Smell is arguably the most ancient sense. Consider for example a bacterial prokaryote, monocellular, anuclear, agella-rotating, as it tumbled through the Archaean (Archaeozoic) seas roughly 2-3 billion years ago. For such an organism, chemotaxis-the ability to redirect its movements in the presence of chemical gradients-was synonymous with survival. This most rudimentary sense of smell was critical for nding chemoattractants like organic nutrients (Figure 5.1a; Adler 1969) and evading chemorepellents like excreted waste (Figure 5.1b; Tso and Adler 1974; Adler 1978). In this manner, the sense of smell was rooted at the earliest evolutionary stages with the machinery of affective processing, to the extent that chemical sensing was indistinguishable from the sensing of biological imperatives.