Smell is the ability of an organism to sense and identify a substance by detecting trace amounts of the substance that evaporate. Researchers have noted similarities in the sense of smell between widely differing species that reveal some of the details of how the chemical signal of an odor is detected and processed.

The sense of smell has been a topic of debate from humankind's earliest days. The Greek philosopher Democritus of Abdera (460-360 B.C.) speculated that we smell "atoms" of different size and shape that come from objects. His countryman Aristotle (384-322 B.C.), on the other hand, guessed that odors are detected when the "cold" sense of smell meets "hot" smoke or steam from the object being smelled. It was not until the late 18th century that most scientists and philosophers reached agreement that Democritus was basically right: the smell of an object is due...