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Introducing the sense of smell

Updated Tuesday 8th March 2011

During the last series of Bang Goes the Theory we invited viewers to take part in our giant, online experiment on the sense of smell. Here we focus on the sense of smell - how it might have evolved, and why it is important.

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Introduction

People were encouraged to obtain one of our scratch and sniff cards (containing four different smells) and to use it while performing a variety of computerised tests. Over 10,000 of you took part! Before we look at the results, let’s start with a description of the way in which our sense of smell evolved.

A sense of smell would have developed very early in evolution. Nerve cells (neurons) were already sensitive to certain chemicals – that’s how one neuron signals to another, by releasing a specific chemical that triggers the next neuron in the chain. It would not be too complex an evolutionary step to develop sensitivity to chemicals in the outside environment. These could indicate the presence of food substances, or something noxious that should be avoided.

All these developments would have taken place in water-living creatures; water made it easy for chemicals to diffuse and chemical reactions to take place. When animals invaded the land it remained beneficial to sample the environment, although the chemicals of interest were now drifting in the air. That’s fine for moving the odour compounds about, but not so good for the chemical reactions that trigger neurons. The solution (in both senses of the word!) was to keep noses wet inside, so that the chemicals could dissolve.

Smelling a flower Creative commons image Icon denniswong under CC-BY under Creative-Commons license

For some creatures a sense of smell is enormously important. For example male moths detect females several miles away, picking up the chemicals (called pheromones) emitted by potential mates. Animals that hunt by smell also have to be very sensitive to odour chemicals. A bloodhound can find a path someone had trodden twenty minutes earlier. What is more, within a few paces up and down, the dog can decide which way the person was travelling, because the scent gets stronger that way.

We humans seem to place far more emphasis upon our eyes and ears – except when it comes to food! When we say, “That tastes good!” we don’t really mean it. Our sense of taste is a very feeble affair, only distinguishing between four (some say five) basic tastes, the principal ones are sweet, sour, bitter and salt.

The fifth is called umami, a kind of ‘tasty’ taste that is particularly found in oriental cooking. Even with that addition, we don’t do very well with taste alone; we are reminded of that every time we get a cold, when our nose is blocked and the food seems tasteless.

Flavour is what we enjoy, and that is a composite sense that combines the signals from our noses and tongues. So, don’t tell the cook the food tastes good – say it has a lovely flavour!

What does smell mean to us?

The question raised by the introduction is whether the enjoyment of food (and flowers etc.) is the only thing we gain from a sense of smell. Because the sense evolved very early, it is linked to a part of the brain which also evolved early. The brain areas that make us conscious are much more recent, so we are not directly aware of what odours may be doing to us.

However, the older part of the brain is linked to the emotional system, so we can experience the effects of smells ‘second hand’ through our emotions and memories. A good example is when a smell of something cooking triggers happy recollections of childhood. Alternatively, for some people the smell of petrol may trigger frightening memories of a car crash.

It seems that we may be tapping the remains of a system that in our evolutionary past would have helped us find food and avoid danger. What of finding a mate? There is some evidence that certain smells make a potential partner seem more attractive.

There are two possibilities here. In part a smell may signal state of health – when choosing a partner both males and females would do well to select a healthy one. The other possibility is a ‘ready to mate’ signal, which is used by many animals. Only the female need emit this odour, to indicate that she is ovulating, so if there is a chemical of this sort we would expect it only to influence men.

Read on for the results of our experiment.

 

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