When you smell something, odor molecules sail up your nose where they bind to proteins — called olfactory receptors — on cells that line your nasal cavity. These receptors trigger signals that your brain interprets as one or more smells.
A team of scientists has identified the olfactory receptors for two common odor molecules: a musk found in soaps and perfumes and a substance prominent in foul-smelling armpit sweat. The research team also found that more recent evolutionary changes in these olfactory receptors make people less sensitive to those smells. So if you’re one of the lucky few not to be overwhelmed by body odor, you probably have evolution to thank. The work was published Thursday in PLoS Genetics.
Olfactory receptors can be traced back hundreds of millions of years and are believed to be present in all vertebrates. Humans have about 800 olfactory receptor genes, but only about half of them are functional, meaning they will translate into proteins that hang out in the nose and detect odor molecules. But within a functional gene, small variations can cause changes in its receptor protein, and those changes can have a huge impact on how an odor is perceived.
“There is a molecule called androstenone,” said Joel Mainland, a neuroscientist at Monell Chemical Senses Center and an author of the new study. “And we know that some people smell that molecule like urine, some people smell that molecule like sandalwood, and some people don’t smell it at all.”
That said, genetic changes aren’t the only thing underlying scent interpretation. “One is genetic and the other is experience, which includes things like the culture you grew up in,” said Hiroaki Matsunami, a molecular biologist at Duke University who was not involved in the research but whose work focuses on smell.
The study by Dr. Mainland and colleagues was a collaboration between scientists in the United States and China. They sequenced the genome of 1,000 people in Tangshan, China, who are members of the Han ethnic group. They did the same with an ethnically diverse cohort of 364 people in New York City. The participants were asked to rate the intensity and pleasantness of a range of common scents on a 100-point scale. The researchers then looked for associations between olfactory receptor genes and smells, as well as variations within those genes and their possible impact on smell perception.
By sampling a large, diverse population of people, the researchers were able to discover smells whose perception was based on genetic differences between people, rather than cultural or experiential factors. That led them to molecules like trans-3-methyl-2-hexenoic acid and galaxolide.
Trans-3-methyl-2-hexenoic acid is considered one of the most pungent compounds in armpit sweat. Galaxolide is a synthetic musk often described as having a floral, woody scent used in perfumes and cosmetics, as well as things like cat litter. The research team was able to identify olfactory receptor variants for those odors, and in both cases, people with the more evolutionarily recent gene variant found the odors significantly less intense.
Galaxolide’s findings were particularly striking, with some participants not being able to smell the musk at all. “It’s really rare to find an effect as large as what we saw for this single receptor on musk odor perception,” said Marissa Kamarck, a neuroscientist at the University of Pennsylvania who authored the study.
dr. Matsunami sees this work as another example of the fact that human scent is more complex than originally thought. He said that while the main findings in the study involved only two scents, they add to the evidence that “odor receptors as a group have an extraordinary variety.”
The authors think their findings support a hypothesis that has been criticized that the primate olfactory system has degenerated over evolutionary time. Kara Hoover, an anthropologist at the University of Alaska Fairbanks who was not involved in this research but who studies the evolution of human scent, is not convinced by that hypothesis in the first place.
“Why is reduced intensity assumed to be degradation?” she asked. “Maybe other things get more intense or smell discrimination gets better. We don’t know enough to draw these conclusions.”
for dr. Hoover, these findings raised other evolutionary questions. “Our species is very young,” she said. “Why so much variety in such a short time? Is there an adaptive meaning?”
