When you really think about it, kissing is pretty gross. It involves saliva and mucous membranes, and it may have historical roots in chewed-up food. Experts estimate that hundreds or even millions of bacterial colonies move from one mouth to another during a kiss. Doctors have also linked kissing to the spread of diseases like meningitis, herpes and mononucleosis.

Yet anthropologists report that 90 percent of the people in the world kiss. Most people look forward to their first romantic kiss and remember it for the rest of their lives. Parents kiss children, worshippers kiss religious artifacts and couples kiss each other. Some people even kiss the ground when they get off an airplane.





So how does one gesture come to signify affection, celebration, grief, comfort and respect, all over the world? No one knows for sure, but anthropologists think kissing might have originated with human mothers feeding their babies much the way birds do. Mothers would chew the food and then pass it from their mouths to their babies' mouths. After the babies learned to eat solid food, their mothers may have kissed them to comfort them or to show affection.

In this scenario, kissing is a learned behavior, passed from generation to generation. We do it because we learned how to from our parents and from the society around us. There's a problem with this theory, though: women in a few modern indigenous cultures feed their babies by passing chewed food mouth-to-mouth. But in some of these cultures, no one kissed until Westerners introduced the practice.

Other researchers believe that kissing is instinctive. They use bonobo apes, which are very similar to humans, to support their idea because bonobos kiss each other frequently. Regardless of sex or status within a social group, bonobos kiss to reduce tension after disputes, to reassure one another, to develop social bonds and sometimes for no clear reason at all.

Scientists, therefore, don't entirely agree on whether kissing is learned or instinctive because there is support for both arguments.





All agree, though, that there are different reasons for kissing and the "messages" are similar in each case. Yes, romance is one of the reasons and the results have psychological, physical, and social factors involved. But romance is not the only reason for kissing.

When a mother kisses her child's bruise to make the hurt go away, psychological, physical and social factors play a part in both people's reactions, too. The same is true when friends kiss as a greeting, worshippers kiss religious symbols or siblings kiss to make up after an argument. Even though some kisses are platonic and others are romantic, they generally have something in common -- they are inspired by and tend to inspire feelings we think of as positive.



Regardless of how people got the idea to kiss or what they mean when they do it, anthropologists are pretty sure that people started kissing thousands of years ago.





But, do other animal species kiss? Aside from the bonomo apes already mentioned, there are signs that other animals do kiss or have their own version of kissing. Many mammals lick each other's faces (pet owner's know pets will lick their human friends' faces), birds touch one another's bills, and snails caress each other's antennae. In some cases they may be grooming or closing in on scent glands, but regardless of the reasons, when animals touch each other in a kissing way, they are often showing signs of trust and affection or developing social bonds.





Is it no wonder then, that we love kissing? This is the reason why even the sight of kissing can stir and move the emotions in an observer.





Photographers and artists have known this for many, many years and, for this reason they often include kissing in their works. Some of what are considered to be classic scenes and masterpieces include strong images of kissing.