Urban Life and the Rise of Chronic Stress in Modern Society
The Rise of Chronic Stress and the Evolutionary Mismatch
Chronic stress is becoming more prevalent, according to evolutionary anthropologist Colin Shaw. He argues that this increase stems from an evolutionary mismatch—our bodies and brains, which have evolved over hundreds of thousands of years for hunter-gatherer conditions, are now exposed to industrialized, urban environments. Is there a solution?
It's the end of July, and it's raining heavily in Zurich. I'm sitting on a foldable chair under the canopy of old-growth beech trees, protected by an umbrella. I take deep breaths as I listen to birdsong breaking through the steady sound of raindrops hitting the forest floor. Water flows down the wide tree trunks, having collected from the branches above. A knotty maze of roots 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. As an evolutionary anthropologist and head of the Human Evolutionary EcoPhysiology (HEEP) research group at the University of Zurich, he gave me an assignment when we arrived at this clearing on the edge of Zürichberg forest: choose your favorite spot, take in the environment, and 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 conducted last summer. The team, consisting of specialists from ecology, immunology, microbiology, cognitive psychology, and exercise science, had 160 people spend 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, hugging trees, playing with the soil microbiome, and everything else."
Before and after being exposed to the natural and urban environments, the researchers measured a range of biomarkers in participants' blood, saliva, and cognitive capacity. In the woods, people showed significantly lower blood pressure, better immune response, and improved psychological state. In contrast, those in the urban environment exhibited 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 jokes that the rain may have even improved the dispersal of phytoncides, volatile organic compounds released by trees that offer immune-boosting benefits to humans—as demonstrated by the forest bathing (shinrin-yoku) movement in Japan.
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 increases 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
As we continue our walk down Letzistrasse into the city, the water gushes down the gutter, 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."
