by Isaac Asimov
The coronary arteries which supply oxygen to the heart itself, are particularly susceptible to atherosclerotic narrowing. The resulting oxygen starvation of the heart gives rise to the agonizing pain of angina pectoris, and, eventually though not necessarily quickly, to death.
The roughening and narrowing of the arteries introduces another hazard. Because of the increased friction of the blood scraping along the roughened inner surface of the vessels, blood clots are more likely to form, and the narrowing of the vessels heightens the chances that a clot will completely block the blood flow. In the coronary artery, feeding the heart muscle itself, a block (coronary thrombosis) can produce almost instant death.
Just what causes the formation of deposits on the artery wall is a matter of much debate among medical scientists. Cholesterol certainly seems to be involved, but how it is involved is still far from clear. The plasma of human blood contains lipoproteins, which consist of cholesterol and other fatty substances bound to certain proteins. Some of the fractions making up lipoprotein maintain a constant concentration in the blood—in health and in disease, before and after eating, and so on. Others fluctuate, rising after meals. Still others are particularly high in obese individuals. One fraction, rich in cholesterol, is particularly high in overweight people and in those with atherosclerosis.
Atherosclerosis tends to go along with a high blood-fat content, and so does obesity. Overweight people are more susceptible to atherosclerosis than are thin people. Diabetics also have high blood-fat levels and are more susceptible to atherosclerosis than are normal individuals. And, to round out the picture, the incidence of diabetes among the stout is considerably higher than among the thin.
It is thus no accident that those who live to a great age are often scrawny, little fellows. Large, fat men may be jolly, but they do not keep the sexton waiting unduly, as a rule. (Of course, there are always exceptions, and one can point to men such as Winston Churchill and Herbert Hoover, who passed their ninetieth birthdays but were never noted for leanness.)
The key question, at the moment, is whether atherosclerosis can be fostered or prevented by the diet. Animal fats—such as those in milk, eggs, and butter—are particularly high in cholesterol; plant fats lack it altogether. Moreover, the fatty acids of plant fats are mainly of the unsaturated type, which has been reported to counter the deposition of cholesterol. Investigations of these matters seemed to show conclusively, in 1984, that cholesterol in the diet is involved in atherosclerosis and people have been flocking to low-cholesterol diets, in the hope of staving off thickening of the artery walls.
Of course, the cholesterol in the blood is not necessarily derived from the cholesterol of the diet. The body can and does make its own cholesterol with great ease, and even though you live on a diet that is completely free of cholesterol, you will still have a generous supply of cholesterol in your blood lipoproteins. It therefore seems reasonable to suppose that what matters is not the mere presence of cholesterol but the individual’s tendency to deposit it where it will do the most harm. It may be that there is a hereditary tendency to manufacture excessive amounts of cholesterol. Biochemists are seeking drugs that will inhibit cholesterol formation, in the hope that such drugs may forestall the development of atherosclerosis in those who are susceptible to the disease.
Meanwhile coronary bypass surgery—the use of other blood vessels from a patient’s body to attach to the coronary arteries in such a way that blood flows freely through the bypass around the atherosclerotic region and supplies the heart with an ample blood supply—has become very common since its introduction in 1969, and very successful. It does not seem to lengthen one’s overall life expectancy, but it makes one’s final years free of crippling anginal pain, and (as they know who have experienced it) that is a great deal.
OLD AGE
But even those who escape atherosclerosis grow old. Old age is a disease of universal incidence. Nothing can stop the creeping enfeeblement, the increasing brittleness of the bones, the weakening of the muscles, the stiffening of the joints, the slowing of reflexes, the dimming of sight, the declining agility of the mind. The rate at which this happens is somewhat slower in some people than in others—but, fast or slow, the process is inexorable.
