Neanderthals were stupid. Medieval people thought the Earth was flat. Humans have a bad sense of smell. All three of these notions are 19th century myths, with the last idea perhaps the most enduring. Yet according to a new paper published in Science, humans actually have a powerful sense of smell, far more important than often thought.
The review by John McGann, associate professor of behavioural and systems neuroscience at Rutgers University in New Jersey, revisits the available evidence concerning human olfaction and questions old saws such as a dogs’ nose being 10,000 times more powerful than ours. Canines are indeed superior at sniffing out urine on lamp-posts, he says, but humans far outperform them when it comes to bananas; different species, it seems, specialise in different smells.
“The fact is the sense of smell is just as good in humans as in other mammals, like rodents and dogs,” says McGann, who heads a laboratory specialising in the neurobiology of sensory cognition, studying how human brains understand the external environment. He estimates humans can discriminate between up to a trillion different odours.
So where did we get the idea that our sense of smell is so terrible? McGann argues it originates with the famous French neuro-anatomist and freethinker Paul Broca (1824–1880), who suggested that the human brain’s frontal lobes, the seat of our intelligence and free will, had grown far larger than those of other species at the expense of other more ‘animalistic’ brain capacities.
In particular, Broca suggested the human brain’s olfactory bulbs, where smells are processed, had reduced in size to accommodate the expanded frontal lobes. Convinced that size correlated with ability, Broca and others thus laid the groundwork for the notion of diminished olfactory ability.
McGann challenges this idea, arguing that comparing the relative and absolute volumes of human olfactory bulbs to other species is not a useful indicator of human smelling power.
A better measure, he posits, is the number of neurons present in the bulbs. Drawing on a variety of research, he says this number is surprisingly consistent across a diverse group of mammals, including humans; despite a 5,800-fold range in body weight, there is just a 28-fold range of olfactory bulb neuronal numbers. So while our olfactory bulbs might be small, they pack a punch similar to other mammals.
Direct comparisons are misleading, furthermore, because current research indicates “different species have different sensitivities to different odorants”. Humans have staggering olfactory power when it comes to certain smells, such as the amyl acetate in bananas or the odorants in human blood. Other species excel at detecting odours relevant to them.
Our powerful species-specific olfaction is central to being human, McGann notes, effecting communication, mate choice and sociality. Research shows we unconsciously smell our hands after shaking hands with a stranger, for example. Environmental odours also heavily influence our emotions and memories.
“Our sense of smell,” he says, “is much more important than we think.”