Economist’s Intelligent Life publication has a fascinating look into the science behind the sense of smell:

“When a chemical molecule enters the nose, it finds its way to specialised receptors embedded in the mucous membrane. Rats, the first creatures to surrender their olfactory blueprint, rely on more than 1,200 receptors. In humans, 350 active receptors, each the product of a single gene, stand guard. Another 600 olfactory ?pseudo-genes? no longer sponsor receptors, an evolutionary consequence of less reliance on smell. All told, olfactory genes comprise about 3% of the human genome, a much larger population than other senses require. Sight makes do with three genes and taste with 29?evidence of smell?s complex challenge.

To understand how receptors distinguish lilac or apple pie from soured milk or cleaning solvent, imagine receptors as an alphabet with 350 letters. Every chemical triggers multiple receptors; resulting combinations spell distinct odours. Do the arithmetic, and you?ll find ample capacity to detect far more than 100,000 chemical odours adrift in the environment. Odour receptors trip neural impulses that notify the olfactory bulb which, in turn, notifies regions of the brain where smell taps primal dimensions.”