Perhaps we ought not complain too loudly about this. If old age and death must come, they arrive unusually slowly. In general, the life span of mammals correlates with size. The smallest mammal, the shrew, may live one and a half years, and a rat may live four or five. A rabbit may live up to fifteen years, a dog up to eighteen, a pig up to twenty, a horse up to forty, and an elephant up to seventy. To be sure, the smaller the animal the more rapidly it lives the faster its heartbeat, for instance. A shrew with a heartbeat of 1,000 per minute can be matched against an elephant with a heartbeat of 20 per minute.
In fact, mammals in general seem to live, at best, as long as it takes their hearts to count a billion. To this general rule, human beings themselves are the most astonishing exception. Though considerably smaller than a horse and far smaller than an elephant, the human being can live longer than any mammal can. Even if we discount tales of vast ages from various backwoods where accurate records have not been kept, there are reasonably convincing data for life spans of up to 115 years. The only vertebrates to outdo this record, without question, are certain large, slow-moving tortoises.
A man’s heartbeat of about seventy-two per minute is just what is to be expected of a mammal of his size. In seventy years, which is the average life expectancy in the technologically advanced areas of the world, the human heart has beaten 2.5 billion times; at 115 years, it has beaten about 4 billion times. Even our nearest relatives, the great apes, cannot match this, even closely. The gorilla, considerably larger than a man, is in extreme old age at fifty.
There is no question but that the human heart outperforms all other hearts in existence. (The tortoise’s heart may last longer but it lives nowhere near as intensely.) Why we should be so long-lived is not known; but as humans, we are far more interested in asking why we do not live still longer.
What is old age, anyway? So far, there are only speculations. Some have suggested that the body’s resistance to infection slowly decreases with age (at a rate depending on heredity). Others speculate that clinkers of one kind or another accumulate in the cells (again, at a rate that varies from individual to individual). These supposed side products of normal cellular reactions, which the cell can neither destroy nor get rid of, slowly build up in the cell as the years pass, until they eventually interfere with the cell’s metabolism so seriously that it ceases to function. When enough cells are put out of action, so the theory goes, the body dies. A variation of this notion holds that the protein molecules themselves become clinkers, because cross links develop between them so that they become stiff and brittle and finally bring the cell machinery grinding to a halt.
If this is so, then “failure” is built into the cell machinery. Carrel’s ability to keep a piece of embryonic tissue alive for decades had made it seem that cells themselves might be immortal: it was only the organization into combinations of trillions of individual cells that brought death. The organization failed, not the cells.
Not so, apparently. It is now thought that Carrel may (unwittingly) have introduced fresh cells into his preparation in the process of feeding the tissue. Attempts to work with isolated cells or groups of cells in which the introduction of fresh cells was rigorously excluded seem to show that the cells inevitably age and will not divide more than fifty times all told—presumably through irreversible changes in the key cell components.
And yet there is the extraordinarily long human life span. Can it be that human tissue has developed methods of reversing or inhibiting cellular aging effects, methods that are more efficient than those in any other mammal? Again, birds tend to live markedly longer than mammals of the same size despitethe fact that bird metabolism is even more rapid than mammal metabolism—again, superior ability of old age reversal or inhibition.
If old age can be staved off more by some organisms than by others, there seems no reason to suppose that humans cannot learn the method and improve upon it. Might not old age, then, be curable, and might not we develop the ability to enjoy an enormously extended life span—or even immortality?
General optimism in this respect is to be found among some people. Medical miracles in the past would seem to herald unlimited miracles in the future. And if that is so, what a shame to live in a generation that will just miss a cure for cancer, or for arthritis, or for old age!
In the late 1960s, therefore, a movement grew to freeze human bodies at the moment of death, in order that the cellular machinery might remain as intact as possible, until the happy day when whatever it was that marked the deathof the frozen individual, could be cured. He or she would then be revived and made healthy, young, and happy.
To be sure, there is no sign at the present moment that any dead body can be restored to life, or that any frozen body—even if alive at the moment of freezing—can be thawed to life. Nor do the proponents of this procedure (cryonics) give much attention to the complications that might arise in the flood of dead bodies returned to life. The personal hankering for immortality governs all.
