Tasty (7 page)

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When modern humans departed Africa around one hundred thousand years ago on their way to populating the world, they carried these genetic variations with them. Taste's small supporting role in this journey still echoes today in the taste for coffee and in produce choices in the supermarket, and shapes flavors throughout the world.

The crossing out of Africa likely occurred at what's now called the Bab el-Mandeb Strait near the southern outlet of the Red Sea, the narrowest stretch of water separating
North Africa from the Arabian Peninsula. Today, the strait is nearly twenty miles wide. But long ago, the Indian Ocean was hundreds of feet lower, and the strait was narrow and shallow, easy to cross by makeshift raft or maybe even by wading. Humans and their immediate ancestors were expert travelers. Similar crossings had occurred before, many times. Some early humans, like the people who tamed fire at the Gesher Benot Ya'aqov cave 700,000 years earlier, had made it as far as what is now Israel, about 1,400 miles to the north.
Homo erectus
had trekked to East Asia. And, as the humans camped on the beach, Neanderthals were hunting in Europe's forests.

These humans were more perceptive than their predecessors. Hundreds of thousands of years of evolution had made their brains larger, and their tastes simultaneously more robust and refined. The world had become one big lab for testing new tools and techniques for the hunt and the hearth. More than a hundred thousand years before the Bab el-Mandeb crossing, early humans started making earth ovens, digging deep pits and lining them with flat stones to focus heat for roasting game, roots, and vegetables. They may have smoked meat to preserve it, infusing it with new flavors in the process. They shared archaic recipes and passed them down the generations, tinkering as they went. These practiced rituals were a hedge against starvation wherever they traveled.

No one knows what drove them out of Africa for the unknown—possibly a famine, or some tribal dispute. Whatever it was, genetic evidence suggests that a small group of hundreds, or at most thousands, of people passed through this or several similar gathering points nearby, in a single exodus. Their descendants spread through the
rest of the world, displacing (and sometimes interbreeding with) the only other human species around, Neanderthals. Virtually all non-­African people in the world today are descended from the relatively small group that crossed at Bab el-Mandeb.

By chance, this founder group had quirky tastes. Genetic detective work indicates that in the Africa of this era, the human population had a fantastic range of sensitivity to bitterness; modern Africans still do. It makes sense that humans evolving in those diverse landscapes, encountering many different habitats and plants, would have a wide assortment of tastes. But the people setting out across the strait lacked this kaleidoscopic variety. Where it concerned Fox's gene, most were tasters, some were non-tasters, and only a few fell in between.

Theories vary as to the exact route early humans took after departing Africa. They may have hugged the shore, or headed northward and inland. At the time, the Arabian Peninsula wasn't the parched desert of today; it contained rivers, lakes, stands of trees, and stretches of savanna. Later, humans likely moved eastward across what is now Iran; then some moved northward, looping back toward Europe, while others pressed east into Asia. They colonized dense jungles and deserts, mountains and islands, from equator to poles, preparing and eating strange new foods wherever they went, eventually reaching the southern tip of South America about twelve thousand years ago.

A record of this seminal journey is now imprinted on our taste genes. The founder group's split between bitter tasters and non-tasters spread across the globe with their descendants, sorting much of humanity into one group or the other. Different habitats, climates, foods, and survival challenges
somehow tuned the bitter division this way and that; some groups grew collectively more sensitive, some less. The evidence can be found in the record of Fox-style taste tests, more than a thousand studies done around the globe over eighty years. Plot the results, and they show that the split between tasters and non-tasters varies by geography.

In northeast Britain, nearly a third of the population can hardly taste bitterness, and among some ethnic groups in India, it's more than half. This may help account for the popularity of beer in Britain, and of bitter melon, the intensely piquant fruit, in Indian cuisine. Continue east, and far more people are bitter-sensitive: in some parts of China, as much as 95 percent of the population. This pattern holds for American Indians, most of whom are descended from Asian migrants. Travel to colder climates, and the balance shifts again. The Inuit of Greenland are among the least bitter-­sensitive of early American peoples, perhaps because there's so little bitterness in their traditional fish-and-seal diet that they lost the capacity to sense it.

