Frying bacon, sweaty socks, Chanel No 5: smell is the most evocative of our senses. So why don't we take it as seriously as sight or hearing? Lyall Gordon explains how a tiny, unappreciated organ in the nose influences our social and sexual lives

WE HAVE a nose for things. We sniff out problems and follow our noses, moving to a successful conclusion in directions that are often far from obvious. And yet we persist in belittling our olfactory abilities, dismissing the human nose as a blunt instrument.

It is true that most animals have a far more acute sense of smell than humans. Dogs are a million times more likely to pick up social scents, hedgehogs 10,000 times better equipped for finding food. But, even with olfactory centres that occupy just one-thousandth of our cranial capacity, we are remarkably good at detecting, recognising and remembering odours.

We can identify our relatives by smell alone, or follow the menstrual cycles of our friends and lovers. We can diagnose disease, detect danger, and distinguish between good and bad food just with our noses. We know, along with Rousseau, that "Smell is the sense of memory and desire." We realise, with Diderot, that smell is the most voluptuous sense. And we acknowledge the truth in Helen Keller's contention that smell, while representing the "fallen angel" of the senses, nevertheless remains a "potent wizard that transports us across thousands of miles and all the years we have lived".

So why the ambivalence? It seems we are missing something. I think we are, and I believe I know what it is. It is an obscure body part. One that has been there, right under our noses, all the time. Science has known about it since 1811. Biologists are familiar with it as the structure in the roof of a snake's mouth which "tastes" molecules collected by the reptile's flickering tongue. And a few dedicated anatomists have tracked down something similar in the nasal passages of possums, anteaters, bats, cats, rabbits, and even a white whale. But, though this body part was also discovered and described in humans a century ago, it has since mysteriously disappeared from the textbooks.

There are passing mentions of it in medical and technical journals dedicated to comparative anatomy and olfactory physiology, where it tends to be dismissed as something vestigial, an anatomical ghost that makes a transitory appearance in the human embryo, vanishing well before birth. And yet it still exists: a study in 1991 of a thousand randomly selected adult human subjects found it in the noses of almost every one.

This elusive feature is the Organ of Jacobson, named after the sharp- eyed Danish anatomist who discovered it nearly two centuries ago. It is easy to miss. The external evidence consists simply of a pair of tiny pits, one on either side of the nasal septum, a centimetre and a half above the human nostril. But the fact that it does exist changes everything. With it, we reinherit the possibility of a powerful and ancient chemical sense: an ability to enter once more into a system of subliminal signalling that continues to give other animals access to a world we thought we had lost as a result of our emphasis on sight instead of smell.

Jacobson's Organ rescues our most underrated sense from obscurity. But it is not simply a supercharger, making us more sensitive to odours. What it seems to do is to open up a channel quite separate from the main olfactory system. It feeds an older, more primal area of the brain, one that monitors airborne hormones and a host of other undercover patterns of information, making physiological changes that have profound effects on our awareness, on our emotional states and on our most basic behaviours. Recent research suggests that this system could be the mechanism necessary for operating a true "sixth sense", one that may account for our sometimes apparently supernatural ability to receive information not normally available to the traditional five faculties.

If all this is true, Jacobson's Organ could be the most important key to unravelling the mysteries of our minds since the discovery of the unconscious.

THE DISCOVERER of Jacobson's Organ, Ludwig Levin Jacobson, was born in 1783 in Copenhagen. He qualified in surgery at the age of just 21 and went to study in Paris with the great anatomist Baron Georges Cuvier. It was probably he who drew Jacobson's attention to the work of Frederick Ruysch, a Dutch embalmer who in 1703 described the anatomy of a number of animals, including a snake with unusual pits in its palate. So Jacobson looked for, in 1809 discovered, and in 1811 published a report on, the organ which still bears his name.

The Organ of Jacobson remained a zoological curiosity until the late 19th century, when interest in it was revived by one of the most charismatic figures in Victorian science. Robert Broom was a Scot and a physician with a very curious mind. In later life he became one of the world's great palaeontologists and physical anthropologists, doing pioneer work on the origins of humans in Africa, but his first enthusiasm was for reptiles, their origins and the significance of Jacobson's Organ. In between practising medicine he scoured museum collections and studied animals in the wilds of Australia and South Africa for material to include in a doctoral thesis, "On the comparative anatomy of Jacobson's Organ", which he submitted to Glasgow University.

"It would seem," said Broom, "that the Organ of Jacobson is the organ in the body that is least liable to become altered by change of habit. I can almost identify an animal by examining this organ and often tell of its affinities."

Recent research confirms Broom's suspicions about Jacobson's Organ. Signs of it appear, like portents, in the embryos of all higher land mammals, although in some, such as tree-living lizards, aquatic turtles and nearly all birds, it never actually materialises. There is no need for it. In others, such as ground-living lizards and most snakes, the organ comes into being, not simply as pouches off the nasal cavity, but as a pair of blind chambers that open directly into the mouth. Why should this be? What happened to make this development not just useful, but necessary?

