A kiss may be just a kiss, but when sweethearts pucker up on Valentine's Day, they will be participating in one of the most bizarre and unlikely of human activities.
Experts say kissing evolved from sniffing, which people did centuries ago as a way of learning about each other.
"At some point, they slipped and ended up on the lips, and they thought that was a lot better," said Vaughn Bryant, an anthropologist at Texas A&M University and an authority on the evolution of human kissing. "You got a lot more bang for your buck."
For most of early human history, smell was more important than any other sense for human relationships, said Sheril Kirshenbaum, author of "The Science of Kissing." People would use smell to determine a person's mood, their health and their social status, she said.
"There were a lot of sniff greetings," said Kirshenbaum, director of the Project on Energy Communication at the University of Texas. "They would brush the nose across the face, because there are scent glands on our faces, and over time the brush of the face became a brush of the lips, and the social greeting was born that way."
Kissing as a romantic sense of expression is believed to have begun in India, where an epic poem called the Mahabharata - believed to have been written about 1000 BCE - included history's first recognizable descriptions of romantic kissing.
"She set her mouth to my mouth and made a noise that produced pleasure in me," the poem said.
Historians believe that at the time, romantic kissing was unknown in the rest of the world, and that it was brought to Europe by Alexander the Great.
In ancient Greece, kissing was a way to communicate status, rank, and loyalty among men in a military or court setting, Kirshenbaum said.
"It was a way to express a sort of a social hierarchy," she said.
Kissing is mentioned in Homer's Odyssey, but as a form of supplication, not romance, she said. For example, Odysseus returns home and is kissed by his slave.
For much of human history, the location of the kiss on the body would demonstrate rank within a royal household or the army. A social equal would kiss a man directly on the mouth, and subservient soldiers, servants, and slaves would kiss the cheek, the hand, the feet, the hem of the robe, or even the ground in front of a person who was considered to be too regal to be kissed at all. This continued into the 18th century.
But by the days of Julius Caesar, the Romans were "kissing fools," Bryant said. The poet Ovid in his poem "Amores," referred to the "savium," which the Romans called the "soul kiss" and which we now refer to as the French kiss, Bryant said.
The Roman Emperor Tiberius tried to ban the kiss because he thought it was a way that people were spreading leprosy, Kirshenbaum said.
"But he was unsuccessful, because people really liked to kiss," she said.
The biggest kissing killjoys of all were the early Christians. Kissing is prominently mentioned nine times in the Bible, but only once, in Romans, does it refer to a romantic kiss. There are kisses of treachery (the Judas kiss), kisses of greeting, kisses of subjection and the kiss of life (in Genesis).
Several popes tried over the years to ban romantic kissing. In 1312, Pope Clement V decreed that "kissing done with the intent to fornicate was to be considered a mortal sin."
At that time, kissing remained unknown in much of the world. When European missionaries fanned out to Africa, Asia and Oceania in the 19th Century, they popularized kissing in places that found kissing abhorrent.
"They carried the word of kissing as well as the word of God to many of these people," Bryant said.
In Japan, the kiss was thought to be offensive when it was introduced by Americans in the 19th century. When the Rodin sculpture "The Kiss" was exhibited in Tokyo in the 1920s, it was hidden behind a bamboo curtain, and kissing scenes were carefully deleted from Hollywood movies shown in occupied Japan after World War Two.
Kissing today takes on many meanings, according to Chicago relationship counselor Jeffrey Sumber, who has written on the social importance of kissing.
"Kissing is communication with bodies," he said. "It is the bridge between our words and our actions."