A. The Olfactory System
When working normally, the sense of smell, or olfaction, enables us to detect a large number of different odorants and to perceive these volatile compounds as odors. Although the stimuli (odorants) are also sometimes called odors, in psychophysics, odor refers to a percept, the result of the process of odor perception, whereas odorant refers to the chemical that elicited the odor.
The airborne molecules from food take one of two paths to sensory cells in the olfactory epithelium: the orthonasal route (through the nostrils, before eating) or the retronasal route (through the nasopharynx, while eating). Both paths are important in food intake, for defense and for pleasure. In the olfactory epithelium, the airborne odorants are detected by olfactory receptors. The receptors lie embedded in the membrane of olfactory sensory neurons, each of which accommodates only one type of receptor.140Binding of an odorant molecule by an olfactory receptor initiates a signal transduction cascade, which ultimately leads to the transfer of the olfactory signal to the brain, where the odor percept is generated.
To be a potential odorant, a molecule has to be volatile enough to reach the olfactory epithelium with airflow. Although most volatiles are odorants, some small molecule volatiles, such as carbon monoxide and carbon dioxide, are odorless. In addition, structurally diverse molecules can elicit indistinguishable odors, while similar molecules, such as stereoisomers, can yield distinct odors (e.g., R(–)-carvone smells like spearmint, but S()-carvone smells like caraway).141,142 To date, the type of odor elicited by a volatile compound cannot be reliably predicted by the structure of the molecule.143
Humans have about 400 different olfactory receptor types, a number greatly exceeded by the number of potential odorants. Thus, it is unlikely that a particular receptor would bind only one type of odorant or that a certain odorant would attach specifically to only one type of receptor. Instead, the olfactory system is thought to make use of combinatorial receptor coding to gain the capacity to recognize the immense amount of odorants; several types of related receptors bind an odorant with varying affinities, and in turn, multiple related odorants can be detected by the same receptor.140 The combinatorial coding suggests that most olfactory receptors are selective (broadly tuned) rather than very specific (narrowly tuned). However, the breadth of tuning varies among olfactory receptors.144,145
Only a few of the human olfactory receptors have been linked with their odorant ligands (i.e., the molecules that they detect).146 Development of automated, high-throughput methods for matching the receptors and their ligands in cell-based model systems (or using computational models) will facilitate confirming the functionality of the receptors. These methods, however, cannot replace the measurement of actual human responses in studies of geneticinfluence on odor percepts. The psychophysical measurement of responses to odorous stimuli remains a time-consuming but essential step when the genetics of the sense of smell and its implications for food intake are studied.
B. Genetics of Olfaction
Humans have nearly 400 potentially functional olfactory receptor genes (OR genes), making this gene family one of the largest in the human genome.147 In addition to these intact genes, which are thought to produce functional olfactory receptors, humans have at least a similar number of nonfunctional OR genes (pseudogenes) and about 60 genes of which both functional and nonfunctional variants are known to exist (segregating pseudogenes). The exact number of functional genes will be known only after the functionality of the corresponding receptors is demonstrated. However, it is obvious that far more genes encode receptors for smell than for taste. The larger number of olfactory receptors likely reflects the need to detect a wider variety of compounds than is the case for taste. Further, the large number of compounds detected by the sense of smell reflects the wider role of this sense: While the sense of taste serves almost exclusively ingestion, the sense of smell has other functions, too. These include sensing environmental dangers (e.g., smoke) and potential interpersonal chemosignaling (e.g., sexual selection).
The heritability of a trait makes the search for genes influencing that trait reasonable. If little or no heritability is found, the underlying genes, if any, are difficult, if not impossible, to locate in gene-mapping studies. While the ability to smell some odorants is heritable, for other odorants, it is not. For instance, the ability to smell food odors like chocolate or lemon is associated with little or no heritability.148,149However, the pleasantness of cinnamon is heritable and has been mapped to chromosome 4 by linkage analysis.150 If the allelic genes that determine the pleasantness of odors like cinnamon are identified, studies of genotype and food intake might be worthwhile.
