Ed Yong – I Contain Multitudes: Book Review & Audio Summary

Book Info

• Book name: I Contain Multitudes: The Microbes Within Us and a Grander View of Life
• Author: Ed Yong
• Genre: Science & Technology
• Pages: 416
• Published Year: 2016
• Publisher: Ecco
• Language: English
• Awards: Winner of the 2017 Wellcome Book Prize, Finalist for the 2017 National Book Critics Circle Award for General Nonfiction

Audio Summary

Synopsis

In I Contain Multitudes, award-winning science journalist Ed Yong takes us on a fascinating journey into the hidden world of microbes that live within and around us. Far from being simply disease-causing villains, these microscopic organisms are essential partners in our survival, shaping everything from our immune systems to our behavior. Yong reveals how microbes have been engineering evolution for billions of years, creating symbiotic relationships that give animals remarkable abilities. Through captivating storytelling and cutting-edge research, he shows us that we are not solitary individuals but complex ecosystems, containing trillions of microbial cells that outnumber our own. This paradigm-shifting book challenges our understanding of what it means to be human and alive.

Key Takeaways

• Microbes outnumber human cells in our bodies and play crucial roles in digestion, immunity, and development—we’re walking ecosystems rather than isolated individuals
• These microscopic organisms have shaped Earth’s atmosphere and environment for billions of years, making complex life possible through photosynthesis and nutrient cycling
• Symbiotic relationships between microbes and animals create remarkable adaptations, from glowing squid to insects that can survive extreme conditions
• Our microbiome begins forming at birth and is essential for calibrating our immune system, with breast milk specifically designed to feed beneficial gut bacteria
• Understanding our microbial partners opens new frontiers in medicine, from treating diseases to rethinking what antibiotics really do to our internal ecosystems

My Summary

The Invisible Architects of Our World

I’ll be honest—before reading Ed Yong’s I Contain Multitudes, my relationship with the word “bacteria” was pretty much limited to reaching for hand sanitizer and avoiding questionable leftovers in the back of my fridge. Like most people, I’d been conditioned to think of microbes as the enemy, those nasty little invaders that make us sick and ruin our day. But Yong completely flipped that narrative on its head, and I haven’t looked at my own body—or the world around me—the same way since.

What struck me most powerfully about this book is how Yong manages to make you feel both incredibly small and remarkably significant at the same time. He opens with a mind-bending perspective on time: if Earth’s entire 4.5-billion-year history were compressed into a single calendar year, humans would only show up in the last 30 minutes of December 31st. Microbes, on the other hand, have been around since March. That’s not just a long time—that’s practically forever in biological terms.

And during all those billions of years, microbes weren’t just sitting around. They were actively building the world we inhabit today. They created our oxygen-rich atmosphere through photosynthesis, established the carbon and nitrogen cycles that make life possible, and colonized every conceivable environment on Earth. From Antarctic ice to underwater volcanoes reaching 400 degrees Celsius, microbes found a way to thrive.

We Are Never Alone

Here’s where things get really personal, and honestly, a bit overwhelming in the best possible way. You’ve probably heard the claim that we have ten times more bacterial cells than human cells in our bodies. Yong corrects this exaggeration with actual numbers, but the truth is still astounding: we have about 69 trillion cells total, and over 39 trillion of them are microbial. We are, quite literally, more “them” than “us.”

But it’s not just about cell count. Consider this: the human genome contains roughly 20,000 genes. When you include all the microbial genes we carry around, that number explodes to over 500 times larger. Think about that for a moment. The genetic instructions governing your body aren’t just coming from your parents’ DNA—they’re coming from entire communities of microscopic organisms that have set up shop throughout your body.

What really fascinated me was learning that each part of your body hosts a different microbial community. Your gut microbiome is completely different from the one on your skin, which differs from the one in your mouth. Yong calls these communities “microbiomes,” and he describes them as ecosystems every bit as complex as a rainforest or coral reef. Each ecosystem has dominant species that act like community leaders, maintaining balance and ensuring everything functions properly.

In my years of reading science books for Books4soul.com, I’ve come across plenty of technical explanations of bodily systems, but Yong’s approach feels different. He doesn’t just list facts—he makes you understand that you are not an individual but a collective. It’s a humbling and somewhat strange realization. Every time I’ve felt alone since reading this book, I’ve had to laugh a little. Alone? I’m carrying around trillions of microscopic companions everywhere I go.

Your Microbes Are Raising You

One of the most profound sections of the book deals with how microbes shape our development from the very beginning of life. Yong explains that breast milk contains over 200 nutrients, including special sugars called human milk oligosaccharides, or HMOs. Here’s the kicker: babies can’t actually digest HMOs. They’re not there for the baby at all—they’re there to feed a specific gut microbe called Bifidobacterium infantis.

