Smell Incredible Design

Learn some amazing facts about the sense of smell
brought to you Darwin-free by Swedish scientists.

Guess your fastest sense. Did you guess the sense of smell? That’s the correct answer – one of several surprising facts about human noses in a press release from Sweden.

Sense of smell is our most rapid warning system (Karolinska Institute). Smell can be a race against time. Our most rapid warning system—the sense of smell—is actually faster when encountering bad odors. That’s because going forward into a bad odor cloud can be a life or death matter. Before you see or hear danger, you might smell it. Engineering a rapid-response organ like the nose was no simple matter.

The olfactory organ takes up about five per cent of the human brain and enables us to distinguish between many million different smells. A large proportion of these smells are associated with a threat to our health and survival, such as that of chemicals and rotten food. Odour signals reach the brain within 100 to 150 milliseconds after being inhaled through the nose.

It was tricky to find this out. Researchers at Karolinska developed new methods to measure the response rate in the olfactory bulb when it detected six different kinds of odors, some bad. Our immediate and unconscious reflex to a bad odor is to lean back and away from it. The lead researcher says,

“The results suggest that our sense of smell is important to our ability to detect dangers in our vicinity, and much of this ability is more unconscious than our response to danger mediated by our senses of vision and hearing.”

The Darwin-free study was published in PNAS by Iravani et al., “The human olfactory bulb processes odor valence representation and cues motor avoidance behavior” (October 19, 2021). The abstract explains the principal finding:

We demonstrate that odor valence is associated with both gamma and beta activity in the OB [olfactory bulb]. Furthermore, we show that unpleasant odors have privileged temporal OB access, as indicated by early beta activity that is linked to a preparatory neural motor response in the motor cortex, which, in turn, is associated behaviorally with a fast full-body avoidance response.

The olfactory bulb is no simple organ. It is one stage of a multi-stage information processing system that helps sort and combine the millions of possible odorants into categories that retain essential information without sacrificing the uniqueness of each odor source.

For a sniff at the complexity of olfaction (sense of smell), see the animation by Illustra Media of “A Pacific Salmon’s Sense of Smell” at the Living Waters film website. Even tiny insects have very acute olfaction. And surprisingly, as our 30 May 2017 report showed, you smell like a dog—meaning, the human sense of smell is not inferior to that of dogs, thought to be champion sniffers. We will reword that as, “You smell good.”