Individual variation in perception of some odors has been attributed partly to specific OR genes. The differences among people in the ability to smell androstenone are at least partially determined by genes,151,152 and an allele of an OR gene, OR7D4, contributes to this trait.153 However, unlike alleles of the taste receptor gene TAS2R38, which account for almost 70% of the person-to-person variation in perception of bitter taste from PTC,41 OR7D4 alleles account for only a small amount of variance in perception of androstenone.153 Two other OR genes have been associated with individual variation in the sense of smell: OR11H7 with isovaleric acid (sweaty odor)154 and OR2M7 with the smell of asparagus metabolites in urine.155 Association between the gene OR2J3 and detection of cis-3-hexen-1-ol (green leaf odor) has also been suggested.156 Why there is relatively little effect of the alleles of a singleolfactory receptor on perception lies in this sense’s complex nature: Many olfactory receptors combine to detect a particular odorant,140 and one odorant may stimulate many receptors, so if one is not working, others may compensate.
Systematic, repeated exposures to individual odorants have been demonstrated to lower detection thresholds (increased sensitivity) to these odorants, suggesting that genes do not entirely determine the perceptions.157,158 One possibility, yet to be proved, is that there are gene-environment interactions in odor perception (i.e., genes influencing the sense of smell are controlled differently in different environments). Whatever the mechanism, the flexibility of the sense of smell could have been evolutionarily appropriate. When first humans moved to new environments and encountered novel odorants from new threats (e.g., toxins) and opportunities (e.g., food sources), the flexible sense of smell may have helped the population to survive.
C. Implications for Food Intake
Although there may also be some innate preferences, smell is probably more flexible and amenable to learning159 when compared to taste. This point is particularly relevant when we consider olfaction as a sentry against spoiled food: The products of fermentation can be perceived as wholesome or harmful, depending on context. As an example, isovaleric acid has a pungent odor that people like if they are told it is from cheddar cheese and dislike if they are told it is from body odor.160 Likewise, people will eat food with a bad smell (e.g., durians or limburger cheese) if they know it is safe and they like the taste. In addition, the pleasant odor of food can stimulate appetite, but the potency of these genetic differences in determining food intake and obesity is unclear.
On the nose: genetic and evolutionary aspects of smell
Among my Christmas presents this year was some perfume – Acqua di Parma, in a beautiful cylindrical buttercup-yellow box. It was from my son, who since his transition to adulthood has developed an interest in such things, and in return we gave him (as requested) Terre d’Hermès. To my ill-educated nose, both smell pretty good – but there’s a more expert source to turn to for an opinion. This is Perfumes: the A-Z Guide, by Luca Turin and Tania Sanchez . Behind its unpromising title lies an entertaining, witty and informative book. The authors write with withering style about the scents they most dislike. So it’s with trepidation you look up the perfume you’ve acquired – luckily those mentioned above both rate a respectable three out of five stars. One-star reviews include ‘smells like a New York sidewalk in July’, ‘less a fragrance than a headache force-field’, and ‘useful as a contraceptive, but little else’.
The ~1800 perfumes sniffed for The A-Z Guide are testament to the lengths to which we humans will go to make us smell like something else. We wash ourselves with soaps and shampoos, anoint our sweatiest bits with deodorants and antiperspirants, and then spray on expensive cocktails of scented chemicals and natural extracts. This is curious, because it appears that nature’s clear intention was for us to smell abundantly of ourselves.
When our ancestors lost their body hair, they retained the associated sebaceous glands designed to anoint each hair with water-repellent secretions. In fact, we have denser aggregates of such glands than almost any other mammal. In addition, we each have 3 million sweat glands capable of exuding 12 litres of cooling fluid daily. There are two varieties – eccrine, which secrete 99% water, and apocrine, which secrete an oily fluid including proteins, lipids, fatty acids and steroids. The apocrine glands are confined to the hairy parts of the body, including the genital area and armpits (axillae). Indeed, in most humans the axillary density is so great that the array of glands is considered an organ. So, with our inherently smelly sebaceous secretions, and our apocrine sweat, made odorous by skin bacteria, we are without doubt the ‘scented ape’ .
