Книга: The Human Age

Meet My Maker, the Mad Molecule

Meet My Maker, the Mad Molecule

Returning to our mystery redhead in the movie theater—what else could we learn about her from a strand of hair or blood sample? Her DNA profile, resembling a supermarket barcode, is a monumental accomplishment, but it’s only a fraction of her story. For a fuller picture of her health and heredity, we would need to include the teeming seashores of her microbes, the rest of her being—in fact, more than her being. Another self, a shadow self. At any moment, she is inseparable from trillions of her single-celled, single-minded, naked companions, some of whom don’t have her best interests at heart.

When she weighed herself earlier today, she may have deducted a pound or two for clothes and shoes. But did she take into account the roughly three pounds of microorganisms that inhabit her crevices and innards? Probably not. She’d need an atomic scale to start with, and anyway microbes are shifty, jumping off her peninsulalike feet and climbing aboard elbows pell-mell; they’re not easy to tally.

Microbes are the most fruitful life form on Earth, colonizing all sorts of ardent unspoken strangers, creating small sulfurous rumblings in the animal belly, reveling in the smell of fish and old shoes, and leaving aftertastes in the mouth stale as bus-station sandwiches. They’re also real workhorses, fluting the air until it’s breathable, promoting photosynthesis on the land and in the oceans, decomposing dead organisms and recycling their nutrients. In industry, we breed them to ferment dairy products and to process paper, drugs, fuel, vaccines, cloth, tea, natural gas, precious metals; they help mop up our oil spills. We yoke them like oxen and set them to work. But just as most of the mass in the universe (94 percent) is “dark matter,” this largest biomass on our planet escapes the naked eye, yet is the invisible Riviera of the visible world.

How remarkable it is that we’re not only renaming our age, we’re on the threshold of redefining ourselves as a completely different kind of animal than we ever imagined. For years, we thought DNA told the whole story. Instead we find that each person is a biological extravaganza of ten trillion microbes and one trillion human cells. It’s amazing we don’t slosh or disintegrate as we walk. Here’s the thing: on a microscopic level we do, while constantly adding new microbes from other people, plumes of dust, and the plants and animals we encounter.

In only the past ten years, our picture of a human being has evolved from a lone animal to a team of millions of life forms working in unison for mutual benefit. Unrelated people may be widespread from Tierra del Fuego to Quaanaaq, but there’s a movement afoot to classify human beings as “eusocial,” a single unit of highly sociable life forms who can’t survive all by themselves. Earth favors similar collectives—ants, bees, termites, coral, slime mold, naked mole rats, etc.—in which individuals pool their know-how to act for “the sake of the hive.” Thanks to the Web and social media, we’re discovering what a bustling rialto each person really is, and also how connected we all remain. Worlds within worlds, each of us is a unique ambulatory superorganism who belongs to one miscellaneous species living on the body of a colossal superorganism of a planet in a waltz of innumerable galaxies sprinkled with other Gaia-like planets and likely their own life forms percolating with untold hangers-on.

A marvel of the Human Age is that, in the past decade alone, we’ve mapped both the DNA in our cells and the DNA in our microbes. In the hunt we’ve discovered that a true view of ourselves as a life form is more untidy than we thought, and unglimpsed by most of us, a cloud of entwined bugs and human cells in a semipermeable frame. Joshua Lederberg, the Nobel-laureate biologist who, in 2000, coined the term “microbiome,” defines it as “the menagerie of the body’s attendant microbes.” Amid the hoopla surrounding the Human Genome Project, he urged, “We must study the microbes that we carry within us and on our surfaces as part of a shared embodiment.”

If the Human Genome Project was a landmark feat of discovery, the Human Microbiome Project is gene cartography’s finest hour. NIH director Francis Collins compares it to “fifteenth-century explorers describing the outline of a new continent,” a triumph that would “accelerate infectious disease research in a way previously impossible.”

