When evolution led mammals to adapt to a nocturnal existence, olfactory bulbs developed, and smell replaced sight as the dominant sense. These aroma circuits or odor pathways are believed to have become the neural outline of the limbic system within the mammalian area. This is the emotional center that generates the parental care of mammals: playfulness, vocal calling and emotion. Emotion and mood originally evolved from neural structures, once allocated only to the sense of smell that merely attracted or repelled. Over time, these two simple responses became the complicated emotions we experience today.