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receptors and molecules bind to one another based on shape.
    “The lock and key analogy isn’t perfect,” Stuart Firestein, a leading olfactory scientist and professor of neurobiology at Columbia University, told me. “But it’s close.” He was sitting over a plate of cheese at a table at Le Monde, a warmly lit restaurant across the street from Columbia’s campus. He cupped his left hand, folding it into the shape of a small vase. He extended the index and middle fingers of his right one out straight and moved them slowly into the depth of the pod to show me how a molecule and receptor would bind. This, he demonstrated, is how they come together in the nose. Like pieces in a complicated puzzle, when a match is found, a connection is made and signals are fired. But how, exactly, that match is made remains unknown.
    I asked him about how the brain makes sense of the signals sent by the receptors. That’s another unknown, he said. We do know that each of the receptor neurons—millions of them, scattered haphazardly through the epithelium, a small piece of tissue in the nasal cavity—always sends its signals to the same spot in the olfactory bulb where it connects with other similar signals in little bundles of neuronal connections called glomeruli . But the understanding ends there. We don’t know what kind of pattern is formed, or how it is read.
    “Olfaction is one system without a special map,” Firestein said. “It demands for us to think in a different way.”
    Significant advance has taken place in the last two decades. Scientists Richard Axel and Linda Buck published a groundbreaking paper in the scientific journal Cell in 1991, for which they later won the Nobel Prize in Medicine. They found that the olfactory receptors were determined by genetics.
    The gene family for olfaction, they discovered, is inordinately large. Working with mice, and later transferring their understanding to humans, they identified a family of upward of one thousand different genes devoted to smell, or close to 3 percent of all those in humans. They found that each olfactory neuron only expresses one type of olfactory receptor gene, which means that each neuron only recognizes a select few odorants in the nose. A large percentage of those genes, it turns out, are called pseudogenes and do not produce working receptor proteins, leaving humans with around 350 rather than 1,000.
    Axel and Buck’s discovery, and subsequent award, brought attention to the science of olfaction, which was then a tiny and unfashionable field. Theirs was an important step toward understanding the biology behind olfactory perception, a complicated process that begins with the static but silent beat of the neuron and can end with the conscious flourish of mood.
    “Axel and Buck changed the way olfaction was looked at,” Firestein told me. This was in part because their discovery peeled away one thick layer of the mystery surrounding the science of the nose. Even more, though, it was because their work inspired buzz—in the media, in the scientific community. It created international excitement. This, in the world of olfactory science? Not a usual thing. Suddenly, there was more room for funding. Rising scientists who would otherwise have gravitated toward a different field were drawn toward the nose. Questions were raised. Studies began. The field expanded.
    This development was important, Firestein explained, because the science of olfaction is a new model system. “This means that olfaction can be used to make generalizations on how other parts of the brain work. If we can understand how each neuron expresses one olfactory receptor gene and only one gene, or how the neurons regenerate, or how the brain organizes and processes each smell, then perhaps we can understand how other areas of the brain function.”
    On my laptop, I watched a video recording of the 2004 Nobel lecture given by Axel, a tall, thin man who bent over the podium like a reed. He spoke