Sense of Smell
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The sense of smell plays a significant and often essential role in social and sexual behavior, learning and memory, identification of food, and detection of hazards. Olfactory receptors, which detect scents from the environment, belong to the largest gene superfamily in mammals, highlighting their apparant importance across species. However, the human sense of smell is commonly believed to be relatively poor compared to that of other mammals, including great apes.
Genetic evidence indicates humans have lost the function of all but 388 of over 1,000 olfactory receptors encoded in the genome. These non-functional genes, called pseudogenes, appear to have accumulated faster and be in higher abundance in the human genome compared to their putative orthologs in chimpanzee, gorilla, orangutan, and rhesus macaque genomes. This loss in the repertoire and function of olfactory componentry indicates reduced evolutionary selection pressure and may reflect reliance on other sensory systems. In another study, a comparison of the human genome to olfactory receptor gene orthologs in the chimpanzee genome indicates the number of intact olfactory receptor genes in both humans and chimpanzees is shrinking, indicating a diminished importance in both species from relaxed positive selection constraints.
Loss of olfactory receptors is linked to the evolutionary appearance of a heightened trichromatic visual system in humans; similarly, primates with trichromatic color vision have a larger fraction of pseudogenes than other mammals, suggesting olfaction is less important in species with trichromatic vision.
Comparative behavioral studies on the function of olfactory sense in humans and other hominids is lacking, particularly for utility in their appropriate environments. Based on genomic comparisons of pseudogenes, great apes should benefit from a larger repertoire of functional receptors. However, behavioral studies indicate human smell perception is equal, or in fact greater, to that of other mammals and great apes, suggesting human sense of smell may, in fact, be relatively good despite a larger proportion of pseudogenes.
Regarding the origins of modern human sense of smell: the unique projecting external nasal structures of the human nose may have negative consequences for human odor detection by absorbing relatively more odor molecules prior to detection by olfactory receptors. The external nose is thought to be an adaptive mechanism to an increasingly arid environment in order to filter and humidify air. The loss of olfactory receptors is also linked to bipedalism, in that elevation of the nose from the odorant-rich ground may have reduced selection pressure on the olfactory system. Changes in sensory receptors which are “non-essential” may enable animals to occupy new ecological niches; the loss of function of olfactory receptors may be a reflection of the importance of a changing human habitat on modern human olfaction.
While human scent detection is popularly believed to be poorer than that of other mammalian species, including that of non-human primates, genetic and behavioral evidence indicate humans have retained comparable sense of smell to that of other great apes.
Cross-regional smell surveys show aging, learning and experience increase variability within the human population-wide repertoire of smell perception and consequent behavior. Differences between male and female attention to odor have been well documented in humans, but not yet shown in great apes. These differences may be indicative of relaxed selective pressure in favor of cultural practices (i.e., 'use-it-or-lose-it' social training for a retained breadth of odor perception in some humans).
It is commonly believed that increased accumulation of olfactory receptor pseudogenes indicates loss of function and thus an overall reduction in the sense of smell in primates.
Gilad's study using a Poisson Random Field (PRF) model for polymorphism and divergence indicates natural selection acts on olfactory receptors in both chimpanzees and humans. Specifically, where most intact human olfactory genes are no longer under strong selective constraint, several olfactory receptors (shared with great apes) maintain important functions maintained by positive selection.
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