If you visit almost any bookshop, you will find substantial shelf space devoted to cookbooks. You will also find an equal and perhaps even greater area devoted to diet books.

Five thousand or even five hundred years ago, this was unimaginable. Humans constantly faced the problem of finding enough to eat. If the meals of the day were provided for, next week or next year was still a worry. This, of course, is true today for most of the world. The United States, along with some other fortunate nations, possesses a cornucopia of food in amounts unthinkable in most periods and places.

At the same time, the nature of what we eat is changing. Although we enjoy amazing choices - strawberries in midwinter, spring salad all year round - we are limited in other ways compared with our ancestors. This is most obvious in the case of animal foods. Our forebears seemed ready to eat almost anything: rabbit, bear, sheep, squirrel, cow, deer, frog, pig, hedgehog, every kind of bird and bird's eggs, and every organ from tongue to brains to lights (lungs) to pancreas (sweetbreads). Some were eaten because of necessity, although others, such as larks' tongues, jugged hare, or plovers' eggs, were considered great delicacies. But if you try looking for these at your local supermarket, you will be disappointed.

The same shrinking choice is true in vegetable foods. A thousand years ago, more than five hundred grains, berries, and vegetables helped to feed the world. Today our staples are a handful of different grains, a couple of dozen fruits, maybe a dozen nuts, and thirty or forty vegetables. Most of these have been vastly modified from their original wild strains, a point I will return to later.

Only when it comes to seafood would I suggest that a wide variety is available, even if only in restaurants and often under ambiguous names. Ocean products, unlike farm products, have not been cultivated and standardized. Many of us don't know what we are eating when the menu says rock salmon, monkfish, or sea perch.

Given the choice, few of us would go back from unlimited amounts of rather limited choices to meager amounts of many different kinds of food. Yet today's increasing specialization invites the question, are we losing something important as we lose diversity, and as less and less of our food is fresh and unprocessed?

At first sight, we shouldn't be. Eating and digestion is a process that sounds as though it should be trivial. In order to survive, the human body needs carbohydrates for energy, fats as a storage medium for certain minerals and use in future production of energy, and proteins for the maintenance, repair, and replacement of body tissues. We also need certain trace substances such as vitamins. What we eat are carbohydrates, fats and proteins, plus the necessary trace substances. It sounds as though we are taking in exactly what we need.

We are, and yet we are not. The carbohydrates, fats, and proteins that we consume were perfect for their original owners - an apple, a potato, a chicken - but they are not suited to human needs. To satisfy those unique needs, the body performs an elaborate process of disassembly and assembly.

Carbohydrates consist of different combinations of oxygen, hydrogen, and carbon. Fats are combinations of the same three elements, though they tend to be much lower in oxygen content. Proteins are more complicated substances, mostly built up of very large molecules containing hundreds or thousands of atoms in very specific configurations. In addition to oxygen, hydrogen, and carbon, proteins contain nitrogen and other elements such as sulfur, phosphorus, and iron. Fats, carbohydrates and proteins all can be used to provide a body with energy, but proteins alone are employed in tissue growth and repair.

Our digestive systems might, in principle, go all the way to reducing these three classes of substances to their separate atomic constituents. In fact, they do not go that far. Carbohydrates are converted to the simplest form of sugar, monosaccharides, in a multi-stage digestive process that begins in your mouth. Fats are broken down to glycerine and fatty acids. Proteins break down to amino acids. And then the complex process of re-assembly and use begins using precise procedures specified by your own DNA. At the end of it, whatever you ate - nuts, meats, fruit, vegetables - has either been used for energy, excreted, or become part of you.

Our dog, whenever possible, prefers to eat human food rather than dog food. Since he is not too smart, we conclude that he has formed a mistaken theory that if he eats what we do, he will turn into a human. In fact, whatever he eats turns into dog. Walter de la Mare was stating a remarkable scientific truth when he wrote in one of his poems, "It's a very odd thing, as odd as can be, that whatever Miss T. eats turns into Miss T."

However, this whole process of disassembly and assembly could be pointing the way to future methods of food production. Why not begin with carbon, oxygen, hydrogen, nitrogen, and other necessary elements, and build food from scratch? We have been changing plants and animals for thousands of years, accelerating their growth, increasing their yield, and improving their taste. We have also used selective breeding to develop hybrids such as plumcots and loganberries, fruit never found in Nature. This is a form of genetic modification. Although the term was not used at the time, the work of hybridization was performed. Today, people's views on genemod foods range from cries of "Frankenfoods" and pressure for a total ban, to a belief that these foods should be permitted broadly and at once on humanitarian grounds since they can help with worldwide hunger.

However, were we to construct food from the basic elements we would avoid the whole subject of genetic modification. Although the idea is an old one and almost certainly feasible, I don't see it catching on any time soon. Aside from the "unnaturalness" that some people would surely claim, it would take a long time to get tastes the way we want them.

We would also face the problem of names. I'm not sure I would care for a slice of dioxy-hydrosulfa-carbo-etc-etc. On the other hand, have you read the list of ingredients on a box of cookies recently?

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