Olfactory receptor

Olfactory receptors (ORs) are a superfamily of G protein-coupled receptors expressed in the olfactory epithelium that detect odorant molecules and transduce chemical signals into neural activity. They are the largest gene family in mammalian genomes: humans possess approximately 400 functional OR genes and 600 pseudogenes, while mice have over 1,000 functional receptors — a testament to the evolutionary importance of smell in nocturnal, spatially navigating species.

The OR gene family expanded through repeated rounds of segmental duplication and divergence, producing clusters of related receptors with slightly different ligand-binding specificities. This combinatorial expansion allows mammals to discriminate thousands of odorants using a repertoire of receptors that individually recognize only a narrow range of molecular features. The evolutionary history of OR genes — expansion, contraction, and pseudogenization — tracks the ecological shifts of lineages: marine mammals that returned to aquatic life lost large numbers of OR genes because olfaction is less useful underwater.

The olfactory receptor repertoire is an evolutionary palimpsest: each species' sense of smell is a record of where it has lived, what it has eaten, and who it has needed to find.