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jawless hagfish burrows into the bodies of dead sea creatures and then devours their carcasses from the inside out. This has proven to be a highly successful evolutionary strategy. Jawless fish, the first vertebrates, appeared about 450 million years ago, roughly 30 million years after the “first bite,” and the fossil record shows they have changed little since then. They are older by about 200 million years than their rival for the title of champion survivor, the cockroach. The hagfish, an outlandish-lookinganimal with an eel-like body and a sucker for a mouth, is sometimes called a living fossil. Humans are descended from some ancient relative of the hagfish; its anatomy and behavior offer a glimpse into the deep past, when the basic couplings between the brain and the senses were first established.
    To early predators, the trilobites, taste and smell would have been virtually indistinguishable. But in jawless fish, they assumed different jobs, and would not reunite until humans appeared on the scene. Taste became a gatekeeper to the body’s inner precincts. But smell reached out into the world. Hagfish swam through a shifting haze of scents. Smell created a picture of their surroundings in their brains: predators, potential mates, their next meal. To humans, the scent of rot usually triggers disgust. But this reaction is subjective. To the jawless fish, it meant survival and satisfaction.
    Where did this additional sensory power come from? Sometimes, mutations in the genetic code do not merely change the body—they add to it. Entire strings of DNA can randomly duplicate themselves; when their biological instructions are carried out, the organism acquires an extra set of something. Redundant tissues can be deadly, mucking up the body’s normal functions. But under the right circumstances, they can bring about significant evolutionary leaps. The original genes continue doing their established jobs, and natural selection works on the copies, which take on new tasks or build new body parts. The German writer and naturalist Johann Wolfgang von Goethe anticipated this powerful evolutionary force in the late eighteenth century, guessing that duplicate parts of the anatomy might transform themselves into something different. The structure of a leaf might be the basis for the flower petal. A skull might be a modified vertebra.
    In the jawless fish, receptors for smell were duplicated and the extras altered to detect new scents. Their immediate ancestors likely had only a handful of smell receptors; hagfish have more than two dozen. As life evolved, this process repeated itself many times over: some animals have as many as 1,300 kinds of smell receptors; humans have more than 300.
    The new sensations bombarding the first jawless fish would have been a cacophony to the brain of the average trilobite. So as the sense of smell grew sharper, the hagfish brain adapted. The olfactory bulb is a way station between the nose and the brain of all animals, converting smells to patterns of nerve impulses. In the hagfish, a new structure grew upward from the bulb, like a flower springing out of the earth. This structure was the forerunner of the cerebrum, the topmost part of the human brain that gives conscious form to virtually everything we do: it processes senses, perceptions, movement, and speech. In humans, the same sets of genes still jointly govern development of the olfactory anatomy and the brain’s basic structure. Smell has been the biological currency of feeling and action for almost as long as animals have had nostrils. It is the human sense of smell that gives flavors their vast range and subtleties. Proust, whose novel In Search of Lost Time is a reverie inspired by the scent and taste of a madeleine cookie dipped in tea, might have been taken aback to hear that carrion feeding was the starting point for humanity’s deep connection between smell and memory.
    Ant Soufflé
    About 250 million years ago,