But another bitter gene had a very different fate. Nicole Soranzo, a researcher at University College London, studied a mutation in the bitter gene TAS2R16 that made people more sensitive to certain substances, including salicin, found in the bark of willow trees; arbutin, in bearberries; and amygdalin, in bitter almonds. This mutation is rare in Africa. Sampling the genes of populations around the world, Soranzo found that natural selection singled it out as a survival advantage elsewhere, and it spread like wildfire. Today upwards of 90 percent of non-Africans have it. Back in Africa, the trait never caught on.

The mysterious contradiction remained. Some forms of bitter sensitivity clearly enhance the odds of survival. But
insensitivity does too. It must have some uses, or it would not be around today. What are they? In humans and apes, at least, bitterness serves as something more than a mere toxic substance alarm.

Alessia Ranciaro, an Italian biologist, was the intrepid leader of a small twenty-first-century taste-testing squad that sought answers. They traveled in Range Rovers from village to village through the bush of Kenya and Cameroon. At each stop, Ranciaro chatted with local elders. She asked for permission to administer taste tests and asked them to help persuade villagers to volunteer. Her team set up tasting stations far more elaborate than Arthur Fox's: each used a thirteen-bottle set of varying concentrations of several bitter compounds. Volunteers took a sip from each, rinsed, and spit. Ranciaro's team began with small concentrations and increased them gradually. “We go on this way until they start to say, ‘I taste bitter,' or ‘It tastes like lemon,' and they started to make funny faces,” said Ranciaro. “When they taste bitter two bottles in a row, we can be confident. That is their final taste.” They tested men and women from nineteen different ethnic groups and every traditional African lifestyle: hunter-gatherers, goat herders, farmers. They also drew blood for DNA tests, so that they could match sensitivities to genes.

Overseen by University of Pennsylvania scientist Sarah Tishkoff, this arduous study aimed high. By returning to Africa, the scientists hoped to finally identify the evolutionary forces behind our diverse bitter tastes. The answer would explain a lot about food, flavor, and human biology.

Tishkoff's team assumed the taste differences had to be tied to what people ate, and surveyed areas relatively untouched by modern society. “They're not eating McDonald's,” said
Michael Campbell, who worked with Ranciaro and Tishkoff. For the most part, the people had been eating the same foods for thousands of years, perhaps as far back as the original exodus from Africa. People eating a meat-heavy diet might benefit less from an aversion to bitterness than a hunter-gatherer group harvesting pungent berries and roots. Over thousands of years, a meat-eating community might sift bitter-tasting genes out of the population.

When the results came in, however, the diet hypothesis—the theory of taste virtually all scientists subscribed to—turned out to be wrong. There was no evidence that food choices influenced genes—at least, not for the past five thousand years. Some older, deeper force seemed to be at work. This raised a provocative question: what if this ancient evolutionary signal was about something more than just taste?

It had been clear for a while that bitter sensitivity was part of a more complicated system in the body that reached beyond flavor. In the 1970s, Linda Bartoshuk, a taste scientist at Yale, noticed that many PTC tasters' sensitivity extended to sour, sweet, and salty tastes. They tended to avoid the powerful kick of chili peppers, wasabi, and ginger, too.

Bartoshuk set aside molecular biology and looked into the mouths of her volunteers. Many bitter tasters had a radically different anatomy from non-tasters, in that they had more fungiform papillae on their tongues. This meant they had more hardwired connections between the mouth and the brain: they perceived more intense taste sensations and more flavor information overall than other people. Some were ten thousand times more sensitive. She dubbed this group “supertasters.” Their flavor experience differed from that of non-tasters. Their food was full of garish neon colors rather than gentle pastels.

