Humans' sense of smell may indeed be gradually fading, according to a study that has found people carry different versions of two scent receptors for musk and body odor.

Olfactory receptors detect airborne chemicals that waft into our noses, but smell receptors vary immensely from one person to the next.

Any two people, on average, will have functional differences in over 30% of their odorant receptor genes, a 2013 study found. Which explains why some people might find some smells pungent or pleasant that the next person can't even detect.

In this new study, Bingjie Li of the Shanghai Institute of Nutrition and Health and colleagues asked 1,000 Han Chinese people and 364 ethnically diverse people from New York to take a whiff of 10 scents, including two odors that people often perceive differently or not at all: a synthetic musk called Galaxolide, and a key molecule associated with body odor from human underarms.

What they found supports a long-standing hypothesis that human sense of smell may have degraded over time due to changes in the genes that encode our smell receptors. Not everyone agrees with that hypothesis, however (more on that later).

Participants rated the intensity and pleasantness of the odors on a 100-point scale, and the researchers looked at genetic variation in their olfactory genes, hoping to find changes linked to how people perceive scents.

"Comparing this perceptual variability with genetic variability allows us to identify the role of single odorant receptors," Li and colleagues write.

The team identified two new smell receptors: one that senses Galaxolide – a 'clean', sweet and powdery smell used in many fragrances – and another that detects the chemical called 3M2H, one of about 120 compounds that comprise human body odor.

Mutations in the genes encoding these receptors affected whether people perceived the scents as stronger or less intense, but genetics only explains a small part of the difference.

Intrigued, Li and colleagues examined these two newfound genetic changes, and another 27 known odor-related mutations, comparing the evolutionary age of when each mutation slipped into our genomes and whether the changes were thought to make human smell receptors less or more sensitive to smells.  

"Summarizing all the published genetic variation that associates with odor perception, we found that individuals with ancestral versions of the receptors tend to rate the corresponding odor as more intense," the authors write, adding that this evidence suggests humans' sense of smell has faded over time.

"While this study was not designed to directly address this hypothesis and may suffer from selection bias, these data support the hypothesis that the primate olfactory gene repertoire has degenerated over time," they write.

This idea is nothing new: there is a long-running theory – though some call it a myth – that humans lost their sense of smell as eyesight became our dominant sense.

Fueling this theory is the fact that humans and other primates have hundreds fewer olfactory genes than mammals that follow their nose such as rats and mice, and about half of the smell genes we do have no longer function.

However, the suggestion that humans' sense of smell is fading slowly over time has been hotly contested, so let's take a look.

Other researchers have made the case before that our sense of smell is not as bad as we might think, pointing to smell-test studies which suggest that even with fewer olfactory genes than other animals, humans' knack of discerning smells outperforms our own expectations and even other animals – depending on the scent.

"Like other mammals, humans can distinguish among an incredible number of odors and can even follow outdoor scent trails," neuroscientist John McGann of Rutgers University wrote in 2017.

What counts, McGann told Ed Yong at The Atlantic in 2017, is not the number of genes encoding smell receptors, or even how many neurons humans have in their olfactory bulb, which sits above the nasal cavity and processes information about odors, but how they work to sense smell. Ours seem to be doing just fine.

Also clouding our understanding is the bias of studies conducted to date, something which the latest study did try to address.

"The typical [olfactory] study focuses on Western participants, who live in a culture where olfaction is not particularly elaborated," psychologist Asifa Majid from Radboud University told Yong in 2017.

"But people in other parts of the world are better at odor detection, discrimination and naming," such as the Jahai people of Malaysia, Majid said.

Maybe most of us are just out of practice.

The study was published in PLOS Genetics.