and no chair. It seems like a small act of defiance against today’s sedentary lifestyle, which is so distant from our ancestral condition.

“As an evolutionary anthropologist, my earlier work focused on Neanderthals and bone adaptation, which was fascinating in its own right,” Shaw reflects. “But the challenges we face today feel more urgent. Those with the resources—financial or intellectual—have a responsibility to invest them in solving these problems. To me, it’s a moral imperative to do the right thing.”