Coffee in the morning ...

Our ability to smell is one of those interesting problems of biology. We are able to instantly smell a molecule our noses never ever encountered before.

'This is impossible' writes Chadler Burr in her book The Emperor of Scent (2004, London, p.10) for the only thing our bodies instantaneously recognise must surely be stored in memory.

Our digestive system, for example, only instantaneously recognises those food molecules our ancestors encountered before. Over the thousands of years our bodies have evolved a system of a one-to-one match between what is known as an enzyme - the agent of digestion - and the molecule it must break down.

We have put lipids, carbohydrates and proteins into our bodies over such a long period of time that our bodies also developed a system for recognising their shape and stored the dimensions in our memory system.

Foreign molecules like plastic our bodies cannot recognise and there is therefore no enzyme to process it. Dogs cannot process chocolate because their ancestors the wolf never encountered it. 'Evolution never selected for dogs an enzyme that recognised the shape of chocolate's molecules' observed Burr.

On the other hand, our immune system has antibodies that must develop the ability to recognise foreign bugs, but it is rarely instantaneous. We are ill for a week with the flu until the body figures out a design that can wrap itself around the bug - and squeeze it to death. If it fails, it kills, like HIV.

Smell does both: it recognises a foreign molecule, as with the immune system, and it does it instantly, like the digestive system. How is it possible? How does smell work biologically?

Like everything else, the ability to smell is unevenly distributed in the human population. It is part of normal human variation. You and I know of people who are famously unable to smell anything and others who smell everything, whether unpleasant or pleasant. Luckily for us, sight is more developed than smell. Unlike dogs, who have a fabulously powerful sense of smell, our sense of smell is a profoundly intimate an experience. Our ability to smell those naturally produced molecules as a turn-on or a turn-off for sex is switched on only at the closest of encounters. Perhaps that is why human beings almost always have sex in utter privacy.

Smell makes ingesting food attractive. We have good reason to believe that cows and horses have a sophisticated smell apparatus that makes grass reek with pleasure, which is just as well for they consume tons of it. Barley, wheat and rice, our staples, are domesticated weeds (grass is an invasive weed), so we do share something with cows.

It turns out that the molecular biology of digestion and immunity is about recognising shape; smell is about the nose recognizing some kind of atomic level vibration. Pretending to be the greatest reverse-engineer named God musing to him or herself, Barr writes: 'Smells are just molecules that fly through the air. You want to catch them. How do you catch them? You make receptors to catch them. Can you make receptors out of flesh? In the human body, there are things called proteins hanging around all over the place. So you build your receptor out of a protein with a pocket that the smelly molecule can fall into.' (p.87) Actually, then, it is both about shape and atomic vibration!

How does smell help survival? It is not really about sex but about sniffing out toxic and decaying food, things that can make you very ill or kill you. For this to work we need an apparatus that instantly determines the biochemical composition of a substance. And the general rule is: if it smells bad do not put it into your mouth!

The toxin-sniffing nose also loves the good smells, which is why the manufacturers of fragrances and perfumes turn over trillions of rands annually. The lowly, ubiquitous, laundry detergent or toilet aerosol refresher contains a synthetically engineering molecule that smells like the flowers of spring.

At the other end of the spectrum, hundreds of biochemists worked very hard to create the famously magic Chanel No.5 with its excess of aldehydes, the citrus-lime of Drakkar Noir and Cool Water, the helionol of Calvin Klein's Escape, the musk in Tommy Girl, not to mention the Iso E Super of Este Lauder's Beautiful and Calvin Klein's Eternity.