by John McQuaid
But the search for taste receptors had dragged on. They had proven almost impossible to isolate: Not only are there relatively few taste-detecting cells to begin with, but it is difficult to coax a reaction from them. The body has a vast apparatus to detect all kinds of cues, from hormones on the inside to heat, cold, pressure, light, and chemicals on the outside. Most of these reactions are very sensitive. It takes only a tiny dose of adrenaline to get a rise out of the receptors that detect it. But taste receptors are about a hundred thousand times less sensitive. This is because they interface with the chaos of the world around us. Given the sheer volume and variety of sensations the tongue encounters in a single meal, the brain would overload if every molecule lit up the taste receptors. Taking a sip of Coke would be like staring into the sun.
The NIH scientists, led by Nick Ryba, had finally leaped many of those hurdles. They were examining taste bud cells while also searching stretches of the genome, hoping to match up a taste receptor protein with the gene that created it. They harvested DNA from the taste buds of rats and mice, whose sense of taste is similar to our own. The trick was finding the right individual gene: a short, specific stretch of code tucked somewhere among vast, unmapped tracts of DNA. Having that blueprint would enable them to clone copies of the receptor so they could easily study its structure and workings.
In a short span of time, science had become quite adept at slicing, dicing, and sorting these once-indistinguishable molecular strands. At NIH, scientists found a way to turn the scarcity of taste receptors to their advantage: They used a technique that plucked only the most unusual DNA snippets from across the tongue, separating them from more generic strings. Some of these had to contain the material for taste. Next, they took each fragment and injected it into taste cells harvested from rodents. If it latched onto the DNA in the cell, it was a taste receptor gene. This was, roughly speaking, like putting a toddler in a room with a woman you think may be the mother: if they hug, then you know they’re related.
It worked: the scientists found half of a rodent gene for a sweet receptor; the second half was found soon after, and then the analogous human gene for sweetness. Their double genes mean that sweet receptor molecules come in two parts that fit together like a train coupling. They are bizarre, Lovecraftian-looking things, tangled skeins of seven coils stuck in the surface of a taste cell. One coil reaches out into the void to snag sugar molecules floating by. When it does, an electrochemical chain reaction begins that travels all the way to the brain, igniting a burst of pleasure.
Elsewhere, scientists were starting to crack another once-intractable problem, the subjectivity of taste. A few years after the sweet receptor discovery, volunteers in an experiment at the University of Groningen in the Netherlands lay on a table with pacifiers connected to long straws in their mouths. They were then slid into a magnetic resonance imaging (MRI) scanner that recorded their brain activity as they sipped bitter tonic water through the straws. Later, they were scanned while looking at photos of people grimacing in response to a taste of a drink, and again as they read brief scripts intended to evoke distaste or disgust. The purpose of this experiment, run by neuroscientist Christian Keysers, was to explore the relationship between tastes and emotions. During the 1990s, the emergence of this type of scanner, called a functional MRI (fMRI), allowed scientists to see which parts of the brain were active when a person ate or drank, smelled an aroma, or read—anything that could be done while one’s head was immobilized.
There were limits to this approach. It showed associations between real-world actions and arcing networks of neurons in the brain, but not exactly what those associations meant. But it was a revealing waypoint between the chemical reaction of taste on the tongue and the mind itself.
Their findings were strange. As volunteers imagined bitterness in a story, or saw photos of a wince of distaste, their brains experienced a “bitter” reaction. These patterns varied slightly in each part of the experiment, reaching out to encompass different parts of the brain. Taste seemed to be a cornerstone of higher functions such as imagination and emotion.
The next twist in this story is still being written; it hinges on certain lingering mysteries. Flavor remains frustratingly paradoxical. Like other senses, it’s programmed by genes; unlike them, it is also protean, molded by experience and social cues, changing over the course of a lifetime. This plasticity is wild and unpredictable: people can learn to like or dislike almost anything, which is why the range of flavors in the world is seemingly infinite, and why the old tongue map was useless.