Bartoshuk's finding could only mean that, in concert with many other genes, taste genes influenced the anatomy of the tongue and nervous system. Hundreds of studies show that the split between bitter tasters and non-tasters extends beyond food preferences: There are more women tasters than men. Alcoholics tend to be non-tasters. Tasting has been linked to diabetes, bad teeth, eye disease, schizophrenia, depression, gastrointestinal ulcers, and cancer. Some of these correlations may be random, but the preponderance of them indicates that bitter taste biology influences the whole body. Since the DNA of taste receptors was decoded over the last decade, it has been found all over the body: along the digestive tract, in the pancreas and liver, in the brain, and in the testicles. (Smell receptors have also been isolated in the liver, heart, kidneys, sperm, and skin.) “You can imagine a simple organism, a protozoan with a sort of mouth where the taste gene is expressed,” said Roberto Barale, a biologist at the University of Pisa, in Italy. “When he starts to evolve and increase his size, the gene which controls expression of taste follows along. So the expression expands throughout the body, from mouth to stomach to intestine. And so on.”

Receptors of all kinds make up a sprawling sensory web that wends through the cells in every living thing. Without them, life would be blind and inert. Certain yeasts use receptors to recognize the sugars they feast on, as well as pheromones, so they can mate. Fruit flies have them embedded on the outside of their bodies to sense changes in light and to trace the scent of ripe fruit wafting by. Plants have them too. Vertebrates have between one thousand and two thousand different types. There is no cell in the human body that doesn't contain them. They alert the body to changes in temperature and chemicals in the water or air. They are the
switches for the body's internal communications, and thus the preferred target for most drugs, medicinal and illegal. They detect the presence of nerves firing, and rising or falling levels of hormones or neurotransmitters that move us to act on appetite, fear, or love. And they power sight, smell, and taste.

Still, the notion that “taste” would be at work all over the body was truly odd. Scientists at the Monell Center and the University of Pennsylvania tried to understand what use bitter receptors might be in the nose. They used cultures of human sinus cells with TAS2R38 receptors. To verify they had the right cells, they dosed them with PTC to see how they would respond. The receptors generated a faint electric current—the same signal they produce when they bond with a bitter molecule on the tongue. Bingo.

Then they gave the cells a sinus infection, flooding them with mucusy gunk. The bitter receptor alarms went off. The neurons sent an electrical message and released nitric oxide, a signaling molecule. Tiny cilia in the sinus cells waved faster and mucus production increased in response. That's how noses expel bad bacteria, or try to. Tasters, it seemed, were less likely to develop sinus infections than non-tasters. This would have been an advantage—unrelated to diet—for early humans colonizing colder climes.

There's also the gut. Like many small communities along the Mediterranean coast, the people of Calabria, the province that comprises the toe of Italy's boot, tend to be especially long-lived. Perhaps it's a consequence of the Mediterranean diet's emphasis on ingredients such as olive oil, fish, and red wine. But it might also have something to do with their sense of taste. Calabrian cuisine makes significant use of bit
ter vegetables, including eggplant, cauliflower, and spinach. The Bergamot orange, more bitter than grapefruit, is a local staple.

Roberto Barale was curious about the Calabrians' bitter tasting genes. As it happened, a group of Calabria residents was already taking part in a study tracking their health over the course of decades. Barale studied them, too, and found something provocative: a mutation in their TAS2R16 bitter gene was more prevalent in the elderly. And the older they got, the more likely they were to have it. Since people without it were dying earlier in life, something about this mutation seemed to contribute to longevity. Barale speculates this may be the result of bitter receptors in the intestines—although their function is currently unknown—somehow facilitating metabolism.

Until recently, most taste studies focused on the biology of the tongue and on perception. Now a new frontier has opened up. The bitter receptors dotting the body are part of a kind of shadow taste system. Unlike those on the tongue, they don't register in consciousness, and what they do is still mostly unclear. Flavor, in other words, is only the capstone of a vast, hidden system. It starts in the mouth with a burst of deliciousness, then disappears in darkness down into the gut, and from there its hand reaches everywhere in the body. It is an infinite mesh of sensors furiously sending and receiving messages as the whole body marinates in the chemical flux of the world.