Alan Mann, a Prinston University physical anthropologist at the annual meeting of the American Association for the Advancement of Science in Boston on February 15.

　　Some thousands of years ago, there was a random mutation. It suppressed the calcification of the third molar, a wisdom teeth. So many of you, and many of your children, have grown to adulthood without the apperance of third molars. Now that particular mutation has increased in frequency so that in some populations more than 40% people have one or more absent third molars. So you might ask the question: How did that develop because obviously you have an apparent third molar if you follow survival of the fetus, then you are gonna fall over holding your mouth and dying in agony.

　　Reproduction of the fetuses – what is it all about? In other words, differential reproduction. How will this work? Well, one evening, a person, who has had serious chronic pain with impacted third molars, perhaps of lower third molar, impacting into the mandibular canal(下颌管), causing a lot of pain. The partner comes up and says: How about a bout of reproduction, dear? And the person says: Not tonight, dear, my jaws are killing me.

　　That is in fact a revolutionary scenario; and because that would produce fewer offspring in those people, the prequency of this feature would increase over time.