Everyone lives in his or her own flavor world, which takes form during early childhood and evolves over the course of our lives. This world is created by the clash of ancient evolutionary imperatives meeting a lifetime’s worth of high-octane processed foods, cultural cues, and commercial messages.
The flavor preferences of my own children, born two years apart, were apparent as soon as they began to eat solid food. Matthew, the elder, relished extremes. He started eating jalapeños in preschool and liked coffee from the time he was nine. Every so often, usually in the summer, he would sit down with a lemon or lime, quarter the fruit, add salt, and devour it with the peel. His sister, Hannah, craved bland, rich flavors, and the foods she ate tended to be white or beige: cheese, rice, potatoes, pasta, chicken. She preferred chamomile tea to coffee, and milk chocolate to dark. Yet both were picky eaters: they knew what they liked and rarely departed from it. Getting them out of their respective comfort zones to try something new was nearly impossible.
This combination of divergent tastes and limited likes turned grocery shopping or restaurant-going into a kind of Rubik’s cube challenge; only pizza satisfied everyone. I made most family dinners, and struggled to get them out of a rut dominated by the same handful of dishes presented with only slight variations in a weekly cycle: pasta, roast chicken, or chicken nuggets for Hannah, hot dogs or shrimp in Szechuan sauce for Matthew. My wife, Trish, and I were more adventurous, but the convenience of this routine dragged us in, too.
The appetites of children are a crucible where the forces of chemistry and culture collide. The sweet tooth, the scourge of modern nutrition and dentistry, is crucial to childhood development. In newborn babies, sugar acts like aspirin, soothing pain. The Monell Chemical Senses Center in Philadelphia, a think tank that studies taste and smell, found that children with a strong taste for sweets also had higher levels of a hormone tied to bone growth. A yearning for sweets pointed early human children to then precious sugars in fruits and honey, and when combined with sourness, to citrus fruits packed with vitamins C and D.
Picky eating is likely a holdover from the same epoch, when humans lived together in small migratory groups and children—thanks to their tendencies to wander and to shove random things in their mouths—faced a constant threat of poisoning. Today, a limited diet is a danger to long-term health, and in its most extreme form pickiness has been labeled an eating disorder, called food neophobia.
Children have strange tastes because they are bizarre creatures. Taste and smell develop earlier than other senses, so a fetus’s sensory universe consists almost entirely of the smells and tastes in amniotic fluid. This makes a lasting impression. In another Monell study, the babies of women who drank a steady diet of carrot juice during their pregnancies or during breastfeeding later took a shine to carrot-flavored cereal.
Then, between birth and the ages of two and three, a baby’s synapses—the connections between neurons that form networks in the brain—multiply from about 2,500 per neuron to 15,000 (an adult has 8,000 to 10,000). This temporarily ties the senses together. Young children live in a fugue of overlapping sensations, one reason why early flavor experiences evoke not just meals but entire moments. As children age, experience gradually trims the thicket of neurons, and better sensory connections emerge. During this process, kids’ tastes vacillate between conservative stretches and probing, adventurous periods.
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nbsp; During the teen years, intense tastes fade along with the physiological demands and evolutionary imperatives of childhood. A subtler palate takes their place, though the original likes and dislikes never quite disappear. This muting allows the range of tastes we can experience to increase, and our reservoir of food memories and associations deepens. Sensations bubble up, synapse by synapse, from chemical reactions in the nose and mouth. Meanwhile, food engages the other senses, tapping the mind’s capacity for learning, understanding, and appreciation. Back and forth it goes: the mind shapes taste, and experience shapes the mind. A version of this dialogue has gone on through billions of meals since life first developed an appetite.