Scented humans may be, but some are more scented than others. A few unfortunate people are homozygous for mutations in the gene encoding an enzyme, flavin-containing monooxygenase 3 , whose job is to metabolise amino-trimethylamine, produced by bacterial action in the gut. In the absence of the enzyme, the chemical is secreted in the sweat, urine and breath – its smell, reminiscent of decaying fish, makes the lives of sufferers very difficult indeed. In the general population travellers and anthropologists have remarked upon differences between the groups they encountered. Some of the early anecdotal accounts are, to modern minds, highly derogatory, and do not bear repeating. The major observation, however, seems real and evolutionarily interesting: in general, East Asians are less smelly than everyone else. This is connected to the number of apocrine glands in the axillary organ; while Europeans and Africans have glands packed so closely that they resemble a sponge, in Koreans (for example) they are either spread thinly or absent altogether .
The genetic basis of apocrine gland density is unknown, but genetics has illuminated population differences in odour via the seemingly unrelated subject of earwax. There are two kinds – the grey and flaky ‘dry’, prevalent in East Asians, and the yellow and waxy ‘wet’. A single nucleotide variant in the genome is responsible for this difference  – individuals who carry an A nucleotide at the relevant position in both copies of their ABCC11 gene (AA homozygotes) have dry earwax, while GA heterozygotes or GG homozygotes have the wet variety. The population distribution suggests positive selection for the A-allele in Asia, but earwax itself seems an improbable candidate. However, earwax emerges from specialised apocrine glands, and analysis of sweat from people carrying different genotypes indicates that the ABCC11 A variant is also responsible for reduced axillary odour , thanks to a failure to transport a smelly-molecule precursor into the sweat . How selection came to act so strongly on this variant is not clear – perhaps choice of partner (sexual selection) was influenced strongly by odour, though why this should be so in some parts of the world but not in others, is puzzling.
There are two sides to smell, of course – as well as production, there is perception, and here most emphasis has been on differences between individuals. At one extreme, some people are born with no sense of smell at all (anosmia). Kallmann Syndrome is usually caused by mutations in the KAL1 gene on the X chromosome, and is often associated with complete anosmia in the male sufferers . This phenotype arises from the failure of neurons in early development to migrate to form the olfactory bulb, where the sense of smell arises; the neurons also fail to reach their next destination, the hypothalamus, with the result that gonadotropin-releasing hormone is not produced. This in turn leads to failure of puberty, and to infertility.
Odour detection is mediated through olfactory receptors (ORs) in the cell membranes of olfactory neurons, encoded by a family of over 300 genes . A combinatorial code of different ORs interacts with odorant molecules, so mutations involving OR genes could lead to specific anosmias – the inability to smell particular odorants. Indeed, genome-wide association studies have found variants within OR gene clusters linked to sensitivity to methanethiol (secreted in the urine after eating asparagus) , androstenone (produced in human sweat, by truffles, and by pigs in the mating season) , and also to the floral-smelling compound β-ionone . The latter probably explains why some people cannot smell β-ionone-rich freesias .
So, what’s the purpose of human scent and our sense of smell? Other animals use scented chemicals (pheromones) to attract the opposite sex, and to indicate fertility – they induce stereotypical behaviours, as anyone who has owned a cat or dog in heat will know. Humans become highly scented animals when they reach sexual maturity, and poets from Catullus to Herrick have written with passion about the fragrances of their lovers. In the nineteenth century rustic Austrian girls used to keep a slice of apple in their armpits during a dance, and would afterwards offer it to their favoured partner to eat, as a token of interest . Despite these sexual connections, compared to other animals, the human sense of smell is of little biological use. The relative proportion of the brain occupied by smell has decreased steadily in the primate lineage from lemurs to humans, and we lack the vomeronasal organ – the ‘second nose’ above the palate that causes our cats and dogs to act in response to pheromones. Plenty of websites offer fragrances such as ‘Alpha Dream’ and ‘PheroMen’, with promises of instant sexual irresistibility. But, despite reports of odour-mediated menstrual synchrony in female roommates , there is little evidence that human pheromones exist.
It seems that our production and perception of smell may be evolutionary vestiges, relegated when we stood upright and our visual systems became of primary importance, and when the need for pair-bonding made advertising female receptiveness disadvantageous. Yet scent enriches our lives, and its animal roots are never far away – among the ingredients of fine perfumes are substances scraped from the anal glands of indignant civet cats, or extracted from the musk glands of rutting male Himalayan deer.