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Broca's great discovery, that the frontal lobes of the brain contain much of what we consider to be uniquely human characteristics, also meant that another part, the olfactory brain, was overshadowed. This created an opposition between the higher intellectual abilities and the lower sense of smell. The British physician and anatomist Sir William Turner took over from Broca and made a new classification: animals had either a sensitive sense of smell, an insensitive sense of smell or no sense of smell at all. Turner placed all animals in one of these three categories, but he ignored the fact that there were no scientific studies yet on which animals had a better or worse sense of smell. It was a guessing game disguised as science. Turner based his classification on Broca's division into osmatic and anosmatic animals. But there was an insidious slippage from Broca's definition of "being controlled by the sense of smell" to Turner's definition of "having a sensitive sense of smell"—which are two completely different things.

We have a bad sense of smell because we are so smart! The notion became part of the legacy of Paul Broca. The irresistibly flattering narrative reappeared in many medical textbooks and treatises around 1900. The American anatomist C. J. Herrick went all out when he wrote that man's sense of smell was "greatly impaired, almost rudimentary" and that "the vastly greater equipment of most other mammals gives them powers far beyond our comprehension." Man's supposedly poor sense of smell even interested Sigmund Freud. For him, it was necessary for modern man to suppress the sense of smell, as it reminded us of our animal urges from when we were four-legged and, like dogs, sniffed each other instead of saying hello.

The sense of smell—primitive and instinctive. Language—reasoning and intellectual. Smell and language were presented as opposites. Even when comparative research results pointed to a well-developed human sense of smell, it was difficult to let go of what had now become dogma. Sir Victor Negus compared the olfactory mucosa of rabbits and humans—the area of the nasal cavity where smells are captured and transmitted to the brain—and found that it was actually larger in humans. This could have led to a reassessment. But strangely enough, Negus came to the opposite conclusion, that "in man the sense of smell is weak and not of great importance," and that studying the human sense of smell was a waste of time. Such was the power of theories that scientists even ignored results that defied their own prejudices. When scientists distanced themselves from the sense of smell, they also distanced themselves from our animal prehistory, and it became a way to emphasize the uniqueness of humans over other animals.

It was at another gathering, 156 years after Broca's Paris meeting, that the myth of man's poor sense of smell was punctured. It happened on Thursday, April 23, 2015, at a hotel in Bonita Springs, Florida, where the Association for Chemoreception Sciences was holding its annual meeting. A small buzz went through the audience when zoologist Matthias Laska from Linköping University changed the PowerPoint slide. The slide showed a funnel-like diagram, a summary of the research conducted so far on the sensitivity of humans and other animals to smell, and its meaning was immediately clear. Laska explained that many different animal species had been tested for their sensitivity to different odor molecules. Of these species, a total of twenty of them had been tested on the same odor molecules that had been tested on humans, making a comparison possible. Who was most sensitive? Sitting in the audience at the time, I read with amazement which species had been tested. Monkeys. Vampire bats. Dogs. And, of course, rats and mice, the most common laboratory animals, which have taught us so much about the functions of the body and brain. But what impressed the audience was how unambiguous the results were. Humans' sense of smell was clearly better than that of most other animals! The scientists in the audience had spent their working lives investigating the senses of smell and taste. Many had probably suspected that the human sense of smell was better than its reputation. But still, Laska's compilation was a revelation: humans have an excellent sense of smell.

Because of Laska's research, the old myth was blown out of the water. Humans were more sensitive than other animals to the vast majority of odor molecules. Human versus rat: 31 to 10. Human versus mouse: 36 to 35. Human versus vampire bat: 14 to 1. Human versus spider monkey: 58 to 23. The only animal that was clearly better than humans was the dog, which is more sensitive to 10 out of 15 odor molecules. Dogs are the olfactory kings of the animal kingdom, and later on you will learn about their special olfactory abilities. Perhaps the most surprising aspect of the comparison is that we humans actually perform so well in the smell tests that we can even give dogs a run for their money.

But perhaps you are wondering how it is possible to investigate the odor sensitivity of a rat. They can't tell us in words which smells they're most sensitive to. And how do we know that the rats really care about the researcher's odor task and aren't thinking about anything else? It is actually quite easy to test their odor sensitivity, but it takes some time. First, a rat must learn to associate an odor with food or drink. The researcher usually lets the animal go hungry or thirsty for a few hours before the experiment; this is not dangerous for the rat, but it increases its interest in exploring its surroundings. The rat is then released into a small chamber with a hole in the wall and a water bottle. When the rat sticks its nose into the hole, a smell machine puffs out an odor, and then the water bottle releases a drop of water. This does not happen every time. Sometimes only clean air comes out of the hole and the rat gets no water. Finally, after practicing for a while every day for a few days, the rat gets the hang of it: when the smell comes, I get a sip of water. Then the researchers see that the rat's behavior changes. When an odor is released, the rat rushes over to the water bottle, but when no odor occurs, the rat pulls its head back and tries its luck again. Once the rat learns the connection, the researcher can reduce the strength of the odor. When the smell is weak enough, the rat reacts as if there is no smell there. Then the researchers have found the rat's odor threshold. Similar tasks can be designed for any animal that can smell and learn—and that's most of them. Elephants, for example, have an excellent sense of smell and they put their trunks into different smell boxes to locate the smell. If they choose the right box, they get a carrot as a reward.