Bite into a lemon and you’ll likely experience a clashing rush of sensations: crushing sharpness, mouth-watering tanginess, and pleasant brightness. But despite its assertiveness—and its role as one of the five main taste profiles (along with sweet, salty, savory, and bitter)—scientists don’t know much about how our acidic taste evolved.
Enter Rob Dunn. The North Carolina State University ecologist and his collaborators have spent years scanning the scientific literature in search of an answer. In a paper published this week in the Proceedings of the Royal Society B, the team reports some clues. Science chatted with Dunn about how, and why, humans like to pucker up. This interview has been edited for clarity and length.
Q: Do other animals like sour foods?
A: With almost all the other tastes, species have lost them through evolution. Dolphins appear to have no taste receptors other than salty, and cats don’t have sweet taste receptors. That’s what we expected to see with sour. What we see instead is all the species that have been tested [about 60 so far] are able to detect acidity in their food. Of those animals, pigs and primates seem to really like acidic foods. For example, wild pigs (Sus scrofa) are really attracted to fermented corn, and gorillas (Gorilla gorilla) have shown a preference for acidic fruits in the ginger family.
Q: Sweet taste gives us a reward for energy, and bitter alerts us to potential poisons. Why might we have evolved a taste for sour?
A: Sour taste was likely present in ancient fish—they’re the earliest vertebrate animals that we know can sense sour. The origin in fish was likely not to taste food with their mouths, but to sense acidity in the ocean—basically fish “tasting” with the outside of their body. Variations in dissolved carbon dioxide can create acidity gradients in the water, which can be dangerous for fish. Being able to sense acidity would have been important.
Q: So how did sour detection become paired to eating and drinking?
A: We’ve lost the ability to produce vitamin C, or ascorbic acid, and liking acidic foods might be a way for us and other primate species to be reminded to ingest it. Another argument is that ancient primates ate way more fermented foods than we recognize. One way to tell if rotting fruits are safe is if they’re acidic, because the thing that makes them acidic is lactic acid bacteria and acetic acid bacteria. The acid in these bacteria kills bad new bacteria, so those fruits are almost always safe to eat.
Q: Fermentation can also create alcohol, so was it the acidity or the buzz that kept animals coming back?
A: In the last 7 million to 21 million years our ancestors evolved a souped-up version of alcohol dehydrogenase, an enzyme that metabolizes alcohol. Our faster version makes it 40 times easier to get calories from alcohol than it would be otherwise. There are also new versions of genes that have a role in recognizing lactic acid that evolve about the same time. These two fundamental evolutionary changes in our ancestors seem to match well. Based on existing data it’s likely that sour taste came first, but we don’t know for sure.
Q: Speaking of booze, some people love sour beer whereas others gag at the thought. How do scientists account for the difference?
A: There’s a lot of early evidence, even in the decades before we knew any genes associated with sour taste, that there are different populations of “sour tasters.” Smell is important here, too, and smell is strongly associated with learning. I would hypothesize that when someone’s learning to like a sour beer— and I think sour beer is wonderful—that the smells are giving you pleasure while you’re noting the taste. But we don’t understand how those differences relate to genes.
Q: Is it possible that we held on to sour taste by happenstance?
A: The only gene so far associated with the sour taste receptor is called OTOP1. It’s also associated with inner-ear function, helping maintain balance, and mutations in this gene are associated with vestibular disorders. The gene for sour taste does these other things, and it’s possible losing it would have other consequences. Something as everyday as sour taste, which you can experience with every meal, still has all these really simple mysteries. It’s wonderful.