“Remember that animals emerged in a world that had already been teeming with microbes for billions of years. They were the rulers of the planet long before we arrived.”
—Ed Yong, I Contain Multitudes
Inside you, there is a garden. It teems with microscopic communities of life forms—microbes including bacteria, viruses, fungi, and protozoa—that are rich in purpose we are only beginning to understand. This garden also envelopes you, growing on all your surfaces to form an invisible barrier. It is the threshold where you interface with the world, a world rich with its own unseen gardens. What I’m talking about here is your microbiome: the world within you.
Humans might act as if the Earth belongs to us, but microbes were here long before us. Bacteria and archaea were the first stirrings of life on Earth—our earliest ancestors. And they comprise much more of life on this planet than we do. Despite the scale and reach of our impact on the planet, humans only make up .01% of its biomass (as of the most recent research on this, published in 2018). By comparison, microbes make up 13-15%, the second highest of all life forms (after plants, which make up more than 82% of the planet’s biomass).
You are about as microbial as you are human. Your body contains around 37 trillion human cells, but it also contains around 39 trillion microbes in your microbiome. The microbes in your gut help you digest complex foods, synthesize essential vitamins, and moderate your metabolism. Your microbiome is also crucial in developing and maintaining your immune system, identifying and distinguishing between harmful pathogens and innocuous antigens. Your well-being is entangled with and dependent on the invisible life that flourishes within you.
Your microbiome may impact not only your physical but mental health, as emerging research is finding. Mounting evidence has uncovered a pathway of communication between your gut microbiome and your central nervous system, called themicrobiota–gut–brain axis. There are 86 billion neurons in your brain and 168 million neurons in your gut, a kind of second brain. Studies have even shown that gut microbiota can produce or stimulate the production of neurotransmitters like serotonin and dopamine that significantly impact your mood.
Microbes sustain the lives of more creatures than just humans. Corals exist as a symbiosis between the coral animal and a community of microbes. Termites rely on gut microbes that are capable of breaking down tough matter like wood. Leafcutter ants create fungal farms where they feed plants to gardens of microbes that break them down and produce nutritious foods for the ants. Bioluminescent beings of the deep, like squids and anglerfish, rely on glowing bacteria to live. Behind some of the most marvelous life forms, invisible microbes are pulling the strings.
Beyond individual lives, microbes are essential for keeping the planet healthy—starting with the very ground we walk on. Soil microbes are especially integral to the global carbon cycle; the microbial health and carbon-use efficiency of soil determines how much carbon can be stored versus released into the atmosphere. They are our allies in fighting the climate crisis. Microbes in the ground also break down organic matter and release nutrients in the process, helping to maintain healthy soil along with the larger balance of life and death that sustains the Earth.
This planet is tended by innumerable invisible hands. When progress feels undetectable, I remind myself that much of life plays out on unseen stages. We must all tend to our inner gardens, as well as the gardens that we are but one small part of. To be human today is to exist at many scales at once, stories we can grasp and ones much larger than us. Then again, one of the most important shifts we can make today might be to shed such isolating and individualistic labels as being human. We are kept alive by communities—we are communities—and so is the Earth.
Biome
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