Cosmos Editorial
We are taking a look back at stories from Cosmos Magazine in print. This article appeared in December 2020.
The sensory pleasures of taste and smell might have played a greater role in evolution than we imagine, writes Annamaria Talas.
Ever wondered why some animals walk or fly for miles in search of salt? Or what drove our ancestors to create better and better tools and learn to tame fire?
Some of the most interesting stories of evolution, according to professor Rob Dunn from the Department of Applied Ecology at North Carolina State University, can be told from the perspective of taste and flavour. Dunn’s upcoming book on the subject – Delicious: The evolution of flavour and how it made us human, co-written with Monica Sanchez – suggests that pleasure is nature’s way of ensuring animals get what they need.
“Pleasure leads us to the food we need, towards sex and reproduction,” he says. “It’s a reward for doing what will keep our species and lineages going. So, every time you taste something, you can think of your tongue as offering you two roads – the road to pleasure, towards what you ancestrally needed, and the road towards danger you should be avoiding.”
Dunn believes research into taste and flavour could lead to fundamental discoveries that are both immediate and relevant to everyone on Earth.
Primatologist Richard Wrangham was the first to hypothesise that cooking was an evolutionary turning point that separated our trajectory from that of chimps, but what was overlooked in his 2009 book Catching Fire: How cooking made us human is why our ancestors began cooking in the first place.
“It’s because it tasted better,” says Dunn. “Chemical reactions in cooking unleashed flavours our ancestors never experienced before and made their food more delicious. In the same way, much has been written about the origin of tool use, but what has gone rarely noticed is that nearly all of the first tools employed by our ancestors were used to find foods that were sweeter, more savoury, or otherwise tastier than those that were readily available.
“Fire was just another tool. Once tools and fire were invented, these foods eventually changed the evolutionary trajectory of our ancestors, but first and foremost, this path was led by flavour and taste.”
Compared to vision, the sense of taste is considered simple and primitive, but only because it remains the least understood.
The cones in human retinas allow us to process millions of hues, but they come in only three basic colours – red, green and blue. In contrast, we have about 50 known taste receptors. Scientists are discovering new tastes almost every year: calcium taste, phosphorous taste, the taste of starch and even the taste of some fats.
Taste evolved to detect poisons and nutritional elements in food: saltiness from sodium chloride; savouriness from amino acids; sweet from carbohydrates; sour from acids. Phosphorous and calcium receptors also help animals replenish their needs in environments where those elements are rare. Scientists at Philadelphia’s Monell Chemical Senses Center posit the existence of many more undiscovered taste receptors – which raises the question: Why?
“Every living thing must solve an equation,” says Dunn. “On one side is the concentration of the different elements in the body of an organism. How much nitrogen? How much phosphorus? On the other side is the chemical composition of the environment. The body needs to satisfy this equation to survive – and [is] solving it through taste and flavour; through being led to what they need and away from what’s dangerous. Taste leads us to reconcile differences between our own bodily equation and what’s most common out in the environment.”
Searching for salty
The significance of taste goes back hundreds of millions of years, to when our evolutionary ancestors crawled onto land. In the ocean, organisms were bathed in nutrients. On land, they faced scarcity: food was harder to find and water didn’t have enough salt for their cells to function properly. The desire for salt became important. In Africa and in the Americas, animals walk for miles just to get to a salt lick, and their reward for effort is a pleasurable salty taste. But taste never exists by itself; it functions in concert with our sense of smell. You can verify this yourself with the jellybean test. Pick up a jellybean in one hand, pinch your nose tightly with the other, then put the jellybean in your mouth and chew it. Let’s say it’s fruit flavoured; you’ll perceive tastes of savoury and sweet but you won’t know which fruit until you release your nose – provided, of course, your sense of smell is intact.
The test relies on a sensory pathway called retronasal olfaction, or “mouth smell”. As we eat and drink, volatile molecules shoot up through the nasal passage to the nose and complete the experience as flavour. “Nose smell” – sniffing – is called orthonasal olfaction.
Taste and smell were thought to run independently until they merged in the brain, but in April 2019, Monell Center cell biologist Mehmet Hakan Ozdener published his discovery of smell receptors on the tongue’s taste cells.
