As Valentine's Day approaches, it’s the perfect time to explore why humans kiss. From a strictly reproductive standpoint, kisses are certainly not required and many cultures have flourished without a single peck. The behavior is only part instinct, having an enormous cultural influence. By tracing the human lip print back thousands of years, we can see its deep cultural traditions.

Gallery: Memorable Hollywood Kisses

The earliest literary evidence we have for kissing dates back to around 1500 B.C. from India’s Vedic Sanskrit texts, the foundations of the Hindu religion. There is no mention of the word “kiss,” but we do find intriguing references to “licking,” and “drinking moisture of the lips.” By the third century A.D., the Vatsyayana Kamasutra (better known as the Kama Sutra), included an entire chapter lavishly describing ways of kissing a lover.

There is no doubt that people in India have been kissing for thousands of years, but they were not the only ones. A Babylonian creation story known as the Enuma elish recorded on stone tablets in the seventh century B.C. contains several kisses in greeting and supplication. More famously, the Old Testament (estimated to have been assembled during the 12 centuries before the birth of Christ) abounds with this gesture. For example, Jacob deceptively kisses his blind and ailing father while dressed as his twin brother Esau, stealing Isaac’s blessing along with the power to rule. A more sensual kiss is described in the Song of Solomon, which reads, “Let him kiss me with the kisses of his mouth: For thy love is better than wine.”

From ancient Greece, we see kissing in Homer’s epics, passed down through oral tradition and finally recorded between the eighth and seventh centuries B.C. Odysseus is kissed by his slaves upon returning home and King Priam kisses Achilles’ hands to plead for the return of his deceased son’s body. Centuries later, The Histories by Herodotus, includes kissing among the Persians. Ethiopian kings were kissed on the foot and Numidian kings were considered too supreme to be kissed at all. Herodotus also reported that Egyptians would not kiss Greeks on their mouths because Greeks consumed their sacred animal, the cow.

The Romans, meanwhile, enjoyed a strong and vibrant kissing culture. They were pioneers of introducing the kiss to other parts of the world via military conquests. Roman poets such as Catullus and Ovid celebrated the kiss and members of the populace were avid mouth-to-mouth practitioners. According to classicist Donald Lateiner of Ohio Wesleyan University, Roman men even seem to have developed a “mouth fixation,” expressing very high standards for who was worthy to be kissed. There were also several important laws on kissing and its significance as a social custom at the time.

With the rise of Christianity, the church grew increasingly concerned about kissing. However, St. Peter referred to the “kiss of charity,” and St. Paul wrote, “Salute one another with a holy kiss.” Priests worried these instructions might encourage carnal sin with the apparent blessing of the church, and some reacted by separating men and women in their congregations.

The Romans were pioneers of introducing the kiss to other parts of the world via military conquests.

Throughout the Middle Ages, kisses served as a demonstration of one’s social standing. A king’s subjects would kiss his ring and robe, his hands, or even the ground before him. Similarly, people pressed their lips to the pope’s ring and slipper. The kiss also served as a sign of trust between feudal lords and vassals. Knights kissed at jousting tournaments and would receive one from the person they protected as thanks for a year of service. During this period, many men did not know how to read and write, so a kiss was used as a legal way to seal contracts. They drew an “X” on the line and kissed it to make it legal, which has carried over into the way we now write “X” to symbolize a kiss today.

During the Industrial Revolution, the hand kiss became popular in England and eventually evolved into the handshake. The advance of globalization integrated kissing further across the globe, and where it wasn’t already practiced, a European version of it would soon arrive thanks to adventurers, tradesmen, and modern technology. By 1872, Charles Darwin theorized that given the diversity and popularity of kissing and related behaviors around the world, humans must possess an innate desire to connect this way. As he surmised, kissing is rooted in our evolutionary past, but significantly influenced by unique social norms and customs.

Sheril Kirshenbaum is a science writer for the Discover magazine blog, The Intersection, and a research scientist at the University of Texas at Austin, as well as the co-author of Unscientific America, with Chris Mooney. She has been a guest on numerous radio shows such as The Brian Lehrer Show and The Bob Edwards Show, and is an adviser and regular contributor to NPR’s Science Fridays.