For five years, a consortium of eighty universities and scientific labs sampled, analyzed, and audited over ten thousand species that share our human ecosystem, thus mapping our “microbiome,” the normal microbial makeup of healthy adults. And the quest continues.

The researchers have found that each of us contains a hundred trillion microbial cells—ten times more than our human cells. When they peered deeper and compared the genes, they realized that we carry about three million genes from bacteria—360 times more than our own human code. Among the hundred or so large groups of bacteria, only four specialize in the human body. They’ve been sidekicks for so long that over time our fate has fused with theirs.

So, odd as it sounds, most of the genes responsible for human survival don’t descend from the lucky fumblings of sperm and egg, don’t come from human cells at all. They belong to our fellow travelers, the bacteria, viruses, protozoans, fungi, and other lowlife that dine, scheme, swarm, procreate, and war all over us, inside and out. Vastly more bacteria than anything else. All alone our moviegoer could be arrested for unlawful assembly. She doesn’t propel a solid body but a walking ecosystem.

They also learned that we all carry pathogens, microorganisms known to spark disease. But in healthy people, the pathogens don’t attack; they simply coexist with their host and the rest of the circus tumbling and roaring inside the body. The next mystery to crack is what causes some to turn deadly, which will revamp our ideas about microbes and malady.

We’ve known about bacteria for 350 years, ever since a seventeenth-century Dutch scientist, Antonie van Leeuwenhoek, slipped some of his saliva under a homemade microscope, which he had crafted with lenses made from whiskers of glass, and espied single-celled organisms crawling, sprawling, flailing about in the suburbs of our gums. He named them animalcules and peered at them through a vast array of lenses (an avid microscoper, he made over five hundred).

In the nineteenth century Louis Pasteur proposed that healthy microbes might be vital, and their absence spur illness. By the time tiny viruses were discovered, only a hundred years ago, people were already driving cars and flying airplanes. But we didn’t have the tools to study the every-colored, shifting, scented shoal of microbes we swim in, play in, breathe in all the day long. Some cross the oceans on dust plumes. Acting as condensation nuclei, they jostle rain or snow until it falls from clouds. Far from being empty, the air, like the soil, throbs with flecks and dabs of life, more like an aerial ecosystem than a conveyor belt for clouds.

We need to reimagine the air, not as a desolate ether but as a lively, largely invisible, ecosystem. As we peer through its glassy expanse to a far trail or up at a billowing cloud, nothing blocks our view, the whole corridor looks vacant, and yet it’s a community pulsing with life. Our eyes merely slide over its tiniest tenants. The sky is really another kind of ocean, and even though we sometimes used to refer to “oceans of air,” we imagined barren currents; we didn’t realize how life-soaked the waves really are.

When David Smith and his colleagues at the University of Washington sampled two large dust plumes that had sailed across the Pacific from Asia to Oregon, they were surprised to find thousands of different species of microbes in the plumes, plus other aerosols, dust particles, and pollutants. All suspended and wafting around the planet, tromboning and floating, interacting with life.

In this panoramic new portrait, the Anthropocene body is no longer an entity that’s separate from the environment, like a balloon we pilot through the world, avoiding obstacles, but an organism that’s in constant conversation with its environment, a life-and-death dialogue on such a minute level that we’re not aware of it. It recognizes the mad microscopic mosaics we really are, molecular bits who trace their origins to simple one-celled blobs, then cellular flotillas that grew by engulfing others in life’s oceanic swap meet. Evolving this way and that, nabbing traits, shedding traits, we went haywire in slow motion over millions of years. Maybe our cells, however much they evolve, retain a phantom sense of those early days as colonial bodies with a shared purpose, more like amoebas or slime mold than mammals. We’re beginning to accept the idea of gypsy organisms that fanfare around us, making catlike raids on each other in dark simmering thickets, species as different from one another as animals adapted to rainforest, arctic, ocean, prairie, or desert. For we, too, have hillocks and estuaries, bogs and chilly outposts, sewers and pulsing rivers for them to quarrel and carouse in.

