The Human Age (38 page)

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.