Recently, a group of researchers embarked on an unusual experiment. They screened the genomes of 1,000 Han Chinese people to find genetic variations that were linked to the way participants perceived 10 different scents, including musk and underarm odor. They then repeated the experiment for six odors in an ethnically diverse group of of 364 people to check their results. They reported that people who had “ancestral” versions of the scent recognition genes perceived the odors as more intense:

Participants carried different versions of the musk and underarm odor receptor genes, and those genetic variations affected how the person perceived the scents. In combination with previously published results, the researchers find that people with the ancestral versions (the version shared with other non-human primates) of the scent receptors tend to rate the corresponding odor as more intense. These findings support the hypothesis that the sensitivity of humans’ and other primates’ sense of smell has degraded over time due to changes in the set of genes that code for our smell receptors.

PLOS, “Humans and other primates have evolved less sensitive noses” at ScienceDaily (February 3, 2022) The paper is open access.

So they concluded that both humans’ scent receptors and those of other primates have degraded over time.

Here’s an intriguing finding:

From these experiments, they pinpointed two new scent receptors — one that detects a synthetic musk used in fragrances and another for a compound in body odor.

Study participants had different versions of the receptor genes for musk and underarm odor, and those variations affected how they perceived the scents.

HealthDay News, “People’s sense of smell may be declining, study suggests” at UPI (February 4, 2022)

If there is a gene receptor that detects “a synthetic musk used in fragrances,” it must have got started within the last few thousand years, not hundreds of thousands of years ago. That in itself is a remarkable find. The human genome may be changing faster than evolution theory is usually thought to allow.

Humans, we are told, have about 800 receptor genes for identifying smells but they vary with the individual and about one quarter of the study participants could not smell the musk. One neuroscientist offers a thought:

“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.”

Corryn Wetzel, “Humans’ Sense of Smell May Be Worse Than Our Primate Ancestors’” at Smithsonian Magazine (February 07, 2022)

Matthew Cobb may be onto something, at least where humans are concerned: We don’t pay much attention to smells unless they are especially attractive or repulsive. We take a very different view of eyesight, noticing slight deviances from what we expect in anything we see.

One commentator, law professor Glenn Reynolds, points out that it’s at least possible that we don’t need our sense of smell as much as we used to. He notes that colorful physicist Richard Feynman (1918–1988) liked to do a sort of experiment as a social icebreaker, to show that our human sense of smell is better than we think. From one account of his icebreaking:

Then I looked at the bookshelf and said, “Those books you haven’t looked at for a while, right? This time, when I go out, take one book off the shelf, and just open it–that’s all–and close it again; then put it back.” So I went out again, she took a book, opened it and closed it, and put it back. I came in — and nothing to it! It was easy. You just smell the books. It’s hard to explain, because we’re not used to saying things about it. You put each book up to your nose and sniff a few times, and you can tell. It’s very different. A book that’s been standing there a while has a dry uninteresting kind of smell. But when a hand has touched it, there’s a dampness and a smell that’s very distinct

“Can humans smell if a book has been opened?” at [skeptics] StackExchange (April; 11, 2011)

Commenters at [skeptics] offered a number of suggestions, including

“I don’t have evidence, but try it — when you open a book which hasn’t been looked at for a while you let air get to it. Makes perfect sense that that will allow dust and other particles to come out, and for some of the moisture in the air to get at the pages.”

“David I think it does make a difference. Paper decays and the act of opening a book will probably scatter decayed paper molecules. Given that only very few molecules are needed to trigger our sense of smell this seems a plausible cause.”

and

“The difference is not, I think, that the book was opened, but that it was handled by a human hand. That would probably leave smell traces that could be identified by a sufficiently sensitive nose. Feynman was testing whether the human nose was incapable of fine sensing like that, or if humans just didn’t bother with it. ”

That was a sharp idea on Feynman’s part. Even if we noticed a difference in smell, that difference might not be thought to convey any information. So it is ignored in favor of human specialties — like reading the book. A dog would definitely notice a similar change in scent but then he does not have access to the type of information that would be, for example, in the book. So he might focus on the scent.

To be sure that our sense of smell had really declined, we would need to see whether concerted efforts to amplify it were successful.

You may also wish to read: Physicist: Migrating birds’ mysterious quantum sense is “spooky.” Birds like the European robin pack a $10,000 lock-in amplifier into a 2 micron cell. This mysterious intelligence, magnetoreception, seems essential to migration, hence, to the survival of many birds. They may even “see” Earth’s magnetic field.