I write these columns several weeks before you read them, on whatever subject happens to be on my mind. I am composing this one after a Thanksgiving dinner involving individual portions of a size apparently designed not so much to sustain life as to extinguish it. Today's subject, naturally, is food.

We like to regard ourselves as the top of the food chain. That is mostly true, although humans are plagued by many parasitic organisms (tapeworms, hookworms, pinworms, mites, fleas, and ticks, to mention only the larger forms) which might fairly claim to be the real top of the food chain, since they prey upon us and we do not prey upon them. I will accept the parasite as king.

However, even our claim for a more limited supremacy was not always justified. A million years ago, plenty of other animals were more than willing to catch, kill, and eat us. Today we may worry a little about shark attacks, while in other parts of the world there is danger from killer whales, sea leopards, polar bears, crocodiles, and the occasional lion and tiger. But on the whole we see ourselves as the killers and eaters.

At the macroscopic level, that is certainly correct. We kill and eat the world's largest animals. Many of them, like the mammoth, saber-toothed tiger, cave bear, moa, mastodon, and giant ground sloth, have been hunted to extinction. In Africa today the largest mammals and birds are increasingly scarce and much of the meat supply comes in the form of "bush meat," provided by monkeys and other small animals. That, too, is becoming harder to find. Are we, as the world population increases, doomed to work our way farther and farther down in size, until all we have left for meat are shrews and hummingbirds and insects?

I don't think so. Unless we all become vegetarians, we will preserve, and value as food, animals able to eat, digest, and thrive on bark, twigs, and grasses. Humans can't do that - and it has been tried. During the disastrous Irish potato famine in the middle of the nineteenth century, a French gentleman proclaimed that it was possible for a human to live on boiled grass. He tried it for three weeks before conceding that he was wrong.

Why can't we eat twigs and grass and paper and bits of wood? Simply because they contain lots of a material that thwarts our digestive systems. That material is cellulose (not to be confused with cellulite, the curse of the more mature lady).

It is a little odd that we are unable to digest cellulose, for three reasons. First, it is the most abundant organic material on earth. Forests are largely made of it. Second, many other animals do digest it. Third, cellulose is a form of carbohydrate known as a polysaccharide, which just means many-sugars, and we can readily, maybe too readily, digest starches, which also are polysaccharides.

A molecule of a polysaccharide is in fact a "food chain" in microcosm - a chain of sugar units (think of them as glucose molecules) joined together because a water molecule has been removed at the place where each pair meets. We relish a number of sugars, such as maltose, lactose, and sucrose, all of which our bodies break down to the simplest forms, monosaccharides such as glucose, before using them. Maltose, lactose and sucrose contain two sugar units, and each is known as a disaccharide. In addition, we also eat and enjoy monosaccharide sugars, such as fructose, and starches, such as potatoes, rice, corn, and wheat, which contain long chains of sugar units.

Cellulose, however, thwarts a human digestion. What is so different about the digestive system of an elephant, a cow, or a rabbit, that makes it able to do what we can't do, and break down the long chains of cellulose into simple single-sugar forms?

The answer lies not in the digestive system's structure, but in its contents. Cellulose is an unusually tightly structured form of polysaccharide, without the side chains of molecules found in starches. The key actors in its digestion are certain bacteria resident in an animal's alimentary canal. These bacteria are able to split the long chains of the cellulose polysaccharide into its single-sugar components, using a process known as hydrolysis. At that point, digestive processes much like our own take over.

The bacteria involved in the breakdown of cellulose are anaerobic, which means that they do not need an oxygen-rich environment. In fact, they will die if there is too much oxygen around. Bacteria like this have been around for a long, long time. They are survivors from a period, two billion years or more ago, when photosynthesis had not yet been developed and the Earth's atmosphere contained little or no oxygen. Instead of producing carbon dioxide as a by-product of metabolism, these bacteria instead generate methane (and are consequently known as methanogenetic bacteria).

Bacteria that break down cellulose need time to do their job. That's why it takes several days for food to pass through an elephant's long digestive tract, and why ruminants like cows chew the cud. Regurgitating food and swallowing into the second, third, or fourth stomach gives several goes at the same piece of vegetation. Rabbits, where the length of intestine available for digestion is limited by the animal's size, have evolved an alternative approach which some humans may find a bit off-putting: they eat their own excreted fecal pellets, which gives them a second chance to extract full nutritional value from those hard-to-digest-down barks and grasses.

Why don't we have these bacteria in our stomachs? Simply because we followed a different evolutionary path, in which our foods were fruits and grains and insects and small animals. We could and can get by very well without needing to digest cellulose.

Animals that digest cellulose produce not just a little methane, but tons and tons of it. Cows emit methane from both ends in large quantities. Cattle feed lots can be detected by airborne methane-sniffers from high altitude. Perhaps it is just as well that humans cannot, like cows and goats and rabbits and elephants, digest cellulose. If we could, our notions of acceptable social behavior after dinner would have to change considerably.

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