‘The world is the totality of facts, not things.’ –Wittgenstein
A fragrant object lies before you—say, a flower, some stinky cheese, or a smelly sock. Molecules break off from the object, evaporating into the air and floating towards your face. A sniff pulls the molecules into your nostril, where they travel through your nasal cavity towards your olfactory neurons, which extend out through holes in your skull into the mucus layer of your nose. The molecules activate receptors studding the outside layer of individual neurons, which send a message to your brain that something smells.
Humans have around 350 types of receptors on 40 million neurons, which in combination allow us to distinguish more than a trillion different odors. Despite this incredible power of olfaction, the human sense of smell is often neglected, considered a holdover from our animal past or a source of unseemly sensations. It doesn’t help that odors are often literally beneath us; the evolutionary argument goes that when early humans started walking upright, our nose got farther away from the smells on the ground, decreasing the relative importance of olfaction for getting around. Evidence contradicting this story came from a 2006 study which recruited undergraduate students to get on all fours and track the scent of chocolate oil dripped on grass. They were remarkably good at it, showing that our “bad” sense of smell isn’t biologically determined—it’s just that we’re out of practice. […]
We know a lot about how a smell signal travels from an activated receptor on a neuron to the brain. This pathway has been worked out in painstaking detail, with cascades of cellular switches and neural action potentials that signal our brain. […] But how does a smelly molecule activate its receptor in the first place? There is still no satisfactory answer to this question despite nearly a century of research.
