When researchers consider the classic five categories of taste—sweet, salty, sour, bitter, and umami—there’s little disagreement over which of them is the least understood. Creatures crave sweet for sugar and calories. A yen for umami, or savoriness, keeps many animals nourished with protein. Salt’s essential for bodies to stay in fluid balance, and for nerve cells to signal. And a sensitivity to bitterness can come in handy with the whole not-poisoning-yourself thing.

But sour? Sour’s a bizarro cue, a signal reliable neither for toxicity nor for nutrition. Really, it’s just a rough proxy for low pH, the presence of acid—the citric in lemons, the acetic in vinegar, and the like. “We don’t need sour to live,” Ann-Marie Torregrossa, a taste researcher at the University at Buffalo, told me. “It’s a weird sense to need.” It has been so scientifically neglected that Rob Dunn, an ecologist at North Carolina State University, considers it something of a “missing taste,” the gustatory litter’s forgotten runt. No one really knows for sure, Dunn told me, “what it’s all about.”

And yet we taste sour, strongly, and are not alone in doing so. When Dunn and his colleagues recently set out to investigate the sensation’s evolutionary roots, he told me, they couldn’t find a single backboned species that had definitively lost the ability to identify acidic foods, be they birds or mammals or amphibians or reptiles or fish. Admittedly, that may be a function of how few animals scientists have surveyed—just several dozen—but already, that makes sour a standout. Cats, otters, hyenas, and other carnivores have lost the ability to suss out sugar; giant pandas are immune to umami; dolphins, which swallow their prey whole, don’t seem to be able to savor sweetness or savoriness, and have booted bitter sensitivity too. But sour sensing appears to have staying power that its cousins do not—which means that it must be doing something important, perhaps something ancient.