Even inside our own cells, we house more twitchy bacteria than anything else, because our mitochondria and chloroplasts were once primitive bacterial cells. They’ve sponged off us for so long that they can no longer exist on their own. Some our body welcomes with open pores because they handle metabolic melodies we couldn’t even hum on our own. It amazes me that we’ve survived with such grace, since we’re born dottily deficient, lacking vital survival skills such as how to digest the very foods we eagerly wolf down. An omnivorous diet helped us endure icy forests and bright broiling terra-cotta landscapes, but we don’t have all the enzymes we need to absorb those foods; our microbes assist.

In the distant past, as Earth bloomed with primitive life, strings and mounds of twinkling single-celled bacteria discovered the mutual benefits of teamwork and became allies. Others took a bolder and more violent step—they gobbled each other up. It’s only at that stage that lilacs, marine iguanas, wombats, and humans became possible. As multicelled organisms grew more and more complex, the imprisoned bacteria adapted and thrived, until they became vital cogs of each complex cell.

The consensus now among evolutionary biologists is that we can’t separate “our” body from those of our resident microbes, which have been fiddling in subtle ways with our nature as a species for millions of years, and influence our health and happiness to a previously unimagined degree. Study after study is showing that microbes profoundly affect our moods, life spans, personalities, and offspring. They influence not only how we are but who we are. How strange that we feel whole, one person whom we can wash and dress and conduct internal monologues with, though most of us is not only invisible but not even what we’re used to defining as human. Planet Human offers a dizzying array of habitats for the unseen and the unforeseen, the hominid and microbial.

Only very recently has the scientific community acknowledged the extent to which our microbes might indeed affect our evolution, and by our I mean the whole mespucha, as they say in Yiddish (the term in biology-speak is “holobiont”). Not just individuals but all their microscopic relatives with their relative points of view. Some hijack our free will, divert our behavior, and become matchmakers. A wasp study is offering fresh insights. By definition, members of a species can mate and produce live offspring. But researchers studying several species of jewel wasp (loaded with ninety-six different kinds of gut bacteria) have discovered that microbes can determine whether unions between different wasp species will succeed. When two distinct species of wasps mated, their offspring kept dying. Until recently, we would have said such a fertility problem was genetic. We know now that it can be microbial. When researchers changed the wasps’ microbes, the species bred favorably and hybridized. Evolution can be detoured by a mob of hidden persuaders.

Once again from the insect world, recent experiments with fruit flies are showing another way microbes can be at the helm, and the too-real possibility that bacteria have played a vital, even scary, role in our evolution. Consider how microbes control the love life of concupiscent humans and lusty fruit flies alike. Ilana Zilber-Rosenberg and her colleagues at Tel Aviv University’s Department of Molecular Microbiology and Biotechnology have discovered that the bacteria inside the gut of a fruit fly sway its choice of mates.

Fruit flies raised on either molasses or starch prefer to mate with others on the same diet. But when the flies are dosed with antibiotics, which kills the microbes in their gut, they’re no longer picky and will mate with any willing male. Among fruit flies, sexy males know all the right dance moves, but they also have to smell sexy, and their pheromone-cologne is modified by the microbes inhabiting the fly. For both humans and fruit flies, the love-wizards of smell are the symbiotic microbes that brew pheromones for us, their larger hosts. Scent rules in human courtship, too, especially among females looking for a mate. Although men seldom report such fussy responses to their partner’s natural smell, women so often do that it’s become a romantic clich?: “There just wasn’t any chemistry.”

Tinker with microbes and you alter stud capital, which in turn alters the genes of the female’s offspring, and so on as generations disrobe or unfold their wings. The object of natural selection isn’t a single plant or animal, Zilber-Rosenberg proposes, but its whole milieu, the host organism plus its microbial communities, including all the parasites, bacteria, fungi, viruses, and other bugs that call it home.

