In today’s hyper-sanitized, screen-driven world, many people have grown disconnected from the natural environments that shaped our physical and mental health for millennia. As we scrub our homes with antibacterial agents, wear shoes on every surface, and live inside temperature-controlled buildings, we have distanced ourselves from the very ecosystems—including bacteria and soil microbes—that once kept us resilient and well. This sanitized lifestyle, though modern and convenient, comes at a cost: rising rates of anxiety, depression, and chronic immune disorders. Human beings evolved in intimate contact with the natural world. Our ancestors hunted, farmed, and raised their children outdoors, constantly exposed to microbes in the soil, air, and on animals. These microbes weren’t just passengers; they were, and still are, essential allies.

The “Old Friends Hypothesis,” proposed by Dr. Graham Rook (2003), suggests that regular exposure to environmental microbes—particularly those found in soil and unpolluted natural environments—is crucial for the proper development of the immune system and for the regulation of inflammation, a process deeply tied to mental health. Chronic low-grade inflammation, often linked to urbanized, indoor lifestyles, has been found to underlie numerous psychiatric disorders, including depression and anxiety. One of the most fascinating discoveries in recent decades has been the connection between the gut microbiome and mental health. The gut-brain axis, a communication system between the gastrointestinal tract and the central nervous system, is heavily influenced by the diversity and health of gut bacteria. Studies like Turnbaugh et al. (2006) and Cryan & Dinan (2012) show that microbial imbalance in the gut can disrupt neurotransmitter production, particularly serotonin and dopamine, both of which regulate mood, stress response, and cognition.

Remarkably, exposure to soil-based microbes—especially Mycobacterium vaccae—has been shown to stimulate the production of serotonin and reduce anxiety-like behavior in mice (Lowry et al., 2007), leading researchers to investigate its potential antidepressant effects in humans. Spending time outdoors—gardening, hiking, or even walking barefoot on grass—restores microbial contact and promotes microbial diversity in the body. Children who grow up playing outside and interacting with animals are consistently shown to have stronger immune systems and lower rates of allergies, asthma, and emotional disorders. The Finnish Allergy Programme (2008–2018) supports this, showing that children raised in biodiverse rural areas have healthier immune and emotional outcomes compared to urban peers. Nature also provides indirect psychological benefits. The Japanese practice of shinrin-yoku or “forest bathing” has been associated with lower cortisol levels, improved parasympathetic nervous activity, and greater feelings of calm and connection (Li et al., 2007). These effects are partly attributed to exposure to forest microbes and natural plant compounds, which reduce inflammation and support immune regulation.

As poet Mary Oliver wrote, “You only have to let the soft animal of your body love what it loves.” And often, that is the earth itself—alive, rich with ancient microbes, healing through every breath, barefoot step, and handful of soil. Reconnecting with the microbial world isn’t just nostalgia—it’s medicine. Let nature in, and wellness follows.

References:

1. Haahtela, T., von Hertzen, L., Mäkelä, M. J., & Hannuksela, M. (2008). Finnish Allergy Programme 2008–2018—Time to Act and Change the Course. Allergy, 63(6), 634–645. https://doi.org/10.1111/j.1398-9995.2008.01645.x

2. Li, Q., Kobayashi, M., Inagaki, H., Hirata, Y., Li, Y., Wakayama, Y., … & Kawada, T. (2007). A day trip to a forest park increases human natural killer activity and expression of anti-cancer proteins in female subjects. Journal of Biological Regulators and Homeostatic Agents, 21(1), 45–55.

3. Oliver, M. (1986). Dream work. Atlantic Monthly Press.

*This article was co-created with the help of AI.