The “last caregiver’s kiss” theory suggests that this type of lip-to-skin contact was common among early primates and served both a hygienic and social function.
With the development of humanity, especially with the loss of hair, the act of grooming diminished, leaving behind the more simplified behavior of kissing. Although kissing is rare in the animal kingdom, it remains an important social practice among humans, and research shows that approximately 90% of cultures involve some form of kissing.
Despite different kissing etiquette across cultures, this practice appears to retain universal biological significance.
Source: Ferra

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