CHAPTER 2
The Birth of Flavor in Five Meals
The first inklings of flavor appeared as early life-forms began to sense the world around them and the taste of nutrients floating by in seawater excited primitive nervous systems. Countless meals were consumed as life evolved over the hundreds of millions of years that followed. Like Russian nesting dolls, our modern tastes contain those experiences. No matter how cultured one’s palate or subtle the ingredients in a dish, a taste summons raw urges out of the deep past, echoing evolutionary twists and long-ago life-and-death struggles over food. Five ancient meals, each taking place at a turning point in evolutionary history, help explain where the sense of flavor, and Homo sapiens’ talent for culinary invention, came from.
The First Bite
The creature resembled a scarab. About an inch long, with a soft, ribbed carapace, it scuttled across the sand in a primordial coastal shallow. Then it sensed a threadbare tapestry of smells, vibration, and shifting light. Its wormlike prey burrowed into the sand, trying to undulate its way to safety. But it was too late. The predator ripped it open with its pincer-like mandibles, sucked it into its mouth and down its gullet, then continued on its way, searching for a sheltered spot to digest.
Evidence of this 480-million-year-old meal was discovered in 1982, when a scientist named Mark McMenamin on a survey expedition spotted a tiny fossil imprint in a gray-green slab of shale. Without giving it much thought, he chiseled the impression out of the rock and bagged it along with dozens of other samples. Then a graduate student, McMenamin was surveying the geology of the Sonoran Desert for the Mexican government, picking over the flanks of Cerro Rajón, a summit about seventy miles southwest of Tucson in the Mexican state of Sonora. The ancient seabed had ended up on a mountaintop.
To the untrained eye, the fossil looked like a series of faint scratches barely a quarter-inch long. When he studied it back at the lab, McMenamin recognized them as traces of the movements of a trilobite, etched into petrified mud. Trilobites were the ancestors of nearly everything in the animal kingdom: fish, flies, birds, humans. They left countless fossils in seabeds, making them a fixture in natural history museums. Many had shells with multiple segments and looked like a cross between a horseshoe crab and a centipede. This fossil’s pattern of markings was well-known, and even had a scientific name, Rusophycus multilineatus. McMenamin kept it and wrote about it in his PhD thesis. He thought little about it until more than twenty years later, when he was a professor of geology at Mount Holyoke College, studying the early evolution of life.
McMenamin was examining the fossil again when he saw something he had previously overlooked. “It had this additional feature, not just the trilobite, but another sinuous trace fossil right next to it,” he said. “These things are rare.” He concluded the fossil contained evidence of an encounter between two creatures. The extra trace was an indication of a smaller, wormlike organism’s attempt to burrow into the mud. From the arrangement of the markings, it appeared the trilobite had been right on top of it. McMenamin employed Occam’s razor: the simplest explanation was that the trilobite had been digging for lunch. This was, he wrote, evidence of the “first bite,” the oldest known fossil of a predator eating its prey.
What did this meal taste like? Is it even possible to imagine?
Before this era, known as the Cambrian Period, flavor did not exist in any meaningful sense. Life on earth consisted mostly of floating, filtering, and photosynthesis. Bacteria, yeasts, and other single-celled creatures nestled in the furrows of granite and between grains of sand. Some joined together into slimy mats of cells. Organisms shaped like tubes or disks rode the ocean’s currents. “Eating” meant absorbing nutrients from the sea. Sometimes one organism enveloped another.
Then, over tens of millions of years—suddenly, in geological terms—the seas filled to teeming with new creatures, including the trilobites, which became the most successful class of organism in the history of life; their dominion lasted more than 250 million years. Their emergence, about 500 million years ago, was when nature as we know it really began: for the first time, life began systematically devouring other life. Unlike their predecessors, these new creatures had mouths and digestive tracts. They had rudimentary brains and senses that allowed them to detect light, dark, motion, and telltale chemical signatures. They used this fancy new equipment to hunt, to kill, and to feed. As Woody Allen’s character Boris remarks in the film Love and Death: “To me, nature is . . . I dunno, spiders and bugs and big fish eating little fish. And plants eating plants and animals eating . . . It’s like an enormous restaurant.”
