Humans like to see themselves as independent beings, believing the brain is the sole commander of behavior. But scientists increasingly find this story far less straightforward. Within our bodies resides a vast, diverse community of microbes. Quietly dwelling in our intestines, skin, and even mouth, they subtly shape how we perceive and respond to the world.

The gut is often called the “second brain.” Trillions of bacteria thrive there, communicating with the nervous system through complex chemical signals. Research indicates these microbes secrete substances that influence neurotransmitters like serotonin, dopamine, and gamma-aminobutyric acid (GABA)—molecules central to regulating mood, anxiety, and well-being. When we feel joyful, it may not only stem from life's smooth sailing but also from the quiet influence of our gut's deep-dwelling inhabitants.

Laboratory studies offer even more intriguing insights. Scientists transplanted gut bacteria from anxious mice into calm mice, and the “new subjects” immediately exhibited similar anxious behaviors. In other words, an individual's psychological state can be transferred through the migration of microbes. This discovery prompts the question: Are some of our personality traits also shaped, at least in part, by microorganisms?

Moreover, diet and lifestyle habits also influence the composition of the microbiome. Individuals who favor high-sugar, high-fat foods often possess gut microbiomes markedly different from those who consume light, fiber-rich diets. These differences extend beyond physical health, potentially influencing emotions and decision-making. Researchers have observed that gut microbial imbalances correlate with increased risks of depression and neurodegenerative diseases. The microbiome seems to act as a silent director, setting the tone for human behavior.

More intriguingly, scientists are exploring this connection to develop novel therapies. Through probiotics, fecal microbiota transplantation, and even customized diets, researchers aim to reshape the microbiome to alleviate anxiety or improve attention disorders. Though these approaches remain in their infancy, their potential sparks significant excitement within the medical community. Perhaps future psychiatric prescriptions will include both psychotherapy and a “microbiome conditioning plan.”

Beyond mental health, the microbiome may also influence social behavior. Research indicates that gut microbiota status affects an individual's desire for social interaction and tolerance for stress. In other words, whether we prefer solitude or seek group integration may not be solely determined by personality—it could also be linked to our internal microbial ecology. While this perspective remains controversial, it has opened entirely new dimensions for studying human behavior.

From a broader perspective, the relationship between humans and microbes reveals a profound symbiotic logic. We are not isolated individuals but “hosts” within a vast ecosystem. Thoughts, emotions, and behaviors do not originate solely from the brain's electrical signals but are part of a symphony orchestrated by countless living beings. Human free will may be intertwined with the murmurs of these microscopic organisms.

The next time we find ourselves puzzled by sudden emotional fluctuations, we might imagine that these aren't merely random surges of the mind, but gentle nudges from microbes deep within our guts. It turns out our inner world is far livelier than we imagined, and some of the “remote controls” are held by these microscopic residents invisible to the naked eye.

（Writer：Seli）