Human lips are a unique evolutionary trait, featuring an outward-turned band of reddish-tissue known as the vermilion, which is exclusive to Homo sapiens among living hominids.
The Anatomy and Vulnerability of Human Lips
Human lips appear red because they are exceptionally thin. Research shows the outer layer is unusually shallow, allowing blood vessels to sit close to the surface and display circulation through a translucent window, which explains why lips pale in cold weather and flush during exertion. However, this thinness carries a biological cost. Lips contain no oil glands, meaning they lack the protective film that shields the rest of the face from drying out, leaving them vulnerable to chapping, sun damage, and environmental exposure.
Evolutionary Theories: Feeding, Speech, and Signaling
Evolutionary biologists have proposed three primary explanations for why humans developed such distinct lips, though none provides a definitive standalone answer. The first theory centers on feeding, suggesting that human infants require muscular, outward-turning lips to secure an airtight latch on relatively flat breasts. Yet, critics note that other primate infants nurse effectively without developing a vermilion border. A second hypothesis links lips to speech, as they act as the final valve for shaping sounds like p, b, and m. While a 2012 study published in Current Biology found that primate lip-smacking shares rhythmic similarities with human speech syllables, chimpanzees also move their lips fluidly without displaying the red tissue.
The third explanation focuses on visual communication. Humans lost most of their body hair, leaving bare faces where facial cues are constantly monitored. On a hairless face, a strip of tissue whose color shifts with blood flow, health, age, and exertion functions as a natural signal. Meanwhile, anthropologists dismiss romantic kissing as a primary driver; a 2015 study published in American Anthropologist surveyed 168 cultures and found that romantic kissing is absent in a majority of them.
Did You Know?
According to a 2017 study published in the Journal of Applied Oral Science, the tactile sensitivity of the human lips approaches that of the fingertip. The human brain devotes a disproportionately large area of the sensory cortex to the mouth, which explains why infants investigate their environment orally long before their hands develop fine motor skills.
Frequently Asked Questions
Are humans the only animals with visible lips?
According to a 2021 study published in Translational Research in Anatomy, the vermilion border is exclusive to Homo sapiens among living hominids, though some other primates possess similar tissue structures under a microscope.
Why do human lips change color?
Because the outer layer of skin on human lips is very thin, underlying blood vessels sit close to the surface, causing the lips to flush with increased circulation or pale when cold.
Did lips evolve for kissing?
No. A 2015 study published in American Anthropologist surveyed 168 cultures and found that romantic kissing is absent in a majority of them, indicating it is an activity practiced with lips rather than the evolutionary reason for their development.
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