THERE IS little in the animal kingdom, with the possible exception of the musk deer's pod or the civet's anal gland, to compare with the human armpit for olfactory potency. It is our major source of body odour, an organ perfectly designed for the job. Apocrine glands there are heaped up, two or three to a follicle, covering a patch the size of a tennis ball, coating the long underarm hairs with their oils, vaporising easily in the warmth, dissolving and spreading with the help of sweat glands that keep the whole area moist and bacterially active.

The secretion of apocrine glands is different from the sebum of sebaceous glands. It is a viscid oil, coloured anything from a milky white to a blood red, depending on diet and perhaps on race. Japanese of the 19th century, when first exposed to European traders, referred to them as batakusai - "stinks of butter". People of European and African ancestry do have the largest armpit or axillary organs, often so densely packed with glands that they look like sponges under the skin. People of Asian origin have smaller organs or no armpit glands at all. In Japan, 90 per cent of the population has no detectable underarm odour, and young men who are unfortunate enough to belong to the smelly minority can even be disqualified from military service on that ground alone.

Apocrine gland secretions, on their own, have no smell. The musky odours of the armpit are entirely the work of bacterial decomposition, which turns fatty oils and hormones into pheromones. Men produce a stronger odour than women with the same bacterial flora on their skin. And women, as their hormonal levels change during oestrus, produce dramatic changes in their own armpit odours. This is all to do with shifts in basic chemistry, and it is surprising that deodorant manufacturers haven't yet begun to market a range of products that cater to this. Calendar Cosmetics: "One for every day of the month. You're worth it!"

BUT there has long been general agreement that underarm odours can be very sexy. The French novelist Joris-Karl Huysmans puts it well in this description of one of his heroines: "The scent of her underarms easily uncaged the animal in men." Stories abound, too, of rustic swains who worked all day in the fields with a kerchief under one arm, and wreaked havoc after dark with this message-laden cloth in a breast pocket, carefully placed under a dancing partner's nose. And word has it that in 1572, Henry III of France inadvertently wiped his face on a sweat-stained chemise, abandoned between dances in the cloakroom at the Louvre by the beautiful young Mary of Cleves, "and from that moment, conceived for her the most violent passion".

Mary, of course, would not have shaved under her arms. She wasn't part of a culture like ours that supports research to show that "24 hours after thorough cleansing with Ivory soap, only one out of 10 shaved axillae could be described as odorous, compared with nine out of 10 unshaved armpits." In their day, even princesses took advantage of the fact that underarm hair provides a large surface area on which bacteria can breed, and acts also as an efficient wick, encouraging the broadcasting of that odour every time the arms are raised, as they must be when dancing with a taller man, or when lovers put their arms around each other and she snuggles into his shoulder.

But our culture requires that we frown on underarm odour and we battle against it with deodorants. So there are no paeans to the armpit in poetry and song, but no shortage of sentimental requests to put heads on, cry on, or lean on, obliging shoulders. Once we were not so squeamish. "I will be arriving in Paris tomorrow evening," Napoleon wrote to Josephine. "Don't wash." Which might translate today as: "Don't disturb the wonderful process of incubation that takes fresh apocrine secretions and allows aerobic diphtheria and other micrococcal bacteria to process it into a priming pheromone with a chemical configuration not unlike musk."

Napoleon said it better. Perhaps he had secrets of his own. We know he drenched himself with Cologne before battle. But that wouldn't have prevented non-odorous steroid precursors in his armpits from metabolising, with the help of time and bacteria, into odorous steroids which are almost identical to the primer pheromones on boar's breath. Small wonder, then, that male armpits in France were once known as "spice boxes", or that whiffs of them can still exert an attraction, both strong and animal, that seems to be beyond our cerebral control - partly because it goes straight from the axillary organ to Jacobson's Organ.

Apocrine glands everywhere on the human skin are the main source of what we now call body odour: the dreaded "BO" that upsets people in crowded trains or locker rooms. But that same bacterial stench, in different circumstances, carries vital information. When volunteers in England agreed to wear T- shirts for 24 hours without bathing or using deodorants, every one of them was later able to identify their own shirt by smell alone, even when put with two others. Most were able also to identify the sex of the volunteers who had worn the other samples.

In America, Richard Doty at the University of Pennsylvania followed this up with a series of carefully controlled studies. In these, underarm odours were collected from men and women who wore gauze pads in their armpits for 18 hours. The pads were then presented in "sniff bottles" to male and female judges who were asked to rate them for strength and pleasantness. They agreed on which were strongest, and correctly identified these odours as male, but failed to agree on whether they were pleasant or unpleasant.

