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Human Sense of Smell Rivals That of Dogs, Says Study (theguardian.com)
One scientific analysis is arguing that the human sense of smell has not only been underestimated over the years, but that it may rival that of dogs and rodents. John McGann, a neuroscientist at Rutgers University in New Jersey and the paper's author, said: "For so long people failed to stop and question this claim, even people who study the sense of smell for a living. The fact is the sense of smell is just as good in humans as in other mammals, like rodents and dogs." McGann has reached this unexpected conclusion after spending 14 years studying the olfactory system. The Guardian reports: McGann identifies a 19th century brain surgeon, Paul Broca, as the primary culprit for introducing the notion of inferior human olfaction into the scientific literature. Broca noted that the olfactory bulb -- the brain region that processes odor detection -- is smaller, relative to total brain volume, in people compared with dogs or rats. The discovery inspired Freud's belief that human sexual repression may be linked to our "usually atrophied" sense of smell. In the latest paper, published in Science, McGann points out that in absolute terms the human olfactory bulb is bigger than in many mammals and a literature search revealed that the absolute number olfactory neurons is remarkably consistent across mammals. McGann goes on to deconstruct other metrics that have been used to support the idea that human smelling abilities are limited. Humans have approximately 1,000 odor receptor genes, for instance, compared to 1,100 in mice, which some had taken as confirmation of mouse superiority. However, other work suggests there is not a tight relationship between the number of olfactory genes and smelling ability. One study found that cows have 2,000 such genes - far more than dogs.
I've studied olfaction, and this just doesn't seem right. This olfactory bulb argument seems like a straw man that no one in the field has been using since... the 19th century. Digging in to the article a bit, it seems the authors of the actual study agree with me, and are using different odors for humans to balance out some of our... differences. Their main point (which is right) is that the human sense of smell is much better than most people realize, and that you can be trained to follow a scent trail, distinguish similar odors, and notice the cognitive effects scent has on you. Anyone who has experimentally studied olfaction for a few years will notice themselves gaining these abilities (it goes away quickly when you're not smelling things professionally several hours a day).
So why is this summary so wrong?
First off, humans only have 400 different olfactory receptors, it doesn't matter if genetics say you should have 1000, you only get 400 (genotype =/= phenotype). Second, you have less "sensor" surface area than other mammals in real terms, not scaled for size. Third, you lack the ability to concentrate scent molecules by varying your rate of breathing like other mammals (this can be overcome by varying breathing through your mouth and nose, but other mammals don't have to do this).