April 13, 2014
We Can Tell Difference Between At Least A Trillion Odors

Never before have any scientists tried to figure out how many different odors humans can tell apart.

Vosshall had long been bothered by the idea that humans were limited to smelling 10,000 odors—an estimate that was made in the 1920s, and not backed by any data. “Objectively, everybody should have known that that 10,000 number had to be wrong,” she says. For one thing, it didn't make sense that humans should sense far fewer smells than colors. In the human eye, Vosshall explains, three light receptors work together to see up to 10 million colors. In contrast, the typical person's nose has 400 olfactory receptors.

What is not clear: How many kinds of receptors are there for smells? The 400 olfactory receptors are 400 different kinds of binding sites? Anyone know?

But no one had tested humans' olfactory capacity. “We know exactly the range of sound frequencies that people can hear, not because someone made it up, but because it was tested. We didn't just make up the fact that humans can't see infrared or ultraviolet light. Somebody took the time to test it,” Vosshall says. “For smell, nobody ever took the time to test.”

How many odors can a dog tell apart?

The researchers used combinations of odorant molecules that humans do not usually encounter.

They used 128 different odorant molecules to concoct their mixtures. The collection included diverse molecules that individually might evoke grass, or citrus, or various chemicals. But when combined into random mixtures of 10, 20, or 30, Vosshall says, they became largely unfamiliar. “We didn't want them to be explicitly recognizable, so most of our mixtures were pretty nasty and weird,” she says. “We wanted people to pay attention to 'here's this really complex thing – can I pick another complex thing as being different?'” The scientists presented their volunteers with three vials of scents at a time: two matched, and one different. Volunteers were asked to identify the one scent that was different from the others. Each volunteer made 264 such comparisons.

It strikes me that with genetic engineering it should be possible to create enhanced humans with a greater sense of smell. Will enhanced smell, hearing, seeing or feeling be most desired for future offspring? Hear a wider range of sounds? Or taste a wider range of food tastes?

Some women already have the ability to see 4 primary colors. So enhancing humans to see more colors does not seem a big stretch.

Just enhancing future offspring so that everyone can see 3 colors would enhance a substantial fraction of all future babies. I was recently surprised to learn that color blindness occurs at rates well above 1%.

Researchers from the Multi-Ethnic Pediatric Eye Disease Study Group tested 4,005 California preschool children age 3 to 6 in Los Angeles and Riverside counties for color blindness. They found the following prevalence by ethnicity for boys:

  • 5.6 percent of Caucasian boys

  • 3.1 percent of Asian boys

  • 2.6 percent for Hispanic boys

  • 1.4 percent of African-American boys

The prevalence of color blindness in girls measured 0 percent to 0.5 percent for all ethnicities, confirming findings in prior studies. However, the numbers were so low overall for girls that researchers say they cannot statistically compare rates between females among the four ethnicities studied.

Those numbers also surprise me due to the racial differences. Did different regions of the world have different levels of selective pressures for the ability to see more or fewer colors? What were the selective pressures that caused those differences? Or uis there some adaptive benefit from seeing fewer colors? Perhaps part of the brain gets freed up for other more valuable purposes?

Share |      Randall Parker, 2014 April 13 09:26 PM 

destructure said at April 15, 2014 2:16 PM:

"Those numbers also surprise me due to the racial differences. Did different regions of the world have different levels of selective pressures for the ability to see more or fewer colors?"

Yep. Lots of animals including deer only see two colors. The reason is very simple. While deer may not be able to distinguish between shades as well, their eyes are much better at picking up movement and seeing through camouflage.

Ronald Brak said at April 15, 2014 9:22 PM:

There are apparently some advantages to colour blindness, or at least people have reported that there are. I can't see it myself, on account of how I'm not colour blind. But it's clear that selection pressure is not strong in either direction, so random genetic drift is more than sufficient to explain differences in incidence in populations. Most molecular evolution (change in DNA) is the result of random genetic drift. There could be some selection operating at a level we're unaware of and perhaps someone has crunched the numbers and investigated mortality among the colour blind and seen if there are any significant anomolies. Of course if one is looking at hundreds of causes of mortality there will of course be anomolies, so further study will be required to determine if they are the result of statistical chance. But I'm guessing if there was anything that had a large effect we would have heard of it by now.

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