My body is crawling with life. Fungi live on my feet. Trillions of bacteria cling to my legs and up among my bits. In this mix live Staphylococcus hominis and aureus, which are found on my skin, and Enterococcus mundtii, the most common denizen of my belly button. But they are just the tip of the “life berg.”

Nearly two hundred species of bacteria have been found residing on human forearms alone. Some are probably bad; some, good; and some, essential. Others may simply be sojourners, pausing en route to some more distant shore. Inside my gut, there might be worms of several different species. There are certainly hundreds of species of microbes in my digestive tract upon which I rely to fully digest my food. There are fungi inhabiting my lungs. There are, I suspect, mites living in the pores of my...

All humans on Earth share their bodies with multiple biomes of many other species from which our existence is inextricable. Cataloging and understanding these organisms and how they are evolving as our lifestyles change—as we give up old, dirty ways for new, clean (and in some ways less healthy) ones—is part of what I write about in my new book, The Wild Life of Our Bodies.

Of course, these microscopic creatures are not the only partners in our dance. I also have a tiger in my body, snakes slithering among my neurons, honeybees and thousands of other species ingrained in my being. These ancestral predators and collaborators have all left deep signatures in my genome.

My relationship to the deadly and damaging organisms that influenced the fate of my ancestors, be they bacteria, or bears, is similar to that which exists between land plants and gravity. Plants that grow on land have evolved hormones that help them sense which way is up so they can grow against gravity. They require special structures in and around their cells to keep their flowers, leaves, and stems from surrendering to the planet’s mass. Evolutionarily, gravity killed, or at least disfavored, ancestral or proto-plants that did not adapt to its inescapable presence.

Yet, when NASA scientists grow plants in the microgravity of space, they become crooked and sick. Without gravity, up and down are rendered meaningless, and the plants suffer.

Something similar has happened with my body, and yours, as modern humans have altered our relationships with our evolutionary dance partners. Parasites and predators were the gravity that attended the evolution of our species. They were the forces which directed our history. The fight-or-flight response, for example, which prompts my body to make snap decisions in times of danger, anxiety, or stress, was honed and shaped by the dependable threat of snakes and other predators. My immune system evolved, in part, to cope with the constant presence of worms. Nor is it simply our ancient nemeses that shaped who we are. My taste buds evolved to lead me toward foods that were once rare but extremely valuable (salts, sweets) and away from those that were deadly.

But through technology and “civilization” we have changed the fate of the species that were such an intimate part of our development. We kill off the predators from around our villages. We eradicate the worms from our guts. We scrub some of the bacteria from our hands and faces.

The Wild Life of Our Bodies recounts the stories of the species whose influence persists in and on our bodies and explores the consequences of the evolutionary and ecological changes we have forced them to make. We may be able to grow a plant on the Moon (or near the Moon, at least), but we cannot escape the gravity of life, the forces that shaped us. As high as we jump, we always fall back down into the rest of life—that web of interactions and wild species from which we are made.

Rob Dunn is a writer and biogeographer at North Carolina State University (www.robrdunn.com). His writing explores the stories of the scientists who seek to understand our lives and the lives of the species with which we interact. His first book, Every Living Thing: Man’s Obsessive Quest to Catalog Life, from Nanobacteria to New Monkeys, won the National Outdoor Book Award for Natural History writing. His own science probes the geography of life—most recently of ants, belly-button microbes, and human diseases. Read an excerpt from The Wild Life of Our Bodies, Chapter 9--"We Were Hunted, Which is Why All of Us are Afraid Some of the Time and Some of Us are Afraid All of the Time."

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