Humans may be slowly losing their sense of smell, according to new study published in PLoS Genetics last week..
When scientists tested individuals' perceptions of various smells, they found evidence that humans' sense of smell is declining over evolutionary time. The team also discovered two new receptors in the nose that help distinguish between certain pleasant and repulsive odors.
When odor molecules in the air stimulate specialized nerve cells that line the nose, the brain interprets it as a scent, or combination of scents. Humans have around 800 olfactory receptor genes that can have minor variations, which change how an odor is perceived. The new results help explain why the fragrance of a specific perfume, for example, may seem pleasant to some and overpowering to others.
“We’re still, I would say, surprisingly ignorant about what all the olfactory receptors do and how they interact with each other to encode olfactory percepts,” says Joel Mainland, a neuroscientist at Monell Chemical Senses Center and author of the research, to the Guardian’s Nicola Davis.
In a collaborative study between scientists in the United States and China, the team first looked at the genes of 1,000 Han Chinese people to see how genetics played a role in scent perception. They exposed the study participants to ten common odors and asked them how they perceived each smell. The researcher then repeated the experiment for six odors in an ethnically diverse population of 364 participants. Each person rated the intensity and pleasantness of a given odor on a 100-point scale, which the scientists then compared their genome.
The study revealed two new receptors: one that detects a synthetic musk used in fragrances, and another that detects underarm odor. Because each participant had different versions of the musk and underarm odor receptor genes, those genetic variations affected how the person perceived the scents. Almost a quarter of participants couldn’t smell the musk scent, for example, Catherine Schuster-Bruce reports for Business Insider.
“It’s really rare to find an effect that’s as large as what we saw for this one receptor on the perception of the musk odor,” says study author Marissa Kamarck, a neuroscientist at the University of Pennsylvania, to Sam Jones for the New York Times.
Kamarck and her colleagues say their results support the controversial hypothesis that primates’ smelling ability has slowly declined over time due to genetic changes. When the team looked at their results in combination with previously published studies on genes and scent, they found that participants with the ancestral versions of the scent receptors—those shared with non-human primates—tended to rate the corresponding odor as more intense.
While the results suggest our ability to detect smells is degrading, more studies are needed to better understand the evolution of human scent receptors.
“It sheds light on a long debate in human and primate evolution—the extent to which sight has tended to replace smell over the last few million years,” says Matthew Cobb of the University of Manchester and author of Smell: A Very Short Introduction, to the Guardian. “There are another 400 or so receptors to study, and the vast majority of our responses to odors remain a mystery.”