But Andy, couldn't it also be argued that the 'silent warning' is equally invalid on the basis that there are no predetors in the womb..? ;-)
Indeed, if it's a subconscious urge that we've kept from prehistoric days, surely the lack of oxygen in the womb is irrelevant?
More importantly, if yawning were something related to a silent defence mechanism, which would surely also go hand in hand with an increased level of adrenalin, how come I yawn a lot when tired (generally in the entirely benign situation of when I'm with my wife, sitting at home, in front of the idiot box) but not when I'm walking down a street full of yobs and yobbettes..?