The kiss may have evolved for reasons that are far more practical - and less alluring.

British scientists believe it developed to spread germs.

They say that the uniquely human habit allows a bug that is dangerous in pregnancy to be passed from man to woman to give her time to build up immunity. Cytomegalovirus, which lurks in saliva, normally causes no problems. But it can be extremely dangerous if caught while pregnant and can kill unborn babies or cause birth defects. These can include problems ranging from deafness to cerebral palsy.

Writing in the journal Medical Hypotheses, researcher Dr Colin Hendrie from the University of Leeds said: “Female inoculation with a specific male's cytomegalovirus is most efficiently achieved through mouth-to-mouth contact and saliva exchange, particularly where the flow of saliva is from the male to the typically shorter female.” Kissing the same person for about six months provides optimum protection, he added. As the relationships progresses and the kisses become more passionate, her immunity builds up.