No trilobites survive today, and fossils do not reveal much about their nervous systems, so assessing their sensory capabilities depends on educated guesswork. Certainly, they could perceive nothing like the complicated flavors of dark chocolate or wine. Human tastes, even the aversive ones, are full of subtleties and associations with other flavors, and to past events and feelings, the whole of our learned experience. Trilobites probably did not feel anything like pleasure, and retained only a few trace memories. Each meal would have tasted more or less the same. Its saliency would have come mainly from the slaking of hunger and the urge to attack.
Still, these primordial elements of flavor were an extraordinary evolutionary achievement, and human tastes share this same basic physiological structure. Of course, that’s something like comparing a mud hut to Chartres Cathedral. But the foundation had been set.
Some big change occurred in living conditions on earth to trigger this predator-prey revolution, which is called the Cambrian explosion. Scientists disagree about what it was. Some believe it was caused by a prehistoric bout of global warming that had melted the polar ice caps after a long deep freeze. The seas rose hundreds of feet, and water rolled far inland, over low hills and rocks with lichens and fungi (trees, grasses, and flowering plants did not yet exist), carving out lagoons and shaping sandbars and shoals, creating warm, shallow cauldrons ideal for life to flourish. Others trace it to a shift in the orientation of the earth’s magnetic field, still others to mutations that brought about the emergence of the action potential, the ability of nerve cells to communicate over distances, or other fortuitous changes in the DNA code.
Whatever the precise sequence of events, an iron link was established between acute senses and evolutionary success. A biological arms race ensued as bodies and nervous systems adapted to rising threats and opportunities. The senses, once mere detection-and-response mechanisms, had to grow more powerful in order to guide complicated behavior. Flavor became the linchpin of this process. From the time of the trilobites to the present, foraging, hunting, and eating food have driven life’s endless bootstrapping, culminating in our big human brains and the achievements of culture. More than vision, or hearing, or even sex, flavor is the most important ingredient at the core of what we are. It created us. The ultimate irony, McMenamin says, is that the introduction of killing into the world, and with it untold suffering, also expanded intelligence and awareness, and ultimately led to human consciousness.
Sweetbreads
Drawn by the scent of decomposition, the jawless hagfish burrows into the bodies of dead sea creatures and then devours their carcasses from the inside out. This has proven to be a highly su
ccessful evolutionary strategy. Jawless fish, the first vertebrates, appeared about 450 million years ago, roughly 30 million years after the “first bite,” and the fossil record shows they have changed little since then. They are older by about 200 million years than their rival for the title of champion survivor, the cockroach. The hagfish, an outlandish-looking animal with an eel-like body and a sucker for a mouth, is sometimes called a living fossil. Humans are descended from some ancient relative of the hagfish; its anatomy and behavior offer a glimpse into the deep past, when the basic couplings between the brain and the senses were first established.
To early predators, the trilobites, taste and smell would have been virtually indistinguishable. But in jawless fish, they assumed different jobs, and would not reunite until humans appeared on the scene. Taste became a gatekeeper to the body’s inner precincts. But smell reached out into the world. Hagfish swam through a shifting haze of scents. Smell created a picture of their surroundings in their brains: predators, potential mates, their next meal. To humans, the scent of rot usually triggers disgust. But this reaction is subjective. To the jawless fish, it meant survival and satisfaction.
Where did this additional sensory power come from? Sometimes, mutations in the genetic code do not merely change the body—they add to it. Entire strings of DNA can randomly duplicate themselves; when their biological instructions are carried out, the organism acquires an extra set of something. Redundant tissues can be deadly, mucking up the body’s normal functions. But under the right circumstances, they can bring about significant evolutionary leaps. The original genes continue doing their established jobs, and natural selection works on the copies, which take on new tasks or build new body parts. The German writer and naturalist Johann Wolfgang von Goethe anticipated this powerful evolutionary force in the late eighteenth century, guessing that duplicate parts of the anatomy might transform themselves into something different. The structure of a leaf might be the basis for the flower petal. A skull might be a modified vertebra.
