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You have looked at your lips every day of your life and probably never once wondered why they’re there.<\/p>\n
getty<\/p>\n
Most of what makes humans anatomically strange is hidden from us. You can\u2019t see your own pelvis tilt or the way your larynx sits low in your throat. Lips are the opposite. <\/p>\n
They\u2019re on the front of your face, you\u2019ve been looking at them since you were old enough to recognize yourself in a mirror, and that familiarity is exactly why the question never comes up. We tend to notice what\u2019s unusual about other people, not what\u2019s unusual about the species.<\/p>\n
So look at a chimpanzee\u2019s mouth instead. It has lips. They\u2019re muscular and mobile, and a chimp can do things with them that no human face can manage. What it doesn\u2019t have is the part you see when you look at yourself, which is a band of pinkish-red tissue turned outward and left on permanent display.<\/p>\n
Anatomists call that band the vermilion. It\u2019s the one spot on your body where the moist lining of the inside of your mouth folds outward and sits exposed to open air. Everywhere else on the body, that kind of tissue stays tucked out of sight. On your face, it’s the first thing anyone looks at when you talk.<\/p>\n
It\u2019s usually described as ours alone, and that\u2019s roughly right, with one caveat worth stating plainly. A 2021 study<\/a> published in Translational Research in Anatomy confirms that the vermilion border is exclusive to Homo sapiens among living hominids, though a few other primates have lip tissue that looks similar under a microscope, and how the human version came to be turned outward is still an open question rather than a closed one.<\/p>\n Our Lip Color Isn\u2019t Pigment, And It Costs Us Something<\/p>\n Lips are red because they\u2019re thin. The outer layer is unusually shallow and the blood vessels underneath sit close to the surface, so what you\u2019re seeing is your own circulation through a translucent window. That\u2019s why lips go pale when you\u2019re cold and flush when you\u2019ve been running.<\/p>\n That thinness has a price. Lips have no oil glands, so they get none of the protective film that keeps the rest of your face from drying out. They chap in winter and burn in summer, and the lower lip is a known trouble spot for long-term sun damage.<\/p>\n This is the part that makes the question worth asking. Evolution doesn\u2019t usually maintain a feature that\u2019s fragile and exposed unless it\u2019s paying for itself somewhere. The argument is over where.<\/p>\n Three Explanations For Our Lips<\/p>\n The most practical explanation is feeding. Human infants nurse from a relatively flat breast rather than a protruding teat, and getting an airtight seal on one takes muscular lips that turn outward, working with the ring of muscle that circles the mouth. A better latch is worth a great deal when you\u2019re a newborn<\/a>.<\/p>\n The problem is that other primate infants nurse without any trouble at all and none of them has a vermilion. Feeding might explain why human lips are so muscular without explaining why they\u2019re inside out.<\/p>\n The second explanation is speech. Your lips are the last valve the air passes through before it becomes a word. Every p, b and m requires closing them. A 2012 study <\/a>published in Current Biology found that the lip-smacking monkeys use socially has a rhythm remarkably similar to the syllables of human speech, which supports the idea that the rhythms of speech grew out of primate facial gestures rather than primate calls. Add a reshaped human vocal tract and you get our range of sounds. But a chimp\u2019s lips move beautifully too, and they stay the color of the rest of its face.<\/p>\n The third explanation is the one aimed squarely at the redness. Humans lost most of their body hair and ended up with a bare face that other people read constantly. On a bare face, a strip of tissue whose color changes with blood flow becomes a signal, whether or not anyone intends it as one. Lip color shifts with cold, with age, with exertion, with health. Most evolutionary biologists consider social and sexual signaling a real contributor here while admitting it\u2019s hard to test something that doesn’t fossilize.<\/p>\n One thing to set aside: lips did not evolve for kissing<\/a>. A 2015 study<\/a> published in American Anthropologist surveyed 168 cultures and found romantic kissing absent from a majority of them, which makes it something we do with lips rather than a reason we have them.<\/p>\n