Fruit flies make appealing test subjects because we share such a bevy of mating behaviors. The dinner date, for instance. What’s the quickest way to a man’s heart? Forget Cupid’s arrow. According to Mom-wisdom, it’s coaxed by a cozy meal, in a penumbra of pleasure that mingles the fragrant food with the cook. If men are anything like fruit flies—and who’s to say they’re not at times; heaven knows women are—Mom was right. For female fruit flies, a dinner date is the ultimate rush. And rush it literally is, since they only live about twenty-five days and can’t afford to be shy. Live fast and die is their mantra, and they need a handy food supply if their large new brood is to survive. Female fruit flies prefer males who favor the same chow. Still, the males need to be in the right mood, and the females are surprisingly picky and manipulative given their short career.

During fruit fly courtship, if the microbe-milled incense is right, the male extends one mandolinlike wing and serenades the female, then engages in that style of oral foreplay many humans do, before mounting her and copulating for twenty minutes or so.

We respond to the same sweet, honeylike aromas that make fruit flies amorous, and so chemists include them in perfumes. Like an insect rubbing its wings together to croon a mating call, many a medieval troubadour used a mandolin to serenade his lady, with whom he’d dine and mate. And remember that sexy tavern scene in Tom Jones, in which the hero and a buxom wench devour a none-too-fresh carcass with carnal abandon? Intriguingly, if a female fruit fly spies a lone mutant (or rather a mutant mutant, say, the one normal fruit fly with quiet brown eyes, which would be the odd-fly-out if all the rest were bauble-eyed), the female hankers for the nonconformist. In the trade, it’s known as “the rare male advantage.”

For fruit flies, too, beauty is in the eye of the beholder, with their microbes adjusting the focus. Did I mention that some fruit flies have come-hither eyes? I don’t mean the dozens of mosaic facets, so evocative of hippie sunglasses, but the zingy psychedelic eye colors lab folk like to endow them with, the better to study mutant genes. As a Cornell grad student, I often stopped by the fetid biology lab to admire the eggplant-blackness of the bellies, the spiky hairs, the gaudy prisms of the eyes—some apricot, some teal, some brick red, some yellow, some the blue of ships on Delft pottery. I still recall the tiny haunting eyes of the fruit flies, like the captive souls of past lab assistants, and the swooping melody of their Latin name: Drosophila melanogaster, which translates poetically as “dark-bellied dew sipper.” Because fruit flies thrive in sultry weather (82°F), the lab offered students a warm den during those numbing upstate winters when ice clotted in beards and mittens, coeds exhaled stark white clouds, and the walkways looked like a toboggan run.

A favorite of biologists hoping to peer into the dark corners of human nature, fruit flies have it all—they’re prowling for mates eight to twelve hours after birth, easy to raise, and able to lay a hundred eggs a day. Plus they share about 70 percent of human disease genes, especially those linked to neurodegenerative disorders such as Parkinson’s and Alzheimer’s.

However, in a sly twist, the last male the female fruit fly has sex with will sire most of her many offspring, and she chooses him only after lots of romps in the orchard or lab, based on his gift for courtship and his scent. As with most animals, from squirrels to spiders, the males pursue but the females choose, and even the lowly fruit fly can be choosy.

So is the human dinner date really just courtship feeding after all, a custom (and microbial picnic) we share with fruit flies, robins, and chimpanzees, which in our chauvinistic, I’m-not-really-an-animal way we’ve coyly disguised? Yes. But what’s the harm in that? There’s a similar meal plan among the annual hordes of Japanese beetles that tat rose leaves into doilies and shovel deep into ready-to-open buds every summer. Gardeners often spy the iridescent scarabs, in twos or crews, perched atop favorite flowers, dining and mating simultaneously. Of course, the ancient Greeks and Romans, who coined the word “orgy” and found that dining lying down leveled the playing field, enjoyed blending sensory delights with equal gusto—banquets of music, food, conversation, alcohol, and sex. As the sage once put it: “Birds do it, bees do it, even educated fleas do it.” No harm at all, unless the process makes you impulsive, deranged, and deadly, which in some cases, depending on the shared microbes, sex can.

Another such culprit is a momentous if commonplace human hanger-on that also bedevils rats, cats, and other mammals and has recently been studied in harrowing detail. Spread along the edges of nature, on the boundary where humans and wild animals mix, the world population of Toxoplasma gondii, a particularly mischievous parasite, is ballooning with our own numbers. One way to catch the infection is to eat undercooked kangaroo meat. Kangaroo was recently approved for human consumption in Europe, and it’s usually served rare in France, followed, predictably, by Toxoplasma outbreaks. Budi may not be a carrier, since orangs are mainly vegetarians, but some nonhuman primates in zoos have acquired the bug after eating meat from infected sheep. Perhaps most surprisingly, the pathogen is increasing its range through human-made climate change. With northeastern Europe’s warmer, wetter winters, more of the pathogen are surviving, and so are its host species. In fact, Toxoplasma gondii may be climate change’s oddest bedfellow.

What would cause a rat to find a cat alluring? The slinky sashay? Batonlike whiskers? Crescent-moon-shaped pupils? A stare that nails you in place? Only a foolhardy rodent would cozy up to a cat. Yet rats infected with Toxoplasma dramatically change their behavior and find cats arousing. Talk about being in over one’s head. There’s nothing in it but the briefest frisson for the rat. The cat feeds its belly. But the protozoan zings along the strange trajectory of its life. Since Toxoplasma can only reproduce inside a cat’s gut, it needs a brilliant strategy to get from rat to cat, and despite its lack of brain power it devised one: hijacking the rat’s sex drive. Toxoplasma-beguiled rats do feel fear when they smell a cat, but they’re also turned on by it, in the ultimate fatal attraction. As with human sexuality, or film noir, a side order of fear isn’t necessarily a deterrent.

The cat hunts again, dines on infected prey, and the odd hypnotists thrive. Only cats further the parasite’s agenda, but other animals can sometimes ingest the eggs without knowing it and become dead-end hosts. That’s why pregnant women are warned not to empty kitty litter or handle cat bedding. Exposure to Toxoplasma can derail a fetus, leading to stillbirth or mental illness. Some studies link Toxoplasma and schizophrenia. Infected women have a higher risk of suicide than parasite-free women. According to Oxford researchers, it can doom children to hyperactivity and lower IQs. And, for some reason, over twice as many pregnant women infected with Toxoplasma give birth to boys.

But these new rat–cat findings are only the beginning of an Orwellian saga steeped in irony and intrigue. Worldwide, scientists are posing questions both eye-opening and creepy. If Toxoplasma can enslave the minds of rats—animals often studied to test drugs for humans—can it also alter the personality of humans? What if that yen to go rock-climbing or change jobs isn’t a personal longing at all, robust and poignant as it may feel, but the mischief of an alien life form ghosting through your brain? Is Toxoplasma to blame for a hothead’s road rage? How about a presidential hopeful’s indiscreet liaisons, or a reckless decision made by a head of state? Could a lone parasite change the course of human history?

So when is a whim not a whim? It feels like we have free will, but is a tiny puppeteer pulling the strings of billions of people? For the longest time philosophers, theologians, and college students debated such questions, then neuroscientists joined the fray, and now a body of parasitologists.

When Jaroslav Flegr, of Charles University in Prague, surveyed people infected with Toxoplasma, he found clear trends and surprising gender differences. The women spent more money on clothes and makeup and were more flirtatious and promiscuous. The men ignored rules, picked fights, dabbled in risk, and were nagged by jealousy. Both sexes got into more than twice the average number of traffic accidents—as a result of either impulsivity or slowed reaction time.

