Sunday, April 1, 2012

Evolution of Lips

Are lips considered to be appendages? The short answer is yes and no. No, because obviously lips do not resemble any limbs in our body. Yes, because lips extend from oral cavities and they do actually resemble protrusions. 

The anatomy of human lips are quite amazing. Unlike other parts of our epidermis, lips only contain about 3 to 5 layers of cellular germ layers. They also do not have a large population of melanocytes (or pigmentation), which makes blood vessels more visible, hence the red coloration. Lips lack sweat glands, therefore become chap a lot quicker than other body parts. These characteristics make lips very prone to pathogenic entrances to our body.

So it seems like lips are very vulnerable gates for lots of troubles in our body. What good do they do?

Food intake. Lips allow us to shut our mouths airtight to keep the food inside while ingesting. Without this sealing action, we would not be able to keep most of our food while breaking them down mechanically.

Suction. Similarly, lips allow infants to create strong suction to hold onto their food sources, even with absence of solid teeth. By moving lips around to make a narrow tunnel-like structure, we can suck in with much higher velocity and pressure (A1v1 = A2v2, fluid continuity equation). This also allows us to whistle. 

Articulation. Lips serve an essential role in producing consonant sounds. Also, without lips, we would not be able to whistle, sing, nor play woodwind or brass instruments. 

Teenage boy having a moment of realization for the evolutionary benefits of his lips.

Sensuality and sexuality. Lips are undeniably one of the body parts that take an extremely important role in intimacy and romance. Because of their thin cell layers, lips have a high density of neurons and nerve endings, making them very sensitive to pressure, temperature, and other stimulations. It is also known that lips are one of the visible representations of fertility for females. The more estrogen a woman produces, the thicker her lips get. Maybe this explains why Angelina Jolie is considered to be one of the most attractive women for many men. Lips can be very erogenous appendages. 

Let's try that again...

Angelina Jolie boasting her estrogen-abundance through her lips.

Do other species have lips as well? Yes! Lips are also prevalent in other species, especially in mammals. Chimpanzees have almost identical lips structures as us. Check out this video of a very seductive female chimpanzee using her lips for attraction.

In other families, lips evolved to be hard and keratinous, forming beaks instead of soft tissues. This solid structure gives them a variety of functions, especially valuable in the absence of arms and fingers, such as killing the prey, feeding the young, courtship, and probing and digging for food. Example of animals with beaks are birds and turtles. Turtles are very interesting animals to study, especially because they lost their teeth as their lips were becoming more and more keratinous. 

Turtles evolved to be toothless but grew keratinous lips, aka beaks.

Birds and their various structures of beaks.

For conclusion, lips and beaks serve important evolutionary functions. Let's be thankful for having lips, and maybe use them for this:

Not for this:



Law Smith, Miriam J.; Deady, Denis K.; Moore, Fhionna R.; Jones, Benedict C.; Cornwell, R. Elisabeth; Stirrat, Michael; Lawson, Jamie F.; Feinberg, David R. et al (2011-09-21). "Maternal tendencies in women are associated with estrogen levels and facial femininity". Hormones and Behavior 61 (1): 12–6. doi:10.1016/j.yhbeh.2011.09.005

Note, Science (2005-11-28). "Why do men find big lips and little noses so sexy? I'll paint you a picture - Comment - Times Online"The Times (London). Retrieved 2007-12-12.

"Lip size key to sexual attraction"BBC News. 2003-03-04. Retrieved 2010-01-15.

Valsiner, Jaan (2000). Culture and Human Development. Sage Publications, Ltd. pp. 134–136.


  1. I never really thought about lips as a body part that aids in sexual selection. I guess it is true though, which is why many women get collagen injected into their lips to make them appear more attractive.

    Crystal Boafo

  2. You mean like this?,r:1,s:0&tx=98&ty=68

    I think they use bovine collagen? Which is quite interesting.

  3. I find it interesting that while our lips are similar to chimp lips, in general, ours are much thicker in their relaxed position. The chimp in the video has to...I'm not sure what you call that action...flex? stretch? anyway, she has to basically open her lips to make them look like ours. In the relaxed position, she has hardly any lips at all. Could it have anything to do the differences in our speech? The articulation function seems like the only one that would apply in this situation.

    Also, what does it mean if a man has thicker lips? Is it the presence of estrogen or some other hormone? Is it selected for or against in sexual selection, generally? Hmm. Maybe I'll look this up tomorrow.

  4. PS I like your photo captions haha

  5. Haha thanks! To answer your question regarding male lip thickness, according to Professor Cunningham from the psychology department of University of Louisville...

    ...However, it is also important that a man's lips should convey the impression that he is virile, and any sign of femininity is a big no-no. At the same time, there should be some hint of generosity and warmth," said Professor Cunningham... So medium-sized lips are probably better on men, than either too small or too large."

    I hope this answers your question. As for chimpanzees, I think they simply find less need for firm lip movements? For instance, we use our lips daily without noticing them, like when we use spoons for a bowl of cereal, or even forks to have that airtight seal. I believe chimps usually eat fruits, insects, seeds, leaves, nuts, or even other animals, but for the majority of the time perhaps they don't need to use them. This is just a wild guess but it's interesting that they have "relaxed" position for their lips.

  6. Effie:

    wow I have never though about this much...are lips part of the integumentary system? We have never classified it as so but I am wondering whether it evolved with the integumentary system since its formation occurs at the same time during embryonic development?

  7. Interesting point- I personally would not consider lips to be part of the integumentary system. I think they serve greater function as sensory receptors than protective layers. Lips do have layers of skins, but they have much fewer cell layers as well as lack other protective elements like hair (I think lips and nipples might be the only parts of the skin that is completely hairless).

    But I think it did originate from integumentary system. In fact, integumentary system is defined as anything that has three major layers of tissue: epidermis, dermis, and hypodermis. I think lips do have these layers.

  8. You left out one of the key functions of lips - emotional recognition. I am a facial researcher and lips allow us to convey emotion even at a great distance (perhaps the reason they are a darker shade). They signal to others whether we are friendly or hostile and quite a few subtle emotions in between.

  9. I wish I could find it, but I remember reading in a primatology journal article back in grad school that some researchers believe that the lips evolved to mimic the human anus (which also amongst many primate species is a source of sexual attraction and point of sexual intercourse). Simply pucker one's lips and look in the mirror.

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  11. Trump sure conveys hostility to others w/his lip expression (80% scowl'g) a picture of his true inner self