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Answer by Adriana Heguy, molecular biologist and genomics researcher:
Interestingly, tongue-curling ability is not solely genetic, and the genetic component may be very small. Monozygotic (identical) twins are not always concordant for tongue-curling ability, so if there is a genetic component, it’s clearly not Mendelian. In other words, it’s not a trait coded by one single gene, and it’s clearly influenced by the environment—in this case, practice. But for some reason this is one of the “myths” about genetics that gets spread around in high school, where it is used as an example of a simple Mendelian trait with a simple dominant-recessive nature.
It’s hard to comment on the evolutionary purpose of an ability so heavily influenced by the environment, and not obviously useful. There are many traits for which we do not have the faintest idea why they exist or what purpose they serve.
In the case of tongue-curling, it’s possible that it’s a case of fine motor control of the tongue. We need to be able to move our tongues to not bite them when we eat, for example, and for swirling food around. For unknown reasons, some individuals are better than others at controlling tongue movement. And since the ability can be acquired by practicing (though not everybody apparently succeeds), it does seem likely that it is indeed a question of motor control. Most people are able to do it. It’s quite common. But it could be that evolution had nothing to do with it.
Or it could be a spandrel; in other words, a side effect of evolution. Maybe the evolution of dexterity or finer motor control of other muscles resulted in tongue “dexterity.” It’s possible that it is an atavism, something that increased tongue muscle control was once useful for tasting or eating certain kinds of foods millions of years ago, and it has not disappeared because the developmental program for fine muscle control is still there. Other animals have very finely controlled tongues, like the giraffe, which uses a prehensile tongue to grab foliage and for self-cleaning. So tongue dexterity is not a human novelty.
Tongue-rolling could be akin to ear-wiggling. Few people can do this; it’s not a simple genetic trait either. For other animals, it’s useful to control the ears so they can hear sounds better. Our own pets, cats and dogs, can definitively move their ears to identify sound direction (useful for hunting or listening for predators). For humans, the trait is vestigial, but since we still need muscles for our ears (called auriculars) and they are part of our facial/head musculature, the trait is unlikely to disappear completely any time soon.
Why is it that some humans developed the ability to curl their tongues and others didn’t? originally appeared on Quora. More questions on Quora: