From http://www.kissing.com/faqkiss.html
Anthropologists think the custom originated in the transfer of food from mother’s to baby’s mouths, a kind of lip contact that adults continued. But as I point out in other chapters on kissing from around the world, customs differ from culture to culture, and there are some places where kissing is really bizarre. Check out Harpers Magazine (Feb. 1996) for more on the origin of kissing.
From http://www.anatomy.usyd.edu.au/danny/anthropology/sci.anthropology/archive/november-1996/0188.html
The founder of human ethology, prof. Irenaeus Eibl-Eibesfeldt has found the
most plausible explanation of the origin of the kiss, I have ever seen:
In ‘primitive’ societies, where you cannot get canned baby-food, parents
chew the food for their babies and stuff it into the mouth of the baby
by mouth to mouth contact, pushing it with their tongue. This is called
kiss-feeding. This behavior pattern has evolved into a ritual of kissing the
baby, transferring only saliva. The function of this ritual is to show
affection rather than to feed.
In many animal species (including Homo Sapiens) mates give each other food
as part of the courtship ritual.
Adding these two observations, it is easy to imagine that our ancestors
millions of years ago have kiss-fed each other as part of their love-making
rituals. Nursing a baby is emotionally involving for parent as well as for
child. This positive emotion is transferred or ‘copied’ into the courtship
rituals by kissing the love-partner.
The etiological connection between baby-nursing and adult courtship behavior is
also reflected it the fact that many people call their lover ‘baby’.
Eibl-Eibesfeldt’s books have been translated into several languages. Look for
the titles ‘Love and hate’ (London 1971), and ‘Human Ethology’ (NY 1989).
Also see: http://battalion.tamu.edu/archives/99a/2-11/fp4.html
http://www.newswise.com/articles/2000/2/KISSING.TXM.html
http://www.gallery127.org/kissing/
http://www.wsu.edu:8001/vcwsu/commons/topics/culture/behaviors/kissing/kissing-essay.html