For humans, nature has never been less threatening. We have conquered many of the diseases that felled our ancestors when they were half as old as the average person lives to be now, and few people die anymore in the jaws of predators. Antibiotics keep bacteria at bay, and the spread of cities and towns has pushed lions and tigers and bears away from our doors. We call this progress. Who would want to go back to, say, a time before penicillin?

But driving other species out of our lives has had some unintended consequences, argues biologist Rob Dunn in his provocative book 

. For most of human history, we lived in proximity to countless other species. We evolved in concert with these life forms; everything from the tiniest microbes to the most fearsome predators shaped the bodies and brains we’re walking around with today. “What happens,” Dunn asks, “when humans leave behind the species their bodies evolved to interact with, whether they be cheetahs, diseases, honeybees, or giant sucking worms?”

The answers Dunn provides aren’t pretty. Without these other life forms, he argues, many features of our own bodies have “become anachronistic or worse.” The unlikely Exhibit A: Crohn’s disease, a chronic inflammation of the bowel that is on the rise in developed nations. Crohn’s and other autoimmune disorders are most common in exactly those places where public health seems to be most advanced—where, for instance, the intestinal parasites that plagued humans for much of history, such as hookworms and tapeworms, have become staggeringly rare. What if, scientists wondered, the absence of these parasites was somehow leaving us more vulnerable to various maladies of the immune system?

The idea sounds crazy—and the experiments conducted to test it even crazier. In 1999, one scientist enlisted 29 patients with intractable Crohn’s and asked them to infect themselves with parasites. Each swallowed a glass of whipworm eggs. Six months later, after the worms had had a chance to take up residence in these patients’ guts, 21 of the volunteers were symptom free.

Soon, other researchers were linking most or all autoimmune diseases to our missing parasites—as well as many allergy and asthma cases. Reintroducing intestinal worms has been shown to alleviate the symptoms of inflammatory bowel syndrome, diabetes, heart disease, and multiple sclerosis. Whatever the exact mechanism—scientists are still working it out—intestinal parasites appear to help train the immune system to work properly. (Perhaps, without parasites, the immune system looks for something else to attack—such as the body itself.) But prescribing parasites is still a counterintuitive and controversial idea that doctors and patients find hard to swallow.

The disappearance of our intestinal parasites is but one stop on Dunn’s breathtakingly broad look at the other species that have shaped human evolution. He moves seamlessly from the physiology of the digestive system to the circuitry of the brain. He explains, for instance, how the disappearance of most of the animals that preyed on humans has left us with outdated fear circuitry. (Primed to detect threats that no longer exist, some brains remain on hyperalert, giving their owners chronic stress and anxiety disorders.)

Dunn does this all in prose that not only evokes the wonder of scientific discovery but illuminates the human struggles behind it. Though the picture he paints is sometimes bleak, he leaves the reader with a bit of hope. We needn’t restore our environments to their original wild state, Dunn says, and it’s a good thing, since that would be nearly impossible. Instead, we could embark upon some targeted rewilding, reintroducing specific parasites to our guts or chugging down cocktails of beneficial bacteria. It wouldn’t hurt to curb our use of antibacterial soaps, as well as antibiotics and other medicines, since they destroy bacteria indiscriminately, without consideration of whether some of the bugs might do us good.

And we could create natural oases, such as urban farms, which could bring certain plants and animals back into our lives. “Given the choice of which and how many species to live around (a choice we still have, though not forever), why not consciously garden a greater diversity of life around us?” he writes. It’s a tall order, but Dunn makes a compelling case that our health and happiness may depend upon it.

 is a science writer who lives in Brooklyn. She is working on a book about biotechnology and the future of animals.

The Wild Life of Our Bodies: Predators, Parasites, and Partners That Shape Who We Are Today

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For humans, driving other species out of our lives has had some unintended consequences.

Animals Are Us

What makes us who we are? The question has captivated philosophers and scientists for millennia, from the ancient Greeks and their humors to the 19th-century phrenologists who believed they could divine someone’s personality by mapping the bumps on his skull. The personality question captivates us still, though the answers have gotten more complicated—and microscopic. But fret not: To help navigate the modern maze of neurotransmitters, animal models, and evolutionary theory, we have science writer Hannah Holmes and her book Quirk.

Today, many psychologists divide our temperaments into a handful of major components known as “The Big Five”: neuroticism, extraversion, agreeableness, conscientiousness, and openness. Each of these factors gets further broken down into six facets—agreeableness, for instance, includes the facets of trust, cooperation, and altruism. How people score on each of these and other dimensions can provide snapshots of their unique personalities.

Holmes devotes each chapter to a single facet (she covers 18 in all), using the same formula to explore each trait. She begins with a peek into a lab where rodents are bred to exhibit one particular facet. In the anxiety chapter, for example, we meet the Nervous Nellies, mice that cower in the dark corners of elevated mazes, afraid to venture into the open. Such animal subjects are helping researchers unpack the biological underpinnings of personality. Want to create one of these fretful rodents? Disrupt a gene that regulates serotonin, a neurotransmitter that’s also been implicated in depression.

Then Holmes shows that the same chemicals and genes influence human personalities. Study a group of anxious female college students, and you’ll find that they, too, have genetic variants that alter the way their brains process serotonin. Finally, Holmes explores why such diversity of personality exists in the first place. “Why,” she asks, “would evolution produce animals that appear to be overly fearful, and overly bold?”

Biologists have long known that, from an evolutionary point of view, a diverse population is a healthy one. A species that includes creatures of all shapes, sizes, and talents is more likely to have some individuals that can survive whatever challenge the world throws at them.

It’s not a new idea, but it’s interesting to consider in relation to personality. Let’s look again at anxiety. When food was scarce, brave animals willing to travel to fill their bellies would have had an obvious evolutionary advantage. But venturing away from home comes with risks—of becoming lunch for other critters, say, or suffering a grievous injury. So when food was plentiful and easy to find, the nervous homebodies would be more likely to survive and pass on their high-anxiety genes.

Holmes has a gift for making complex science clear and accessible, but Quirk has one flaw: It exhibits more than a whiff of biological determinism. Consider the discussion of altruism. Researchers have found that a neurotransmitter known as vasopressin is connected to altruism. The gene responsible for the vasopressin system comes in two forms—a short gene, which leads to less vasopressin, and a long gene, which leads to more of it. On average, Holmes writes, “people with two copies of the short version of the vasopressin gene give the least; those with one long and one short copy are middling; fools who give the shirt off their back are most likely to have two copies of the long version. Vasopressin and altruistic behavior look as though they’re bound in holy matrimony. Or in biological matrimony, at any rate.”

The genetic effect on altruism is indeed remarkable, but DNA is not the end of the story. To assess generosity, scientists often use a game in which players are given a small amount of cash and asked to decide how much to contribute to a common pool of money. Noting, for example, that subjects from cultures in which offering charity is insulting tend to give little, Holmes concedes culture’s influence on this experiment.

It’s an observation that warrants more than a couple of sentences in a chapter chock-full of molecular biology. Even if Holmes wanted to maintain her rigid focus on genes and neurotransmitters, it would have been worthwhile to remind readers that brains and genes are just part of the puzzle. With personality, as with most human traits, it’s nature and nurture. Holmes’s book, interesting as it is, addresses only half of the equation.

 is a science writer who lives in Brooklyn. She is working on a book about biotechnology and the future of animals. 

"Quirk: Brain Science Makes Sense of Your Peculiar Personality"

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The question has captivated philosophers and scientists for millennia.

Emily Anthes