The sense of smell is called olfaction. Many animals have a sense of smell far superior (macrosmatic) to humans. Humans are called microsmatic because of their modest olfaction abilities. Yet the human nose is capable of smelling over 10,000 different odors, some in the range of parts per trillion of air. The human nose is actually the main organ of taste as well as smell. The taste buds on our tongues can only distinguish four qualities – sweet, sour, bitter and salt -all other ‘tastes’ are detected by the olfactory receptors high up in our nasal passages.  It’s the smells of food that make them taste good or bad. We have about 5 or 6 million of these receptors while a dog, for example, has about 220 million of them.

Females, in general. have a better sense of smell than males do. Female humans are able, during ovulation, to smell some part of the HLA genes of potential sex partners and prefer partners with HLA genes different from their own. The human leukocyte antigen (HLA) system is the name of the major histocompatibility complex (MHC) in humans. They contain a large number of genes related to immune system function in humans. They are important in disease defense  and may be the reason for mate selection choices by females, as different sets of HLA genes in offspring produce healthier immune systems.

Also, odor is a valuable indicator of disease. From an abscess to viral infections and most everything in between, disease smells, though we may not necessarily all be able to smell it consciously. Sweet or strong body odors were warning clues that our ancestral females in search of good genes for their offspring may have been wired to pay attention to.  Females' better sense of smell, therefore, may well have been due to their need to carefully evaluate a potential mates’ genetic qualities. A poor mate choice for male ancestors may have meant as little as a few minutes wasted, but a human female's mistake could result in a child unlikely to survive.

Humans can identify individuals who are blood related; mothers, fathers and children. Blindfolded women and men o a lesser degree can find their spouse by smell alone. Blood related individuals have similar sets of genes.

Each of us has an odor that is, like our fingerprints, unique. Part of that odor is pheromones, which are chemical substances that the human body secretes. These unique odors change the sexual or social behavior of another individual by attracting or turning off any attraction. The “attracting” powers of pheromones, the scented sex hormones, play an important role in people finding people with whom they have “chemistry”.  A male’s appeal to a female human may depend in a large part on how many immune system genes he shares with her and the less the better.

Memories of childhood events are often triggered in great detail by smells.  The perception of smell is not only of the sensation of the odors themselves but of the experiences and emotions associated with these sensations. Smells can evoke strong emotional reactions and it can change or enhance moods, learning ability, and sleep.