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    An odor molecule must be light and lively to make it up the human nose. Most never do. Dogs smell more than we do because they have immense receptor sheets, lower stature, big snouts, and floppy ears. A bloodhound's ears are like the string mops sailors use to swab the deck: they don't let much get by them. A dog's world is saturated in smell. So is a reptile's world, but unlike snakes and lizards, dogs have excellent odor discrimination. A dog can deconstruct smell mixtures in the same way a human can inventory the contents of a room at a glance. There's the sofa, the coffee table, bookshelves, and so on. A person can't identify the ingredients in a bouillabaisse by smelling it unless he or she is a trained professional nose, as perfume designers are called. Dogs can.
    So how do odorants and receptors pair up? It's critical that they do, because this hooking-up is what ensures that an odorant's electrical signal is sent to the brain. The prevailing theory is based on shape. An odor molecule and its receptor fit together like a lock and key. The odorant is the key that "unlocks" the receptor, which then sends an electrical signal along one of the nerve axons that run through the tiny holes in a wafer-thin section of the skull called the cribiform plate. Once inside the skull, the axons join together in clusters called glomeruli (pronounced "gluh-mehr-ya-lie" and less than expertly drawn by me) that transmit smell signals to the two olfactory bulbs located inside the brain just behind the nose and eyes and above the olfactory receptor sheet.
    In the same split second that an odor molecule binds with its receptor, a signal is sent to the correct glomerulus in the olfactory bulb. A team led by Howard Hughes Medical Institute investigator Lawrence Katz of Duke University found that each glomerulus detects individual odorants only, and the olfactory bulb passes these pieces of information on to more advanced brain regions to make readable maps of the whole. The brain has to listen to each musician's melody to hear a symphony, explained Da Yu Lin, who took over the project in 2005 after Katz's death. "The whole is the sum of its parts."
    Or is it? Peter Mombaerts of Rockefeller University collaborated with researchers at Yale to engineer mice that lacked a certain protein cell in the glomeruli of the olfactory bulb. The researchers don't know how the protein works, but without it, mice can't tell odors apart; smells sent on to the higher brain don't make sense. This suggests the olfactory bulb has a sorting role.
    Next question: how does the brain combine sensory patterns with relevant memories, feelings, and thoughts into a single experience? In the olfactory (or piriform) cortex, where smells are consciously perceived, the odor is assigned certain characteristics specific to the smeller, such as whether or not he likes it; whether a lemon smells sour or a caramel roll sweet; and whether a particular coffee roast is pleasantly bitter or acrid.
    How does the product of olfaction—this odor map created in the olfactory cortex—collaborate with inputs from other brain regions and result in (for example) someone lifting a mug of hot Kenyan to her lips, sniffing it once or twice, blowing on the surface, and sipping? What is the underlying logic of smell's passage from the receptor sheet to the neocortex, the thinking brain?

6. The Breakthrough
    T HE GENES THAT CODE for olfactory receptors are the air-traffic controllers of smell. The modern conception of genes began in 1953, when James Watson and Francis Crick discovered the chemical structure of DNA, a double helix; the formation explained how genetic instructions could be stored and passed on from one generation to the next. Crick and Watson used the first image of DNA to propose what has come to be called the central dogma of molecular biology: in a nutshell, genes can make proteins (which are the building blocks of living organisms), but proteins can't make genes.