The microbiome is inﬁnitely more versatile than any of our familiar body parts. Your cells carry between 20,000 and 25,000 genes, but it is estimated that the microbes inside you wield around 500 times more. This genetic wealth, combined with their rapid evolution, makes them virtuosos of biochemistry, able to adapt to any possible challenge. They help to digest our food, releasing otherwise inaccessible nutrients. They produce vitamins and minerals that are missing from our diet. They break down toxins and hazardous chemicals. They protect us from disease by crowding out more dangerous microbes or killing them directly with antimicrobial chemicals. They produce substances that affect the way we smell. They are such an inevitable presence that we have outsourced surprising aspects of our lives to them. They guide the construction of our bodies, releasing molecules and signals that steer the growth of our organs. They educate our immune system, teaching it to tell friend from foe. They affect the development of the nervous system, and perhaps even inﬂuence our behaviour. They contribute to our lives in profound and wide-ranging ways; no corner of our biology is untouched. If we ignore them, we are looking at our lives through a keyhole.
This book will open the door fully. We are going to explore the incredible universe that exists within our bodies. We’ll learn about the origins of our alliances with microbes, the counter-intuitive ways in which they sculpt our bodies and shape our everyday lives, and the tricks we use for keeping them in line and ensuring a cordial partnership. We’ll look at how we inadvertently disrupt these partnerships and, in doing so, jeopardise our health. We’ll see how we might reverse these problems by manipulating the microbiome for our beneﬁt. And we’ll hear the stories of the gleeful, imaginative, driven scientists who have dedicated their lives to understanding the microbial world, often in the face of scorn, dismissal, and failure.
We won’t focus only on humans, either. We’ll see how microbes have bestowed on animals extraordinary powers, evolutionary opportunities, and even their own genes. The hoopoe, a bird with a pickaxe proﬁle and a tiger’s colours, paints its eggs with a bacteria-rich ﬂuid that it secretes from a gland beneath its tail; the bacteria release antibiotics that stop more dangerous microbes from inﬁltrating the eggs and harming the chicks. Leafcutter ants also carry antibiotic-producing microbes on their bodies, and use these to disinfect the fungi that they cultivate in underground gardens. The spiky, expandable pufferﬁsh uses bacteria to make tetrodotoxin – an exceptionally lethal substance which poisons any predator that tries to eat it. The Colorado potato beetle, a major pest, uses bacteria in its saliva to suppress the defences of the plants that it eats. The zebra-striped cardinalﬁsh houses luminous bacteria, which it uses to attract its prey. The ant lion, a predatory insect with fearsome jaws, paralyses its victims with toxins produced by the bacteria in its saliva. Some nematode worms kill insects by vomiting toxic glowing bacteria into their bodies; others burrow into plant cells, and cause vast agricultural losses, using genes stolen from microbes.
Our alliances with microbes have repeatedly changed the course of animal evolution and transformed the world around us. It is easiest to appreciate how important these partnerships are by considering what would happen if they broke. Imagine if all microbes on the planet suddenly disappeared. On the upside, infectious diseases would be a thing of the past, and many pest insects would be unable to eke out a living. But that’s where the good news ends. Grazing mammals, like cows, sheep, antelope, and deer would starve since they are utterly dependent on their gut microbes to break down the tough ﬁbres in the plants they eat. The great herds of Africa’s grasslands would vanish. Termites are similarly dependent on the digestive services of microbes, so they would also disappear, as would the larger animals that depend on them for food, or on their mounds for shelter. Aphids, cicadas, and other sap-sucking bugs would perish without bacteria to supplement the nutrients that are missing from their diets. In the deep oceans, many worms, shellﬁsh, and other animals rely on bacteria for all of their energy. Without microbes, they too would die, and the entire food webs of these dark, abyssal worlds would collapse. Shallower oceans would fare little better. Corals, which depend on microscopic algae and a surprisingly diverse collection of bacteria, would become weak and vulnerable. Their mighty reefs would bleach and erode, and all the life they support would suffer.
Humans, oddly, would be ﬁne. Unlike other animals, for whom sterility would mean a quick death, we would get by for weeks, months, even years. Our health might eventually suffer, but we’d have more pressing concerns. Waste would rapidly build up, for microbes are lords of decay. Along with other grazing mammals, our livestock would perish. So would our crop plants; without microbes to provide plants with nitrogen, the Earth would experience a catastrophic de-greening. (Since this book focuses entirely on animals, I offer my sincerest apologies to enthusiasts of botany.) ‘We predict complete societal collapse only within a year or so, linked to catastrophic failure of the food supply chain,’ wrote microbiologists Jack Gilbert and Josh Neufeld, after running through this thought experiment. ‘Most species on Earth would become extinct, and population sizes would be reduced greatly for the species that endured.’
Microbes matter. We have ignored them. We have feared and hated them. Now, it is time to appreciate them, for our grasp of our own biology is greatly impoverished if we don’t. In this book, I want to show you what the animal kingdom really looks like, and how much more wondrous it becomes when you see it as the world of partnerships that it actually is. This is a version of natural history that deepens the more familiar one, the one laid down by the greatest naturalists of the past.
I Contain Multitudes by Ed Yong is available now (The Bodley Head, £20)