Goats kissing. Fir0002/Flagstaffotos. Wikimedia Commons

Some researchers believe that kissing began millions of years ago as a result of mouth-to-mouth feeding, with mammal mothers chewing food and then "forcing it" into the mouths of their young.

"From these observations, it is claimed that humans also learned kissing from exchanging food between mothers and their offspring," Texas A&M University anthropologist Vaughn Bryant, who has long researched the history and spread of kissing, told Discovery News.

"However, if this were true and it were innate, then why didn't all humans kiss? We know that many cultural groups did not kiss and knew nothing about it until they were shown," Bryant said.

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1895 illustration of the epic Mahabharata. Chore Bagan Art Studio / Wikimedia Commons

While the true origin of kissing remains a mystery, historians have found in India the earliest references to the practice.

Four major texts in the Vedic Sanskrit literature suggest an early form of kissing. Dating from 1500 B.C., they describe the custom of rubbing and pressing noses together.

"Eventually, someone slipped and found that the lips were very sensitive and found it pleasurable. That's one theory on how it started," Bryant said.

About 500 years later, the epic poem Mahabharata contained references of lip kissing.

"She set her mouth to my mouth and made a noise and that produced pleasure in me," it said.

The historic reference continues with the Kama Sutra, a classic text on erotica written during the early fifth century A.D, where descriptions of kissing techniques abound.

Bust of Alexander the Great, 2nd century. Acquired by Henry Walters, 1912, Walters Art Museum / Wikimedia Commons

Around 326 B.C., kissing began spreading from India, thanks to the conquering armies of Alexander the Great.

"They learned about kissing from the Indians. After the death of Alexander, his army split up and his generals went to various areas of the Middle East," Bryant said.

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A couple kissing. Terracotta figurine made in Tarsus, Roman Era. Jastrow/ Wikimedia Commons

The Romans were the ones who popularized kissing, spreading the practice to most of Europe and parts of North Africa.

"They were devoted 'kissing' missionaries," Bryant said.

For them, a kiss wasn't just a kiss. There was the osculum, which was a kiss of friendship often delivered as a peck on the cheek. There was the basium, a more erotic type of lips-to-lips kiss, and, finally, the savium. This was the kiss of passion that later became known as the "French kiss."

Kissing was so much a part of the ancient Roman culture that related laws were passed.

"One stated that if a virgin girl were kissed with passion in public, she could demand to be awarded full marriage rights from the man," Bryant said.

Detail of a miniature of Love kissing the Lover, 1490 – 1500.Master of the Prayer Books / Wikimedia Commons

In the Middle Ages, all of Europe was kissing. However, the practice was governed by one's rank.

People of equal rank, both male and female, would kiss on the lips; people of lesser rank would kiss one's cheek, hand, knee, foot or the ground in front of the person.

"The greater the rank difference, the further from the lips one would kiss that person," Bryant said.

As many did not know how to read and write, a kiss was also used to seal contracts. People drew an "X" for their name on the document and kissed it to make it legal. That's the origin of the X put on Valentines or letters to symbolize a kiss.

By the 1300s the Catholic Church became greatly concerned about kissing, fearing that it would lead to carnal acts.

At the Council of Vienna in 1311-1312, Pope Clement V forbade the so-called "holy kiss" during church services. Still today, a handshake is used as a gesture of peace in Catholic Church in place of the holy kiss.

The Cavalier's Kiss, Frederic Soulacroix (1858-1933). Wikimedia Commons

The great "age of kissing" in England and much of Europe ended in the mid- to late 1600s, when it was replaced by other gestures, such as bows, curtsies and topping one's hat.

According to Bryant, the Great Plague of 1665 in London, rather than the strict Church laws, played a role in the change.

"It encouraged many to stop kissing for fear of spreading the disease," he said.

Between 1760 and 1840, during the Industrial Revolution, the hand kiss became popular in England and eventually evolved into handshaking.

An iconic scene from the film, "The Kiss."Wikimedia

In 1896, a silent film called "The Kiss" showed for the first time two actors, May Irwin and John Rice, repeatedly kissing.

The scene caused a scandal, as this contemporary review testify: "The spectacle of the prolonged pasturing on each other's lips was beastly enough in life size on the stage, but magnified to gargantuan proportions and repeated three times over -- it is absolutely disgusting."

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One of the most iconic images of a kiss. New York City celebrating the surrender of Japan, 1945. Lt Victor Jorgensen/ US Archives

Romantically, passionately, longingly, million of kisses are given on Valentine's Day.

Wondering how many type of kisses are there today? German neuroscientist Onur Güntürkün spent two years around airports, railway stations, parks and beaches, watching people kissing.

He recorded 124 "scientifically valid kisses."