Human sense of smell is among the most often underestimated yet astonishing senses. The human nose contains about 400 different types of odor receptors, and through the combination of these receptors, the brain can distinguish trillions of different smells โ far exceeding the ability of any man-made instrument to analyze complex chemical mixtures.
The sensitivity of the human nose is extraordinary. For certain chemical compounds, humans are able to detect their presence in the air at concentrations less than one part per trillion (1 ppt) โ meaning one molecule of the substance in one trillion molecules of air. A compound called geosmin, which is responsible for the smell of soil after rain (petrichor), can be detected by humans at as low as 0.4 parts per trillion.
Evolution has given humans a highly sensitive sense of smell for practical reasons: detecting spoiled food, avoiding predators, identifying sick people, and possibly for mate selection through chemosignals. Studies show that humans are able to recognize the smell of people who have a different major histocompatibility complex (MHC) from their own โ a sign that smell plays a role in mate selection involving genetic diversity.
Unlike other senses, the sense of smell is the only sense where information is sent directly to the limbic system (the emotional center) of the brain before reaching the cerebral cortex (the rational thinking center). This is why smell is so powerful in triggering strong memories and emotions โ the smell of baked bread may immediately bring back childhood memories because of this direct neural pathway.