Everything that we’ve learned about the gut microbiota challenges traditional scientific beliefs, which is one reason why it has become a topic of so much interest and controversy, both in the world of science and the media. It is also the reason why some people are posing deeper, more philosophical questions about the impact of the microbiome: Are our human bodies just a vehicle for the microbes living in it? Do the microbes manipulate our brains to make us seek out foods that are best for them? Should the fact that we humans are outnumbered by nonhuman cells change our concept of the human self?

While such philosophical speculations are fascinating, the implications of what the science of the human microbiome has revealed so far in the last decade are equally profound. Just as the Copernican Revolution in the sixteenth century fundamentally changed our understanding of the world’s position in the solar system, and Darwin’s revolutionary theory of evolution proposed in the nineteenth century has forever changed our position within the animal kingdom, the human microbiome science is forcing us again to reevaluate our position on earth.

According to the new science of the microbiome, we humans are truly supraorganisms, composed of closely interconnected human and microbial components, which are inseparable and dependent on each other for survival. And most concerning is the fact that the microbial components are vastly greater than our human contribution to this supraorganism. Because the microbial component is so closely connected through a shared biological communication system to all the other microbiomes in the soil, the air, the oceans, and the microbes living in symbiosis with almost all other living creatures, we are closely and inextricably tied into the earth’s web of life. The new concept of the human microbial supraorganism clearly has profound implications for our understanding of our role on earth and for many aspects of health and disease.

How we hear them…

The microbes not only inhabit the inside of your gut; many of them sit on a razor-thin layer of mucus and cells that coats the inner lining of your intestine. In this unique habitat they are barely separated from the gut’s immune cells and the numerous cellular sensors that encode our gut sensations. In other words, they live in intimate contact with the major information-gathering systems in our body. This location allows them to listen in as the brain signals the gut how stressed you are, or when you feel happy, anxious, or angry, even if you are not fully aware of these emotional states. But they do more than just listen.

As incredible as this may sound, your gut microbes are in a prime position to influence your emotions, by generating and modulating signals the gut sends back to the brain. Thus, what starts as an emotion in the brain influences your gut and the signals generated by your microbes, and these signals in turn communicate back to the brain, reinforcing and sometimes even prolonging the emotional state.

When the first publications on this topic—mostly animal studies—appeared in the scientific literature some 10 years ago, I was skeptical of the results and implications, which just seemed to be too far outside the conventional view of medicine. However, after my research group at the University of California, Los Angeles, under the leadership of Kirsten Tillisch completed our own study in healthy human subjects, we were able to confirm the results of the animal studies—and I became determined to further explore the question of whether the interactions between the gut microbiota and the brain could affect our background emotions, social interactions, and even our ability to make decisions.

Is the proper balance of microbes a prerequisite for mental health? And when these connections between the mind and gut are altered, can they raise a person’s risk of developing chronic diseases of the brain? These questions are fascinating not only from a scientist’s perspective, but also from a human one: a better understanding of the gut–brain connection is urgently needed in view of the impact that many brain disorders have on human suffering and health care costs.