An Immense World: How Animal Senses Reveal the Hidden Realms Around Us

by Ed Yong

Unless you actually stuck your nose over some benzaldehyde, you couldn’t guess that it smells like almonds. If you saw dimethyl sulfide drawn on a page, you couldn’t foresee that it carries the scent of the sea. Even similar molecules can produce immensely different smells. Heptanol, with a backbone of seven carbon atoms, smells green and leafy. Add another carbon atom to the chain and you get octanol, which smells more like citrus. Carvone exists in two forms that contain exactly the same atoms but are mirror images of each other: One smells of caraway seeds and the other of spearmint. Mixtures are even more confusing. When mixed, some pairs of odors still smell distinct, while others produce a third smell that’s unlike the two parents. Meanwhile, perfumes that contain hundreds of chemicals don’t smell any more complex than individual odorants, and people typically struggle to name more than three ingredients in a blend. Noam Sobel, a neurobiologist who studies olfaction, has come closer than anyone else to wrangling this complexity. While I was writing this book, he and his team developed a measure that analyzes 21 features of odorant molecules and collapses these into a single number. The closer this smell metric is for any two molecules, the more similar their odors. This isn’t quite the same as predicting scent from structure, but it’s the next best thing—predicting scent from similarity to other scents.