In a study carried out by Birmingham University, a male hormone called androstenone was sprayed on a seat in a dentist's waiting room, and the pattern of patient movements was recorded on the following days. This androgen has no perceptible odour, but in a practice with equal numbers of patients of either sex, far more women used the seat than men, and all seemed more willing than other female patients to wait their turn.

I HAVE been concentrating on the legendary armpit, but it is hardly the only source of human odour. In every primate ever observed in the field, males are very obviously attracted to the genital area of females at certain stages of the oestral cycle. Male macaques, mangabeys and baboons investigate females by touching their vaginas with a finger and sniffing it. As ovulation approaches, marmosets, woolly monkeys and all the apes will lick and smell the vaginas of the females directly. And at ovulation, when the females themselves are more receptive and most sensitive to odours, they and the males become immersed in mutual genital investigation.

In human females, vaginal secretions are highly complex, containing more than 30 compounds, mostly fatty acids; some odour-bearing, some not; but all subject to bacterial decay. Taken together, and in a series of combinations that vary dramatically throughout the cycle, there is sufficient information in such secretions to enable anyone with an active nose and sufficient curiosity to predict the time of ovulation and behave accordingly. The jury is still out, but studies show that intercourse in our species takes place more often near ovulation than at any other time, whether or not the man knows his partner's cycle.

The naso-genital relationship seems always to have been on our minds. Virgil tells of adulterers being punished by amputation of the nose - a fitting penalty, perhaps, given the importance of smell in the sort of sexual attraction that begins in puberty with an explosive growth of hormones. In this sense, smell is a secondary sexual character, and the nose one of our most important sex organs - one that may even have had to be constrained at a critical stage in our evolution.

Michael Stoddard at the University of Tasmania is one of the very few to have considered the role of olfaction in our early history. He points out that our distant ancestors lived in small family groups, as many primates still do today. But sometime in the Miocene, starting perhaps 10 million years ago, major changes took place. We became more vertical, more bipedal, more mobile, more predatory and less hairy, and began to live in larger groups.

As a result, males took on more responsibility for feeding the family, and joined with other males in pursuit of the best high-calorie resources. Females, faced with the painful problem of giving birth to babies with increasingly large heads, shortened their pregnancies and found themselves having to deal instead with longer periods of dependence. These changes put a greater premium on maintaining pair bonds and, to that end, sex began to play a larger part in our lives, over and beyond that of simple reproduction. So the sexes came to look very different from each other. Males grew bigger in every way, until they had the best-developed genitalia of any primate. Females became sexually receptive for longer periods and advertised their maturity by developing breasts in advance of lactation. And both sexes became more odorous than any other primate has ever been.

Humans are the only mammals who bond in pairs and live in large groups. It isn't natural, and Michael Stoddard suggests that evolutionary pressures dealt with the problem by finding a way of allowing private sex to continue at a high level, without attracting too much attention from rival males. And that the solution that arose was that humans became more secretive about ovulation - and our feelings about body odours changed.

There is something in this idea. It would help explain our continuing ambivalence about our own smells, our insistence that humans should not smell like humans, and our tendency to borrow the smells of other species. We men certainly have become so alienated from the natural cyclic odours of our women, that we have become uneasy about them, saying that we find them repugnant and making them the focus of some of our most inflexible taboos.

The truth, however, is that we continue to be very smelly apes. Our armpits and sex organs go on producing strong, musky odours, and we carry with us all the olfactory apparatus necessary to perceive them. If there is a switch that turned the tide, and turned part of our sensitivity off back there in the Miocene, it was clearly a mental one: something in the newer parts of the big brain.

The old, wide range of olfactory cues continues to wash over us, but it is obvious that we no longer respond to them in quite the same way. We have, in a sense, adopted a reptilian solution to the problem, down- grading the sensitivity of the nose - in effect, gone into a state of partial olfactory denial. But, like the reptiles, we still have access to a second system, one that has been allowed to persist because of its more private nature. We have a small but apparently functional Organ of Jacobson that operates at close quarters with a different, more down-to- earth array of messages. And it is becoming very subversive.

Most of the action stimulated by Jacobson's Organ takes place at an unconscious level. Its effects are felt primarily in the emotional centres of the brain, spreading outwards through the pituitary gland and the gonads. It has the capacity to bypass the neural blockage and produce the sort of feedback that brings things once suppressed back to conscious attention. And it has the sneaky habit of reminding us that the old strictures, the mind games of the Miocene, are no longer absolutely necessary. With it, perhaps we still have the chance to regroup, to re-enchant our lives, and become more aware of the sexy world of scent that is just waiting to be rediscovered.

1999 Lyall Watson. Extracted from `Jacobson's Organ and the Remarkable Nature of Smell', published by the Penguin Press on 30 September 1999. All rights reserved

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