“A man’s kiss is his signature.” — Mae West

There’s the romantic kiss, the social kiss, the air kiss, the kiss that conveys affection, familial ties or deep respect. The ancient Romans were big on romantic kissing, but the Greeks viewed kissing more as a way to pay homage or give social recognition. In Biblical times, washing and kissing another’s feet was considered a gesture of humility and respect. The Black Stone of Mecca, a relic considered holy by Muslims, has been polished smooth by centuries of kissing. Today, kissing is a near universal behavior, with many of us puckering up under the mistletoe, to seal a marriage and to ring in the New Year.

Humans have been kissing — for any number of reasons — for a very long time, according to scientists. But where did the custom come from and why exactly do we do it?

There are several evolutionary theories that could explain why people are attracted to the lips of other people, Sheril Kirshenbaum, biologist and author of The Science of Kissing, said at the ideacity 2011 conference in Toronto.

One theory harks back to our hunter-gatherer ancestors. When foraging for food, the colour red was viewed as a reward signal, Kirshenbaum said. Hence the subliminal ‘come hither’ message of red lips.

Another possible origin of human kissing is based on the Freudian thought that it can provide the same sense of comfort, calmness and attachment that babies feel when they nurse. Similarly, another theory says that kissing reminds us of the pre-historic practice of “premastication”, wherein a mother chewed the food herself and then fed her baby by mouth.

Infants frequently use their lips to explore the world around them, and indeed, our lips are full of sensory information, according to Kirshenbaum. When we kiss someone, for example, we gather all sorts of personal data and not just visually — but in terms of taste, scent and touch. All of our senses become engaged which, particularly for women, can help us judge if a person is an adequate mate. Studies have shown, for example, that on a subconscious level, a woman can smell when a man’s group of genes that manage the immune system (called MHC) are compatible with her own. This DNA compatibility increases the likelihood of producing healthier and stronger offspring.

Passionate mouth-to-mouth kissing also triggers a cascade of chemical changes in our body. If ‘the Earth moves’ when you kiss someone, neuroscientists say this could be attributed to a rise in love hormones such as Oxytocin, which is responsible for feelings of attachment and bonding. Kissing can also cause a spike in Dopamine and Serotonin, which are linked to craving and desire and that ‘can’t-wait-to-be-with-someone’ feeling we have when we think we’re falling in love. Romantic kissing can also cause a surge in adrenaline levels.