When this microbe digests HMOs, it releases proteins that babies can digest, including anti-inflammatory compounds that coat the gut and calibrate the developing immune system. In other words, breast milk is designed to feed the bacteria that will then help feed and protect the baby. It’s not just mother and child—it’s mother, child, and an entire microbial support network all working together.

This completely changed how I think about child development. We often talk about nature versus nurture, but Yong introduces a third factor: microbes. They’re neither purely genetic nor purely environmental—they’re somewhere in between, passed from mother to child but also acquired from the environment, and absolutely essential for proper development.

The research Yong presents on mice is particularly illuminating. He describes a family of gut microbes called Bacteroides thetaiotaomicron that activate specific genes during development. These microbes ensure that mice form the right blood vessels and that their guts develop the capacity to break down toxins and synthesize nutrients. Without these microbial partners, development goes wrong.

As someone who’s always been interested in how we become who we are, this added an entirely new dimension to my understanding. It’s not just our genes and experiences shaping us—it’s also the invisible microbial communities that colonize us from birth.

Superpowers Through Partnership

If you need convincing that microbes are more than just passive residents, Yong provides example after example of animals that have gained remarkable abilities through their microbial partnerships. These symbiotic relationships are so important that many animals literally cannot survive without them.

One of my favorite examples from the book involves the Hawaiian bobtail squid. These small squid have a special light organ on their underside that houses bioluminescent bacteria called Vibrio fischeri. The bacteria glow, and the squid uses this light for counter-illumination—when hunting at night, the glow on their underside matches the moonlight filtering down from above, making them invisible to predators looking up from below. It’s nature’s own cloaking device, and it only works because of a microbial partnership.

Then there’s the example Yong opens one section with: the tentiform leaf miner moth. In autumn, when most leaves turn yellow and fall, some remain green. Why? Because leaf miner moths form their cocoons on leaves, and they carry a microbe called Wolbachia that produces hormones to keep the leaf green and attached to the tree. The moth gets to complete its development in safety, all thanks to its microscopic partner.

These aren’t isolated curiosities—they’re examples of a fundamental principle of life on Earth. Symbiosis with microbes is everywhere, giving animals abilities they could never have evolved on their own. Some microbes help insects digest plant matter that would otherwise be indigestible. Others produce nutrients that their hosts can’t make themselves. Still others provide protection against parasites or predators.

Reading about these partnerships made me think differently about evolution itself. We tend to imagine evolution as a slow process of random mutations and natural selection, with individual organisms competing for survival. But Yong shows that evolution is also about cooperation, about forming partnerships that benefit everyone involved. Microbes can share genes with each other rapidly, passing along useful adaptations much faster than traditional evolution through reproduction. This means that acquiring new abilities doesn’t always require millions of years—sometimes you just need to partner with the right microbe.

Rethinking Health and Disease

After learning all this, I found myself reconsidering everything I thought I knew about health and medicine. If we’re ecosystems rather than individuals, then disease isn’t just about a foreign invader attacking us—it’s often about our ecosystem falling out of balance.

Yong doesn’t dive too deeply into medical applications in the sections I read, but the implications are clear. When we take antibiotics, we’re not just killing bad bacteria—we’re carpet-bombing our entire internal ecosystem. Sometimes that’s necessary, but it comes with consequences. We’re wiping out beneficial microbes along with harmful ones, potentially disrupting systems we don’t fully understand.

This has practical implications for how I think about my own health. I’m more cautious now about unnecessary antibiotic use. I’m more interested in probiotics and fermented foods that might support my gut microbiome. I’m even more forgiving of a little dirt—the old saying about kids needing to “eat a pound of dirt” before they grow up might have more truth to it than we realized. Exposure to diverse microbes, especially early in life, might be essential for developing a robust immune system.

Of course, Yong is careful not to oversell the microbiome as a cure-all. The science is still developing, and there’s a lot we don’t understand. But the fundamental shift in perspective—from seeing ourselves as sterile individuals to recognizing ourselves as complex ecosystems—opens up entirely new ways of thinking about health, disease, and treatment.

Why This Matters in Our Modern World

Living in our sanitized, antibiotic-saturated modern world, we’ve become somewhat disconnected from the microbial reality that shaped all life on Earth. We wage war on bacteria with antibacterial soaps, hand sanitizers, and cleaning products that promise to kill 99.9% of germs. But Yong’s book suggests we might be fighting the wrong battle.

Our modern lifestyle may be disrupting our microbiomes in ways we’re only beginning to understand. The rise in autoimmune diseases, allergies, and inflammatory conditions in developed countries might be linked to impoverished microbiomes—the result of C-sections (which bypass the birth canal where babies normally pick up their first microbes), formula feeding, excessive antibiotic use, and overly sterile environments.

This doesn’t mean we should abandon hygiene or modern medicine. But it does suggest we need a more nuanced approach. Not all bacteria are bad. In fact, most are neutral or beneficial. We need to distinguish between harmful pathogens and helpful partners, and we need to think about how to preserve and support our beneficial microbes even while fighting disease.

