This is an excerpt written by Carl Zimmer from the May 2010 issue of Discover magazine

“Noam Sobel of the Weizmann Institute of Science in Israel set out to pin down how the nose processes odor molecules by determining the relationship between the structure of a molecule and the way it smells. The scientists began by building a database of 1,500 odor-producing molecules, cataloging 1,664 different traits – their size, the strength of the chemical bonds between their atoms and so on.

To see whether this yardstick is universal, neurobiologist Nathalie Mandairon and her colleagues at the University of Lyon in France decided in 2008 to test Sobel’s odor-ranking system on mice. The researchers picked a set of odors and observed how long the animals sniffed them. The closer an odor was to the pleasant end of Sobel’s yardstick, the longer the mice spent smelling it. Sobel’s odor yardstick seems to have uncovered a fundamental truth about the nature of smell. When the noses of our distant ancestors evolved into sophisticated molecular detectors, he suggests, they started to process key traits to come up with a simple measurement – an internal yardstick. Our ancestors felt pleasure when they sensed smells that signified desirable things (a mother’s teat, a ripe piece of fruit) and moved toward them. They felt disgust or fear at odors that signified danger (rotting corpses or a predator’s feces) and moved away.

Our sense of smell is not a purely automatic system, however. An odor, Sobel argues, is more than the physical properties of a molecule; it is also the emotions these properties summon up. This is where learning comes in. We can learn to fear certain smells that signal danger, just as we learn to make associations with dangerous sights or sounds. At a deeper level, our brains can revalue the same smell for different conditions. The smell of bacon may be alluring if you are hungry, but after a fourth helping, the same smell can start to get sickening. Scientists can see this revaluing take place within the brain as emotion-regulating regions change their activity. As a result, we can use pleasure and disgust as a guide not just for finding the right kinds of food but also for eating the right amount. These links to emotion and learning also help explains why a single whiff can summon up powerful memories.

Using Sobel’s yardstick, scientists may be able to start untangling the paradox of smell: why we are so good at distinguishing odors and so bad at naming them. Our brains appear to have evolved an elegant way to reduce the dizzying variety of molecules in our environment into a simple scale based on what matters most about scents – whether they smell good or bad. By giving different odor molecules a place on the yardstick, we can distinguish fine gradations among them. Although this method ma be helpful and efficient, it doesn’t give us much information that we can use in putting a name to different smells. Imagine that someone showed you pictures of different types of fruit and had you name each one. Now imagine that the pictures zoom in on a single patch of color on each piece of fruit. You might be easily able to tell the difference between two shades of red without being able to say which belonged to a strawberry and which to a raspberry.”