Rats have proclivities and tastes. Humans have those in spades, as well as sentiments and reveries. But mindset doesn’t matter. All warm-blooded mammals respond to thrill, anticipation, and reward—especially if that includes a wallop of pleasure. Many of the odd behavioral changes scientists attribute to Toxoplasma tap the brain’s dopamine system, and that’s what Toxoplasma zeroes in on, rewiring networks to favor its own offspring, even if that means death for the host. Cocaine and other euphoriants use the same dopamine system. As the Stanford neuroscientist Robert Sapolsky explains, “the Toxoplasma genome has the mammalian gene for making the stuff. Fantastic as it sounds, a humble microbe is fluent in the dopamine reward system of higher mammals.

“This is a protozoan parasite that knows more about the neurobiology of anxiety and fear than twenty-five thousand neuroscientists standing on each other’s shoulders,” Sapolsky adds, “and this is not a rare pattern. Look at the rabies virus; rabies knows more about aggression than we neuroscientists do.… It knows how to make you want to bite someone, and that saliva of yours contains rabies virus particles, passed on to another person.” It’s an extraordinary genetic tool for a witless one-celled creature to wield.

Marine mammals and birds are spreading the parasite via water currents and ribbons of air. How many of us may already be unwilling hosts? According to the Centers for Disease Control and Prevention, 10 to 11 percent of healthy adults in the United States tested positive for Toxoplasma, and the true figure (most people haven’t been tested) is thought to be 25 percent of adults. Some scientists estimate that in Britain, a decidedly cat-loving country, half the population has been infected, in France and Germany 80 to 90 percent, and in countries that favor undercooked meat even more, with nearly everyone an unwitting mark—destiny’s child, to be sure, but also Toxoplasma’s zombie.

According to Nicky Boulter, an infectious disease researcher at Sydney University of Technology, eight million Australians are infected, and “infected men have lower IQs, achieve a lower level of education, and have shorter attention spans. They are also more likely to break rules and take risks, be more independent, more antisocial, suspicious, jealous, and morose, and are deemed less attractive to women.

“On the other hand, infected women tend to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with noninfected controls. In short, it can make men behave like alley cats and women behave like sex kittens.”

What does it take to slant an opinion? Advertising, group pressure, financial gain, a charismatic leader? How about a real lowlife, a wheeler-dealer who delights in messing with your mind and harbors primitive drives? Enter the saboteur skillful enough to slowly and subtly change the personality of whole nations—a humble microbe. Some researchers speculate that between a third and half the people on Earth now have Toxoplasma in the brain. And it’s only one of the many microbes that call us home. Is it possible that what we chalk up to cultural differences may be different degrees of mass infection by a misguided parasite? Kevin Lafferty, a parasite ecologist with the U.S. Geological Survey, also theorizes that cultural identity, at least “in regard to ego, money, material possessions, work, and rules,” may reflect the amount of a parasite in a population’s blood.

If you’re now eyeing your tabby with raised eyebrows, there’s no need to panic. Even invisible dictators can be deposed, and Toxoplasma responds well to antibiotics. In any case, would it have a greater influence than family dramas, pharmaceuticals, TV, college, climate, love, epigenetics, and other factors in human behavior? It’s probably one spice among many. After all, a slew of elements and events influence us from day to day, changing us in cumulative and immeasurable ways. Toxoplasma may be but one, and it doesn’t lurk in all cat owners or devourers of steak tartare. It may ring its changes only in the presence of certain other microorganisms. How can you tell the dancer from his dance of microbes?

In the garden, all the plants and animals have their own slew of microbial citizens, some sinister, others helpmeets. That takes some getting used to. It’s a big paradigm change, one future generations will understand from childhood and capitalize on. In health and medicine, they’ll focus on the human ecosystem, our whole circus of human cells, fungi, bacteria, protozoa, and archaea working together, untidily perhaps, but in concert.

When I was growing up, scientists only grew microbes in small petri dishes in their labs, and all bacteria were nasty. In just a decade, we’ve begun seeing the big mosaic and we’re even starting to think in terms of microbes for improving the planet in precise ways: fixing the health of endangered species with wildlife probiotics, ousting invasive species using certain bacteria, sweetening groundwater that’s been tainted by pollutants, cleaning up oil spills with voracious grease-loving microbes, helping agriculture feed more people without fertilizers by employing bacteria that make the crops grow faster and more robustly.

