Andrea Marks in Scientific American:
The smell of coffee may urge you out of bed in the morning, and the perfume of blooming lilacs in the spring is divine. But you do not see police officers with their noses to the ground, following the trail of an escaped criminal into the woods. Humans do not use smell the way other mammals do, and that contributes to our reputation for being lousy sniffers compared with dogs and other animals. But it turns out the human sense of smell is better than we think. In a review paper published in Science last week neuroscientist John McGann of Rutgers University analyzed the state of human olfaction research, comparing recent and older studies to make the argument our smelling abilities are comparable with those of our fellow mammals. McGann traces the origins of the idea that humans have a poor sense of smell to a single 19th-century scientist, comparative anatomist Paul Broca. Broca, known for discovering Broca’s area—the part of the brain responsible for speech production—noted that humans had larger frontal lobes than those of other animals, and that we possessed language and complex cognitive skills our fellow creatures lacked. Because our brains’ olfactory bulbs were smaller than those of other mammals and we did not display behavior motivated by smell, Broca extrapolated these brain areas shrank over evolutionary time as humans relied more on complex thought than on primal senses for survival. He never conducted sensory studies to confirm his theory, however, but the reputation stuck.
Scientists built on that tenuous foundation over the years, McGann says. Geneticists saw supporting evidence for humans’ limited olfactory abilities because we have a smaller fraction and number of functioning olfactory genes—but again this was not well tested. The idea that color vision took the evolutionary pressure off olfaction was later debunked when no link was found between that evolutionary development and smell loss. In addition, the size of olfactory bulbs, both in absolute terms and in proportion to the brain, does not relate directly to smelling power as scientists once thought. Now that more sensory tests are being done, the results are mixed. Experiments conducted in previous decades have found humans are just as sensitive as dogs and mice to the aroma of bananas. Furthermore, a 2013 study found humans were more sensitive than mice to two urine odor components whereas mice could better detect four other sulfur-containing urine and fecal-gland odors tested. A 2017 study also revealed humans were more sensitive than mice to the smell of mammal blood.