The human lips are fleshy, muscular folds framing the mouth, bordering the exterior facial skin and the internal oral mucosa. Lips perform a wide range of functions, from managing food intake to facilitating complex communication. The lips of Homo sapiens are distinct among primates due to their unique, everted structure and exceptional sensitivity. This raises the scientific question of why human lips evolved to possess such a prominent, exposed, and highly specialized anatomy compared to other hominids.
Structural Uniqueness of Human Lips
The most distinguishing feature of human lips is the vermilion border, the sharply defined transition line between normal facial skin and the reddish lip tissue. This tissue’s epithelium is far thinner than facial skin and is only lightly or partially keratinized. The thin outer layer makes underlying structures more visible, contributing to the lip’s characteristic color.
The reddish hue is caused by a high density of micro-capillaries, or tiny blood vessels, which are much closer to the surface than in other areas of facial skin. Furthermore, the vermilion lacks typical skin appendages found elsewhere, such as hair follicles, sweat glands, and sebaceous glands. The absence of these oil-producing glands means the lips are unable to self-lubricate, which is why they dry out and chap easily.
Primary Mechanical Functions: Ingestion and Sealing
The primary function of the lips is to act as a muscular valve for the mouth, controlling the intake of food and liquids. This sealing capacity is powered primarily by the orbicularis oris muscle, a complex, multi-layered structure that encircles the mouth opening. Contraction of this muscle allows for precise closure and pursing of the lips.
This muscular control is particularly important in early life, playing a direct role in infant survival through suckling. The lips create the necessary vacuum seal required to draw milk during feeding, which is a highly coordinated mechanical action. The orbicularis oris also works in coordination with the cheeks and tongue to position food between the teeth for mastication.
Specialized Role in Complex Communication
The precise musculature of the lips allows them to contribute significantly to two distinct forms of human communication: verbal and non-verbal. In speech, the lips function as articulators, shaping the vocal tract to produce specific phonemes. They are essential for forming all labial consonants, which require the involvement of one or both lips.
Verbal Communication
Bilabial sounds, such as /p/, /b/, and /m/, are created by bringing both the upper and lower lips completely together to obstruct the airflow. Labiodental sounds, like /f/ and /v/, are produced by positioning the lower lip against the edge of the upper teeth. The fine motor movements provided by the orbicularis oris are necessary for the rapid and subtle changes in shape required to distinguish between these different sounds.
Non-Verbal Communication
The lips are a highly visible component of non-verbal communication, conveying emotion through minute changes in shape. The lips can retract for a smile or protrude into a pout, movements that signal a wide range of social cues. The distinct, highly pigmented border makes these subtle movements more easily detectable by others.
Sensory Input and Evolutionary Significance
The lips are one of the most sensitive areas of the human body, serving as a primary sensory organ for environmental exploration and food assessment. This high sensitivity is due to a dense concentration of nerve endings, including specialized mechanoreceptors within the vermilion tissue. Meissner’s corpuscles are located in the dermal papillae of the lips and are responsible for detecting fine, discriminative touch and low-frequency vibration.
This sensitivity allows the lips to quickly assess the temperature, texture, and size of an object before it enters the mouth. This protective mechanism ensures only suitable items are ingested. The unique structure and sensitivity of the human lip have led to several theories regarding their evolutionary appearance.
One prominent hypothesis suggests the exposed vermilion border is a form of honest signaling, related to sexual selection. The highly vascularized, reddish tissue can indicate a person’s current physiological state, such as oxygenation, blood flow, and general health. This visual cue may have become a factor in mate selection over time, as a brighter, fuller lip color could signal reproductive fitness.
Another theory proposes that the development of complex spoken language may have played a role in the lip’s unique anatomy. The constant friction between the lips required to produce the numerous bilabial sounds in human speech may have contributed to the gradual loss of hair follicles and the eversion of the skin. In this view, the exposed, hairless vermilion border is an anatomical result of the specialized requirements for human vocal communication.