Charlie Stross has a reputation among science fiction authors as one of that community’s most outspoken critics of interplanetary travel. Any dreams about humans permanently colonizing any world but Earth, he maintains, foolishly ignores how every part and process of our bodies has evolved for complete interdependence with every aspect our home planet, well beyond obvious stuff like gravity and oxygen. You can’t just pop a plexiglas bubble on your head and fly to the stars like a cartoon spaceman; bereft of the only environment nature designed it for, your body will fail in short order. Literal extensions of the planet they evolved on, fragile humans simply cannot live anywhere but here.

The Stross viewpoint came to mind repeatedly as I read Ed Yong’s I Contain Multitudes: The Microbes Within Us and a Grander View of Life. The book surveys modern science’s rapidly growing understanding of microbiology, with a particular focus on the microbiome: the island-universe that each and every person and animal on earth becomes for invisible creatures beyond number. (Before reading this book, I don’t think I’d heard it expressed that an adult human hosts more microbes than there exist stars in our galaxy, and by many orders of magnitude.) And within my own lifetime-so-far, the role of microbes vis-à-vis animals has itself evolved from filth that lurks everywhere and sometimes makes us sick to an elemental presence wholly intertwined with the development and sustenance of complex terrestrial life (and which sometimes makes us sick).

You, and every friend you have with a spinal cord, evolved in a world caked with microbes, and you all carry this forward by allowing them to thoroughly colonize every inch of your outside-world interface — not just your visible parts, but down the ducts and tubing of your nose, your mouth, and all your miles of guts. In the gut especially, evolution has designed many of our most crucial life-sustaining processes to invite the inevitable little darlings to make themselves at home and help us help them in the most literally symbiotic ways, while also making sure they keep out of deeper tissues where they’d only cause trouble. Anywhere we go, they gotta come with us. We can’t live without them.

Beyond that, in our roles as microbiomes, we really do live second lives as different worlds. While all humans carry the same broad categories of bugs, we are all so different in the details, the makeup of our teeming trillions unique to each of us. The “flavor” of our tiny symbionts likely influences our relationship, our interfacing, with the outside world at least as much as our genetics. And on that note, with especial delight did I read about horizontal gene transfer. Microbes, far too simple for something as mechanically involved as sex, instead play in Darwin’s great game by just casually passing their DNA back and forth. It struck me as a squishy biological version of quantum mechanics, the rules about how mutation and evolution works breaking down and getting weird when viewed at a sufficiently small scale.

Through HGT, fecal-transplant therapy (about which this book holds a whole chapter), and other microbe-specific wonders lies dizzying future potential, one where we encourage the strains in our bodies to learn new tricks that also benefit we-their-hosts. Yong interviews scientists who let themselves imagine a twenty-years-on time when we might ingest tailored microbial sachets that reprogram our gut-buddies to boost our health and eat our diseases. Somewhat ironically, I can’t deny finding this idea cleaner than the still-foretold future of injected nanobots picking the plaque from our arteries and so on. How elegant, to retrain the living tools we were all quite literally born into, rather than reinventing them from scratch.