Not only do all animals use chemical stimuli, but they do so by using similar mechanisms (Ache and Young, 2005; Bargmann, 2006; Jacobs, 2012, Fig. S1). Eisthen documents four convergences in the olfactory system in insects, crustaceans, nematodes, mollusks, and vertebrates: odorant binding proteins in the fluid overlying olfactory receptor (OR) neurons, G protein-coupled receptors as odorant receptors, a two-step pathway in the transduction of odorant signals, and the presence of glomerular neuropils in the first central target of the axons of OR cells (Eisthen, 2002).
Such structural similarities in olfactory systems remain a remarkable and somewhat mysterious phenomenon. The olfactory system presents other problems: OR projections segregate and project to receptor-specific glomeruli, but beyond the glomerulus, there is no obvious topography (Sosulski et al., 2011). The unpredictable variation in the number of OR genes across species is also mysterious. The numbers must be significant, as OR genes represent the largest multigene family in mammals, representing 4% to 5% of the entire proteome (Niimura, 2009). At present, there is no accepted hypothesis to explain this variation, which can range from 1,500 chemosensory receptors in the nematode worm (Caenorhabditis elegans), 130 in Drosophila melanogaster, 900 in the laboratory mouse, to 350 in humans (Bargmann, 2006).
Thus, the study of olfaction is a world of paradoxes: the independent scaling of the OB, the function of convergent neuro-architectures, and the diversity of OR genes. However, perhaps these paradoxes arise from the assumption that the primary function is discrimination. If instead the OS hypothesis is correct, the structural similarities may be explained by convergent cognitive processes for spatial navigation. Likewise, variability in OB size and OR gene number could reflect the species’ use of odorants in spatial navigation. To explore this proposal, first it is necessary to consider how olfaction differs from other senses.
THE PECULIAR CASE OF OLFACTORY PERCEPTION
By its physical properties, the chemical world must be encoded differently. As Bargmann (2006) concluded, “the visual system and auditory system are stable because light and sound are immutable physical entities. By contrast, the olfactory system, like the immune system, tracks a moving world of cues generated by other organisms, and must constantly generate, test, and discard receptor genes and coding strategies over evolutionary time.” Olfaction’s genius for tracking moving targets has important implications. As Osorio et al. (1994) concluded: “the mammalian neocortex with its protean powers has evolved from the olfactory forebrain of primitive vertebrates [Sarnat and Netsky, 1981]. Perhaps