The hope is that, just as with genes in the Human Genome Project, if researchers can identify the core microbes that most humans share, then it will be easier to divine which species contribute to specific complaints. This offers a new frontier for fighting illness, one easier to manipulate than the genome, and safer to barge in on than deeply embedded organs like the heart or liver.

New studies suggest that a single pathogen is rarely enough to trumpet disease, because different microbes form alliances. “The real pathogenic agent is the collective,” says David Relman, an infectious disease specialist at Stanford University. This has sparked a new way of thinking about illness called “medical ecology,” which recognizes the collective as the key to our health. In the past, we thought of all bacteria as bad, a contagion to be banished, a horde of invisible dragons. Ever since the end of World War II, when antibiotics arrived like jingle-clad, ultramodern cleaning products, we’ve been swept up in antigerm warfare. But in a recent article published in Archives of General Psychiatry, the Emory University neuroscientist Charles Raison and his colleagues say there’s mounting evidence that our ultraclean, polished-chrome, Lysoled modern world holds the key to today’s higher rates of depression, especially among young people. Loss of our ancient bond with microorganisms in gut, skin, food, and soil plays an important role, because without them we’re not privy to the good bacteria our immune system once counted on to fend off inflammation. “Since ancient times,” Raison says, “benign microorganisms, sometimes referred to as ‘old friends,’ have taught the immune system how to tolerate other harmless microorganisms, and in the process reduce inflammatory responses that have been linked to most modern illnesses, from cancer to depression.” He raises the question of “whether we should encourage measured reexposure to benign environmental microorganisms” on purpose.

A baby is born blameless but not microbe-free. Mom coats her with helpful microbes as she squeezes down the birth canal, including Lactobacillus johnsonii (a bacterium one expects to find in the gut, not the vagina), a bug essential for digesting milk. I was bottle-fed formula, but breast-milk-fed babies grow stronger immune systems because breast milk, often the first source of nourishment, teems with more than seven hundred species of hubbub-loving, life-enhancing bacteria. Researchers are thinking of cobbling them into infant formula to help ward off asthma, allergies, and such autoimmune triggermen as diabetes, eczema, and multiple sclerosis. Babies pick up other useful bacteria in Mom’s dirt-and-crumb-garlanded home and landscape. At least, they should.

Doctors are embracing the idea of personalized medicine based on a patient’s uniquely acquired flora and fauna, as revealed in his or her genome, epigenome, and microbiome. No more antibiotics prescribed by the jeroboam on the off chance they might prove useful. Instead, try unleashing enough beneficial bacteria to crowd out the pathogen. No more protecting children from the hefty stash of derring-do white-knight bacteria they need but we’ve learned to regard as icky.

Patients whose gut flora have been wiped out by certain antibiotics are prey to Clostridium difficile, an opportunistic weasel of a bug that causes severe, debilitating diarrhea. Once it has taken up residence, it’s miserably hard to expel it and restore the good bacteria. What does seem to help, though it’s not an image to dwell on, is fecal transplants from a healthy person—an enema full of bacteria to recolonize a stranger’s intestines, join the Darwinian fray, and triumph over the pathogens by acting like sailors on leave.

When Kathy Lammens, a stay-at-home mom with four young children, learned that her nine-year-old daughter’s battle with colon disease might lead to a colostomy bag, she began looking for alternative therapies. After much research, she decided on do-it-yourself home fecal transplants, tendering one five days in a row.[33] Twenty-four hours after the first, all of her daughter’s symptoms improved. Now Kathy, a robust believer, offers a YouTube video with instructions.

