There's a reason sweethearts don't give each other boxes of radicchio for Valentine's Day, and it's related to the reason we don't refer to lovers as bitterhearts: humans, like most animals, have a soft spot for things that taste sweet.
How we developed the fondness for sugars, and how sugars came about in the first place, is the topic of a lecture being given by evolutionary biologist Jason Cryan at the New York State Museum in Albany this evening. The lecture and cooking demonstration is part of the museum's popular Cooking the Tree of Life series, which began last February to commemorate the bicentennial of Charles Darwin's birth. I asked Dr. Cryan to give us a preview of his presentation.
F&T: How far back in evolution does the "sweet tooth" go?
JC: That depends! Some experiments have demonstrated that motile bacteria orient themselves toward sweeter solutions, so one inference is that the "sweet tooth" goes back REALLY far! If we restrict ourselves to talking about primates, then studies show that we (primates) have a distinct preference for ripe fruits vs. unripe ones; this is thought to be a response to the fact that plants load up their fruits with sugar upon ripening, after the seeds in the fruit have matured enough to survive being eaten and dispersed later through the digestive system (thus, there's a two-way relationship that has evolved between fruit-producing plants and seed dispersers, each getting what they want out of the relationship). Since many of the natural sugars we're talking about are plant-derived, then it's likely fair to say that sugars have been around for as long as plants have (although I can't say exactly how long that has been!).
The evolutionary explanation for the sweet tooth revolved around that idea that we have physiologically associated a sweet taste with high-energy foods which would have helped our earliest ancestors survive better in their environment (getting more "bang-for-the-buck"....if an individual has to spend time and effort foraging for food, it's better to obtain energy-dense food items than energy-poor food items). When one considers our ability to taste, our ability to perceive "sweet" is relatively weak, while our ability to perceive "bitter" is generally considered much stronger (in fact, the strongest of our taste reception, on average). Perception of "bitter" is thought to be an evolutionary strategy of quickly identifying plants that contain potentially harmful toxins (produced as secondary plant compounds). Thus, evolving a low tolerance to "bitter" and a high tolerance to "sweet"' might have promoted our ancestors to actively seek out sweet tasting foods.
F&T: Do all animals like sweets?
JC: As far as I know, all animals tested like sweets EXCEPT for cats (including everything from house cats to lions and tigers). It gets complex, but basically the ability to detect sweets relies on the interaction of two proteins in taste buds that are encoded by two separate genes in the genome of animals. There's experimental evidence showing that cats have lost function in one of these genes, and have consequently lost the ability to taste "sweet"; evolutionarily speaking, this loss may have come about by the fact that cats have evolved an exclusively carnivorous diet, and therefore a mutation preventing their ability to detect "sweet" plant products would not have been any disadvantage to their fitness.
F&T: Do some animals like sweets more than others?
JC: I don't know that we know enough about preferences in different species; but it is clear that there is variation among individual humans to perception of sweets. You probably have heard about so-called supertasters. Essentially, there's a lot of variation in humans when it comes to the number of taste receptors (taste buds) we have on our tongues (from something like 500-10,000!). Those with denser (or more numerous?) taste receptors typically detect various tastes at a significantly lower threshold than those with fewer taste buds. That variation in ability to detect tastes leads some people to think that great slice of chocolate cake is just "too sweet," while others think it "just right"!
A lot of the rest of the presentation will describe the difference between natural selection and artificial selection, and how we (humans) have domesticated various crops, selecting to maximize certain plant traits (specifically, for this lecture, "sweetness"); this has led to crazy, "extreme" evolution of supersweet grasses (i.e., sugarcane varieties) and cultivated fruits with as much as 10X more sugar content than their wild relatives (and often without seeds and with a significant reduction in plant fiber...how's THAT for extreme evolution!).