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On May 19, 1859, the Society of Anthropology of Paris held their inaugural meeting. Paul Broca took the podium, well aware of the spy sitting in the audience. He was familiar with the other audience members. Nineteen respected colleagues, well-dressed and mustachioed. Together with Broca, they formed a significant part of Paris's scientific elite. They all knew each other. But among them there was also an unknown man. The spy was sent, Broca knew, by the chief of police on the orders of the bishops of the French senate. The spy's mission? To ensure that the society did not fall into "atheism and materialism."

By this time, modern science had rocked the world. Charles Darwin, Broca's role model, had recently presented the seeds of the theory of evolution. In a presentation of his research, Darwin had argued that animal species changed through evolution, and this notion would lead to the controversial conclusion that humans were related to apes and other animals. On the Origin of Species would be a bombshell when it was released in the fall. Broca was a doctor and physiologist, and like Darwin, he was fascinated by animal breeding. But the time was not quite right for his ideas. Broca stirred up controversy in the French biological society with a paper on how hares and rabbits could undoubtedly produce a common offspring. According to Broca, these hybrids showed that the boundaries between different animal species were not as strict as previously thought. The established view was that animal species were created by God, each with a specific purpose in creation. The paper was considered both embarrassing and blasphemous. Biologists were not ready for this kind of innovation.

After long negotiations, Broca had managed to get permission to form his own scientific society, and organize the first meeting of la Société d'Anthropologie de Paris, the world's first association to understand the innermost nature of man by studying his behavior and brain. The society was an initiative of freethinkers among Broca's colleagues, and they were not popular in all camps. According to the men of the Church, freely seeking the truth about humans risked giving rise to subversive ideas, such as that humans do not have free will. Paul Broca's own research laboratory had long been under surveillance. The authorities did not allow societies of more than twenty people, and all members of the society had been thoroughly vetted by the authorities.

Paul Broca studied the brains of many different human and animal species and understood better than anyone else at the time how they differed from each other. That is, he didn't understand much. Among his papers were detailed studies of the brains and craniums of everyone from the Italian writer Dante Alighieri to "the murderer Lemaire," whose particular characteristics and actions Broca thought he could understand from their brains. Political ideas were rampant in brain research at the time, and the presence of the Church often required researchers to package their findings creatively to avoid criticism. Broca's investigation of the differences between French and German brains made him popular with patriotic countrymen. The French had wider heads and therefore better brain functions, Broca claimed. But a particularly big research problem was the fact that different animal species had such similar brains. It was common knowledge among experts that the brains of humans and apes were similar, and this fact risked leading to the uncomfortable conclusion that they were cast in the same mold, that they were related and that humans might lack a unique and free will—thus questioning an important dogma for the Church. It was important to avoid conflict with churchmen, and after the Paris meeting, Broca navigated the minefield. Likening humans to other animals could lead to problems. The solution came down to the sense of smell.

The human brain, Broca said, was characterized by large, powerful frontal lobes. People who suffered damage there lost the qualities we consider uniquely human: decision-making, social skills, language skills. Simpler animals did not have such impressive frontal lobes, nor did they exhibit anything resembling human intelligence. What the simpler animals had instead, Broca said, were olfactory brains. In the rat and mouse, the olfactory bulbs, the buds that receive the olfactory signals from the nose, were mighty bulges in front of the rest of the brain. But in humans, Broca argued, the sense of smell had receded to make room for the frontal lobes. He wrote: This lobe, enlarged at the expense of others, occupied the cerebral hegemony; intellectual life is centered there; it is no longer the sense of smell that governs the animal, it is intelligence enlightened by all the senses. Broca came to divide animals into two categories: the osmatic animals, which have the sense of smell as their main sense, and a few anosmatic animals, which, like humans, are not controlled by their sense of smell. The division was based on the opposition between the sense of smell, which Broca considered "bestial," and higher intelligence. It was intended to explain how humans differed from other animals: we humans are governed by reflection and thought and have suppressed the primitive sense of smell. The division was also colored by the racial theories of the time. Some of Broca's anthropological colleagues had started mapping how smells had important cultural and religious meanings in other, non-European cultures. The importance of smell was interpreted as a clear sign that these peoples were more primitive than Europeans. Contrasting the primitive sense of smell with the intelligence and civilization of the Europeans was a compelling comparison, understandable and appealing to both the public and the most zealous controllers of the Church. The neuroscientist John McGann has argued that Broca's distinction between osmatic and anosmatic animals was so appealing that it came to shape the view of the human sense of smell for 150 years. So strong that even today young people often value their cell phones more than their sense of smell. The problem is that this comparison led us astray.