2. Smelling with genes

‘Smelling with genes’ begins by looking at the 1991 Nobel Prize-winning discovery of genes in rats that encoded proteins functioning as olfactory receptors. After this groundbreaking discovery, researchers were soon able to identify similar genes in other vertebrates. Experiments with androstenone, a smell secreted in human sweat and the saliva of male pigs, reveal that humans’ DNA sequences can be predicted by their response to certain specific smells. Smell receptor genes of all kinds evolve quickly in humans and animals. What do variants in our olfactory gene repertoire tell us about our ancestors and the processes behind our evolution?

1. How we smell

‘How we smell’ explains the processes, mechanisms, and anatomy behind smell or olfaction. What is the dimensionality of smell? Ancient Greek and Roman philosophers argued that pleasant smells were made up of round atoms and unpleasant smells of pointed ones; while the detail is incorrect, the theory is accurate. When we smell something, olfactory neurons send a response into the brain where they converge with cells with the same receptor type to form a ball-shaped structure, the glomerulus. Higher order neurons then combine signals across glomeruli to extract olfactory information from the environment. All animals with a brain share this basic wiring diagram for detecting smells.

6. Smell in culture

‘Smell in culture’ explores the social, political, and historical significance of smell, including smells in literature and politics. Since prehistory, humans have manufactured scents and often associated them with the supernatural. Originally used in worship and against infection or evil spirits, perfume is now commonly used. Smell has been used as a weapon in political discourse to enforce divisions between ethnic groups or social classes. Different cultures perceive smells in different ways. Much of how we feel about smells depends on our own cultural conditioning. Can smells be used in urban planning? How might the planet Mars smell?

7. The smell of the future

How is the climate crisis affecting our sense of smell? ‘The smell of the future’ looks at the effect of chemical changes in the atmosphere and their disruptions to the behaviour of birds, bees, and fish. Humans in highly polluted areas have decreased olfactory function, which can affect mental health. Recent attempts at building artificial noses have met with varying degrees of success. Some of these electronic noses can perform useful functions such as sniffing out diseases. There is no magic solution to loss of smell or smelling things which are not there (phantosmia) but there are some techniques that patients can use to help a lost sense of smell return.

5. The ecology of smell

Ecology is complex. ‘The ecology of smell’ demonstrates that smell is no exception. Some organisms even use olfactory trickery and chemical confusion to ensure their needs are met. The consequences of this can be minor, as in a flower or plant delivering a simulacrum of sex rather than the promised pollen or nectar—or it can be catastrophic, as in parasitoids taking over a host’s body to consume it from the inside. Some ecological relationships are beneficial rather than exploitative, such as the relationship between humans and fruit flies. We can use smell to disrupt insects’ reactions and urges, as in the use of sprays against mosquitoes.

4. Smell, location, and memory

Is there scientific evidence that smells can unlock memories? ‘Smell, location, and memory’ demonstrates that there are connections. These connections can be positive or interesting (babies whose mothers consume certain foods during their gestation seem to remember certain tastes) or negative (smells can trigger trauma). Smells are more efficient than images at helping us recall childhood memories. Alzheimer’s patients often experience a decline in olfactory function and people with damage to their frontal lobes can have difficulty remembering or classifying smells. Rats’ sense of smell can be trained and improved over time and olfactory learning in insects can significantly improve their chances of survival.

3. Smell signals

‘Smell signals’ looks at pheromones—chemical substances released by animals that cause a specific reaction in another of their species. The clearest examples of pheromonal communication come from insects, including bees, moths, and fruit flies. Scientists have found it harder to identify pheromones in vertebrates. There is chemical communication between animals, and examples of pheromonal signalling in mice, goats, and rabbits. For pheromone evolution to occur, both stimulus and receptor must change simultaneously. Pheromones are generally not proteins, so are not directly affected by genes. While humans are quick to accept the idea that they have pheromones, there is no decisive evidence.