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.