Of course, this is presuming that we do, in fact, kiss, and, frankly, the jury's still out on that. Welcome to Oberlin, right?
Anyway, here's an empirically adequate and plausible account of the evolutionary processes that led to kissing:
They say the gesture allows a bug named Cytomegalovirus, which is dangerous in pregnancy, to be passed from man to woman to give her time to build up protection against it.
The bug is found in saliva and normally causes no problems. But it can be extremely dangerous if caught while pregnant and can kill unborn babies or cause birth defects.
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 the best protection, he added.
As the relationships progresses and the kisses become more passionate, the woman's immunity builds up, cutting her odds of becoming ill.
By the time she becomes pregnant, the odds of her unborn baby becoming infected are much lower.
Of course, as with much of evolutionary biology, there are competing empirically adequate accounts:
Previously scientists have claimed that kissing acts as a form of evolutionary quality control, with saliva holding clues to fertility, health and genes.
It seems to me that the flying spaghetti monster has us pretty well wired up.
But seriously, though, kissing, like so much of evolutionary biology seems rather arbitrary and sort of bizarre. As Wallace Shawn wrote in the August 2009 Harper's (sub. requ.):
There may be an adaptive value to each particular choice that evolution made, but from our point of view as human beings living our lives, the various details present themselves to us as arbitrary. It can only be seen as funny that demagogues give speeches denouncing men who insert their penises into other men's anuses--and then go home to insert their own penises into their wives' vaginas! (One might have thought it obvious that either both of these acts are completely outrageous or none of them are.
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