Long before there was any official "Theory of Evolution," most people believed that humans were gradually improving in every significant way: bigger, stronger, smarter, and more civilized. All this in spite of convincing evidence that in some ways we are less capable than our early ancestors and most animals.

Take, for example, the sense of smell. A dog would argue (if a dog could argue) that we don't really have one. We can't distinguish our friends by their individual odors, or even tell when some human has recently left a room.

Most of us are aware that our eyesight deteriorates with age, and our hearing, too. But most of us neither know nor care that the number of a person's olfactory receptors - the basic sensors for smell - goes down by about one percent per year, and with it our sensitivity to odors. In writing recently about the ways that technology can compensate for the deterioration with age of our senses, I didn't put either taste or smell on the list; and our sense of taste, it turns out, depends heavily on our sense of smell. Without the help of the nose, we distinguish only four basic tastes: bitter, sweet, sour, and salty.

I didn't leave smell off the list of things to be improved by technology because I thought it was unimportant, or because a smell-enhancement device up your nose seems vaguely ridiculous in a way that contact lenses or a hearing aid do not. I left it off because our understanding of smell is primitive - about at the level that our conceptual models of vision and hearing were in the sixteenth century.

We now know that a simple sound wave can be characterized by just two numbers: its pitch and its loudness. We also know that any sound, no matter how complex, is made up of a sum of simple sound waves. Similarly, a "simple" light wave can be characterized by its intensity and its wavelength (it also has a polarization, which is not important for our purposes). The light that we see is made up of a sum of simple light waves. There are volumes more to be said about both sound and light, and massive works have been written about each, but in a few sentences we have just defined the underlying models.

And the underlying model for smell? We don't have one, and it's not for lack of trying. One attempt is through a "smell prism" which tries to define all odors as combinations of six basic ones: fruity, flowery, resinous, spicy, foul, and burnt. It has not been very successful. Also, note how vague these categories are, and how they depend on individual perception. We can't pin a physical variable to the idea of a flowery smell, as we are able to pin a wavelength to a color or to the note of a flute.

Perhaps the problem lies in the lost ability of our noses to distinguish detail. We live mainly in a world of sight and sound. A dog's world is defined as much or more by the smells that surround it. A smart dog might well be able to provide a "taxonomy of odors," from which a basic theory and ultimately a mechanical device which matches the dog's own nose in accuracy and sensitivity might be built.

Or maybe we need to look beyond the canine world to find the true connoisseurs and categorizers of aromas. The sensitivity champions of the animal kingdom are not mammals, they are insects. Certain kinds of male moths, such as the emperor moth, Eudia pavonia, or the silkworm moth, Bombyx mori, can detect a single molecule of a specific chemical. That chemical, a kind of hormone known as a pheromone, is unique to each moth species and is emitted by a female ready to mate. The male moth can home in on such a female from more than five miles away.

Impressive as this is, my money stays with the mammals. To draw an analogy from the world of sound, the mating moths are tuned to hear a single specific note, far away and almost infinitely faint, while they remain deaf to everything else. The tracking bloodhound, on the other hand, is more like the conductor of a full symphony orchestra, able amid a great volume of sound to hear and locate a note of a single instrument that is off in pitch or intonation.

Humans can't compete with either the moths or the hounds; and yet I wonder if in some ways our sense of smell is better than we realize or want to admit. Of all our senses, the olfactory system is the only one that has a pipeline straight into the cerebral cortex, without passing through the preliminary processing center known as the thalamus. We all know how strongly evocative particular smells can be, pulling up long-ago memories apparently from nowhere. (I once walked through the research facility of International Flavors and Fragrances, and was overwhelmed by the variety of odor-driven associations that came unbidden to my mind in just a few hundred feet.)

Now, moths are not the only creatures that generate pheromones intimately linked to mating. Almost all animals do so - including humans. Most of our sweat glands are used to help control body temperature, but one class of them, known as the apocrine glands, also secrete pheromones. Normally we are unaware of them, or we seek to mask them with other odors. However, I wonder how successful we are at this. The basis for the attraction felt by particular men and women has never been explained, and it certainly depends on far more than physical appearance. Furthermore, specialists on the human brain believe that only perhaps five percent of what we do is known to our conscious mind and controlled by conscious decision-making.

It seems to me very plausible that our much-maligned and undervalued olfactory centers have far more say in our lives than we know. They pick up pheromones and pass that message along to certain centers of the brain in charge of mating decisions, while nothing of this gets through to our conscious thought processes.

Maybe "love at first sight" ought to be replaced in our romantic phrase books by "love at first scent."

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