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.