“[Our laboratory can grow fully functioning taste cells outside the human body,” says Ozdener. “One day, inspired by my son’s idea that snakes smell through their tongues, I exposed human taste cells to odours. Surprisingly, the cells responded. After my discovery was published, I received a number of emails from people all over the world who had total [nasal] smell loss, confirming that they could smell through their tongue. What this means is that the integration of taste and smell begins on the tongue, not in the brain.”
Library of flavour
Our taste and smell senses are magnitudes more sophisticated than vision. Nonetheless, that’s the ambition of molecular geneticist Robert Margolskee, Monell Center director and one of the world’s leading chemosensory researchers. He believes that, given time, we’ll be able to digitise taste and smell. “Our dream is to move from the analogue experience of flavour to a digital library,” he says. “Once we have that, we could share that information instantaneously and very accurately everywhere in the world. This could be the foundation of a new industry that is able to create any kind of meal from scratch.”
The connection between taste and vision is much closer than we think. In April 2020, a team of scientists led by molecular biologist Craig Montell at the University of California Santa Barbara published research that further extended knowledge of receptors called opsins.
These proteins have been known as light sensors since the 1870s, but in 2017 Montell and his colleague Nicole Leung published research that revealed opsins had multiple functions. Through studies involving fruit flies, the pair documented the role of opsins in sensing temperature and sound; and “there are hints”, they wrote, “that opsins have light-independent roles in a wide array of animals, including mammals”.
It turns out that opsins are involved in taste sensing as well. In a 2020 paper, Montell, Leung and colleagues reveal that fruit flies use opsins to detect bitter tastes, and that this role is independent of their light and other sensing. Since the ability to sense chemicals in the environment is much older than vision, this discovery points to taste receptors having a much deeper evolutionary role.
Whole body taste sensation
It’s well documented that the senses of taste and smell are ancient. They extend back billions of years to the Archean age of bacteria. Eons later, taste receptors were grouped into tastebuds that are part of the peripheral nervous system. Contrary to the concept of the tongue’s “taste map” – which places sweet taste at the tip of the tongue and bitter at the back – tastebuds house receptors for every taste all over the tongue.
In 2013, Monell Centre molecular neurobiologist Peihua Jiang published research that suggested this is because there are stem cells hiding at the base of tastebuds. The question is how these progenitors develop into different taste cells. In humans, tastebuds are the only organs other than the liver that are capable of complete regeneration. Nature has been very clear about the importance of taste receptors. Far from being restricted to the tastebuds, standalone taste receptors have been discovered in many parts of the body, including the lungs, pancreas, intestines, nose and even gingiva, the gum that covers the bone ridge surrounding the teeth. These cells look and function like the taste cells in tastebuds, detecting salt, savoury, sweet, sour and bitter – but for reasons that have nothing to do with taste.
Geneticist Paul Breslin from Rutgers University in New Jersey, US, says taste has two principal evolutionary roles: “One is the conscious perception of taste, which allows us to sample what’s coming in the mouth and be aware of that. Is it nutritious? Is it poisonous? The other is the unconscious role of taste. In the gastrointestinal tract, they coordinate and regulate digestion. In their absence, digestion simply doesn’t happen. In the lungs and nose, these solitary taste receptors are the first responders against invading pathogens. They sense the presence of pathogenic bacteria and alert the immune system.”
In some people the bitter taste receptors are broken. These people typically like things like broccoli, tea, coffee and dark chocolate. We put it down to personal preference, but it’s because they’re not good at detecting bitterness. It’s a genetic glitch that’s present not just in the tastebuds but also in the nose’s solitary taste-like cells. This bias in taste also has health consequences: coffee-and-broccoli lovers tend to develop chronic rhinosinusitis because they are less likely to be able to detect invading pathogens. Unaware, they go to their GP for antibiotics and as soon as they stop taking them, the infection returns.
From the dawn of time, chemosensors helped organisms to survive. In multicellular organisms, taste cells coordinate and regulate digestion, and are in constant communication with the immune system. In mammals, taste and smell are connected to pleasure, ensuring that animals eat what they need.
“For millions of years, what we needed was rare in the environment,” says Rob Dunn. “But now, a stroll to the supermarket provides everything we need, in excess. [In evolutionary terms] this change happened in the blink of an eye. We can’t wait for natural selection to correct our taste for our unhealthy diet. Understanding taste and flavour is the only way to solve the mismatch between what pleases us and what’s good for us.”