At a recent lunch with scientists who study the evolution of diet, a geneticist casually passed around a vial filled with strips of paper to lick, as though she were handing out toothpicks. Some people tasted nothing, whereas others puckered at the bitter flavor to varying degrees. That's because humans vary genetically in their ability to taste a bitter chemical known as phenylthiocarbamide, which elicits the same response as bitter flavors in Brussels sprouts, cabbage, and broccoli. A new study finds that if our close relatives, the Neandertals, were at that lunch, they too would have had varied tastes for bitter food, suggesting that differences in the ability to detect bitterness stretches back at least half a million years.
The vast majority of living humans--at least 75% of people in the world--can detect bitter flavors. They have inherited either one or two copies of the "major taster" variant of the TAS2R38 gene, which mediates how protein receptors on the surface of the tongue detect bitterness. But about 25% of humans are insensitive to these bitter flavors. This has prompted researchers to wonder when and why this feature evolved in human evolution.
In the new study, a team of Spanish researchers examined the DNA from a Neandertal from El Sidron cave in northern Spain. The team reports online today in Biology Letters that it sequenced the amino acids encoded by the TAS2R38 gene and found that 55% of the DNA included the taster version of the gene and 44% included the nontaster version. "This Neandertal was a taster although slightly less of a taster" than someone with two copies of the gene, says evolutionary biologist Carles Lalueza-Fox of the Institute of Evolutionary Biology in Barcelona, Spain, who led the study.
The fact that both versions of the gene were present suggests that, like humans, some Neandertals were tasters whereas others were not. The discovery of the same gene variants in both species of humans shows that they were inherited from a common ancestor that lived more than half a million years ago before Neandertals and modern humans diverged. In fact, chimpanzees also have diversity in bitter taste detection but use different genes to mediate this sensitivity. Toxins in bitter-tasting plants can harm the thyroid in large quantities, and thus "the standard assumption is that these bitter chemicals are bad for us--and thus being a taster would be advantageous," says anthropological geneticist Anne Stone of Arizona State University in Tempe, who was not involved in the study. "Why then is nontasting something that has evolved independently in humans [as well as] chimpanzees?"
One idea is that some bitter foods may contain medicinal properties, says Stone. In any case, the researchers suggest that the evolution of bitter-taste perception is a vivid example of "balancing selection"--a process in which multiple versions of a gene are maintained in a population, perhaps because different variants are advantageous in different environments. Lalueza-Fox adds that the study shows that Neandertals, renowned for their reliance on meat to thrive in frigid habitats, clearly also "had some significant intake of vegetables in their diet."