For me personally, this book has influenced small but meaningful changes in my daily life. I’m less obsessive about sterilizing everything. I eat more fermented foods like yogurt, kimchi, and sauerkraut. I spend more time outdoors, exposing myself to environmental microbes. I think more carefully before taking antibiotics, making sure they’re truly necessary. These aren’t dramatic changes, but they reflect a shift in mindset—from viewing microbes as enemies to recognizing them as partners.

The Writing That Makes It All Accessible

I should take a moment to appreciate Yong’s writing itself, because it’s a big part of what makes this book work. Science writing can easily become either too technical (losing general readers) or too simplified (frustrating those who want real depth). Yong strikes a beautiful balance.

His background as a science journalist for publications like The Atlantic and National Geographic really shows. He knows how to find the story in the science, the human element in the research. He introduces us to the scientists doing this work, shares their excitement and frustrations, and helps us understand not just what they discovered but how and why.

The book is full of vivid imagery and unexpected connections. When he describes microbes as being so small that a million could fit on the head of a pin, it gives you a visceral sense of scale. When he compares Earth’s history to a calendar year, it makes deep time comprehensible. These aren’t just rhetorical flourishes—they’re essential tools for helping readers grasp concepts that are genuinely difficult to wrap your mind around.

That said, I can see why some readers might find certain sections dense. Yong doesn’t shy away from scientific detail, and there are passages that require focus and attention. He’s writing for a general audience, but an engaged one. This isn’t a book you can skim while half-watching TV. It rewards careful reading and thinking.

Comparing Perspectives on the Microbial World

Having read quite a few popular science books over the years, I found myself comparing I Contain Multitudes to other works that touch on similar themes. Books like Bill Bryson’s A Short History of Nearly Everything cover some of the same ground regarding Earth’s history and the origins of life, but with a broader scope and lighter touch. Bryson is more of a tourist guide through science, while Yong is more like a specialist guide focusing on one particular ecosystem.

For readers interested specifically in gut health, books like Giulia Enders’ Gut: The Inside Story of Our Body’s Most Underrated Organ take a more medical and practical approach. Enders focuses primarily on human digestion and health, while Yong takes a much wider view, looking at microbes across all species and their role in evolution and ecology.

What sets Yong’s book apart is its scope and its fundamental argument about what it means to be an individual organism. He’s not just writing about interesting microbial facts—he’s arguing for a complete reconceptualization of life itself. We’re not individuals but collectives, not autonomous but interdependent, not separate from nature but intimately woven into it through our microbial partners.

Questions Worth Pondering

As I finished the book, I found myself sitting with some big questions. If we’re truly ecosystems rather than individuals, what does that mean for concepts like identity and selfhood? When I make a decision, how much of that is “me” and how much is influenced by the microbial communities in my gut? Research has shown that gut bacteria can influence mood, behavior, and even food cravings. It’s a bit unsettling to think that my desire for certain foods might be coming from bacteria that thrive on those foods.

There’s also the question of how we should approach medical interventions. If disrupting our microbiome has consequences we don’t fully understand, how do we balance the immediate need to fight disease against the long-term need to maintain a healthy microbial ecosystem? It’s a question that doesn’t have easy answers, but it’s one worth grappling with.

Final Thoughts From My Reading Chair

I Contain Multitudes is one of those rare books that genuinely changes how you see the world. After reading it, I found myself noticing things I’d never paid attention to before—the fermentation bubbles in my sourdough starter, the way my dog rolls in the grass (picking up environmental microbes), even the slightly musty smell of autumn leaves (decomposition, powered by microbes).

Yong has given us a gift: a new lens for understanding life on Earth. He shows us that we’re never alone, that we’re part of ancient partnerships stretching back billions of years, and that the invisible world of microbes is just as important as the visible world we normally focus on.

Is the book perfect? Not quite. Some sections can feel dense, and readers without much science background might occasionally feel lost in the details. Yong also focuses heavily on the fascinating science while sometimes leaving practical applications as an afterthought. But these are minor quibbles about a book that achieves something remarkable: making you fall in love with organisms you can’t even see.

For anyone interested in biology, health, or simply understanding the world we inhabit, I Contain Multitudes is essential reading. It’s the kind of book that makes you want to grab friends and family and say, “Did you know…?” followed by some mind-blowing fact about the microbial world.

I’d love to hear from others who’ve read this book. Has it changed how you think about your own body? Have you made any changes to your lifestyle based on what you learned? And for those who haven’t read it yet—what’s your relationship with microbes? Do you think of them as enemies, or are you ready to welcome them as partners? Drop your thoughts in the comments below. Let’s keep this conversation going, because understanding our microbes might just be one of the most important scientific frontiers of our time.