One study has revealed that mice with autism don’t host the same gut microbes that mice without autism do, and they seep behavior-altering molecules through the body and brain. But researchers find that dosing the mice with the beneficial bacterium Bacteroides fragilis eases the symptoms, and so human trials will follow. Another study discovered that if heart patients don’t eat enough protein, the good gut microbe Eggerthella lenta will steal some of a patient’s dose of digoxin, an important heart stimulant.

Some of global warming’s unwelcome guests are tiny winged buccaneers carrying invisible stoles of misery. Mosquitoes in Africa and South America are rambling farther north, injecting dengue fever, malaria, West Nile virus, and yellow fever into parts of the world unfamiliar with such scourges. Perfusing our clothes and bedding with insecticides isn’t safe, but the diseases infect hundreds of millions of people each year. So the Michigan State microbiologist Zhiyong Xi has been working on the problem in a novel way, by rearranging microbes. When he noticed that mosquitoes carrying dengue fever and malaria were missing the mosquito-loving bacterium Wolbachia, he tried infecting the mosquitoes with a heritable strain of Wolbachia, and sure enough, the next generations didn’t carry either illness, and the lifesaving trait was passed on to their offspring.

It’s intriguing to imagine the role a simple microbe may play in someone’s relationships and career, and it reminds us that nothing life ever does is simple, or boring. How many threads weave a fleeting thought, let alone a hankering? It also reminds us of the fierce beauty of Earth’s organisms, whatever their size, creatures unimaginably complex, breathtakingly frail and yet sturdy, durable, filled with the self-perpetuating energy we call life. A big brain isn’t required to concoct sly, world-changing strategies.

AS I GLANCE out at the yard, I’m charmed by nature’s details: the magnolia tree’s fuzzy buds fattening up for spring; the melting snow on the lawn that’s left hundreds of grass follicles; long arcs of wild raspberry canes covered in their chalky lavender winter mask. But I’m also struck by the everythingness of everything in cahoots with the everythingness of everything else. When I look at my hand now, I scout its fortune-teller’s lines, and the long peninsulas of the fingers, each one tipped by a tiny weather system of prints; I see it whole, as one hand. But I also know that only a tenth of what I’m seeing is human cells. The rest is microbes.

When all is said and done, both our parasites and we their innkeepers are diverse—no one hosts the same reeking and scampering microbial zoo. Our microbes can change either in ratio or in kind at the drop of a cookie or in the splash from a locker-room puddle or through an ardent kiss, and then we have to adapt quickly. So it’s possible that some diseases really are inherited, but the genes that bestowed them were bacterial. When you think about it, for a major trait to evolve—something grand like the advent of language or the urge to explore—only one gene has to change on the Y chromosome of one man. That would be enough, over many many generations, to create a predisposition or a trend in an entire culture. It all depends on the highjinks of the maddening microbe.

Maybe this should also remind us how much of a pointillist jigsaw puzzle a personality really is. As a friend approaches with a smile, we greet a single person, one idiosyncratic and delightful being who is recognizable—predictable, even, at times. And yet every “I” is really a “we,” not one of anything, but countless cells and processes just barely holding each another in equilibrium. Some of those may be invisible persuaders of one sort or another: protozoa, viruses, bacteria, and other hobos. But I like knowing that life on Earth is always stranger and more filigreed than we guess, and that both the life forms we see and those we cannot see are equally vibrant and mysterious.

Where does your life story begin? When does the world start whittling your personality and casting your fate? At birth? In the womb? At the moment of conception, when DNA from your mother and father fuse, shuffling an ancient deck of genetic cards and dealing out traits at random from Mom or Dad? Long before womb-time, it would seem, much farther back, before your parents’ courtship, even before their parents’, in a crucible of choices, daily dramas, environmental stresses, and upbringing. Our genome is only one part of our saga. The epigenome is another. The birdlike microbes singing in the eaves of the body are yet another. Together, they’re offering a greatly enriched view of the terra incognita inside us. In the process, sometimes loud as headlines, but more often silent as the glide of silk over glass, how we relate to our own nature is subtly changing.