text: Certain anthropological discoveries revealed that in those earlier times of human evolution, mothers used to feed their infants mouth to mouth by chewing food, the same way birds feed their own. This act seem to have been slowly took form in kissing lip to lip. I believe this particular act is so sensual as it combines all five senses in to a singular activity. closer proximity of the person's face fill our vision, their moaning are heard by ears, nose smells the partner's aroma and taste and touch are with the lips and tongues entangled with each other.... I hope this is the right answer that will quench your curiosity