Actually, it makes little sense to freeze intact bodies, even if all possible revival could be done. It is wasteful. Biologists have so far had much more luck with the developing of whole organisms from groups of specialized cells. Skin cells or liver cells, after all, have the same genetic equipment that other cells have, and that the original fertilized ovum had in the first place. The cells are specialized because the various genes are inhibited or activated to varying extents. But might not the genes be deinhibited or deactivated, and might they not then make their cell into the equivalent of a fertilized ovum and develop an organism all over again—the same organism, genetically speaking, as the one of which they had formed part? Surely, this procedure (called cloning) offers more hope for a kind of preservation of the genetic pattern (if not the memory and personality). Instead of freezing an entire body, chop off the little toe and freeze that.
But do we really want immortality—either through cryonics, through cloning, or through simple reversal of the aging phenomenon in each individual? There are few human beings who would not eagerly accept an immortality reasonably free of aches, pains, and the effects of age—but suppose we were all immortal?
Clearly, if there were few or no deaths on earth, there would have to be few or no births. It would mean a society without babies. Presumably that is not fatal; a society self-centered enough to cling to immortality would not stop at eliminating babies altogether.
But will that do? It would be a society composed of the same brains, thinking the same thoughts, circling the same ruts in the same way, endlessly. It must be remembered that babies possess not only young brains but new brains. Each baby (barring identical multiple births) has genetic equipment unlike that of any human individual who ever lived. Thanks to babies, there are constantly fresh genetic combinations injected into humanity, so that the way is open toward improvement and development.
It would be wise to lower the level of the birth rate, but ought we to wipe it out entirely? It would be pleasant to eliminate the pains and discomforts of old age, but ought we to create a species consisting of the old, the tired, the bored, the same, and never allow for the new and the better?
Perhaps the prospect of immortality is worse than the prospect of death.
Chapter 16
* * *
The Species
Varieties of Life
Our knowledge of our own bodies is incomplete without a knowledge of our relationship to the rest of life on the earth.
In primitive cultures, the relationship was often considered to be close indeed. Many tribes regarded certain animals as their ancestors or blood brothers, and made it a crime to kill or eat them, except under certain ritualistic circumstances. This veneration of animals as gods or near-gods is called totemism (from an American Indian word), and there are signs of it in cultures that are not so primitive. The animal-headed gods of Egypt were a hangover of totemism, and so, perhaps, is the modern Hindu veneration of cows and monkeys.
On the other hand, Western culture, as exemplified in Greek and Hebrew ideas, very early made a sharp distinction between human beings and the “lower animals.” Thus, the Bible emphasizes that Adam was produced by a special act of creation in the image of God, “after our likeness” (Genesis 1:26). Yet the Bible attests, nevertheless, to man’s remarkably keen interest in the lower animals. Genesis mentions that Adam, in his idyllic early days in the Garden of Eden, was given the task of naming “every beast of the field, and every fowl of the air.”
Offhand, that seems not too difficult a task—something that one could do in perhaps an hour or two. The scriptural chroniclers put “two of every sort” of animal in Noah’s Ark, whose dimensions were 450 by 75 by 45 feet (if we take the cubit to be 18 inches). The Greek natural philosophers thought of the living world in similarly limited terms: Aristotle could list only about 500 kinds of animals, and his pupil Theophrastus, the most eminent botanist of ancient Greece, listed only about 500 different plants.
Such a list might make some sense if one thought of an elephant as always an elephant, a camel as just a camel, or a flea as simply a flea. Things began to get a little more complicated when naturalists realized that animals had to be differentiated on the basis of whether they could breed with each other. The Indian elephant could not interbreed with the African elephant; therefore, they had to be considered different species of elephant. The Arabian camel (one hump) and the Bactrian camel (two humps) also are separate species. As for the flea, the small biting insects (all resembling the common flea) are divided into 500 different species!
Through the centuries, as naturalists counted new varieties of creatures in the field, in the air, and in the sea, and as new areas of the world came into view through exploration, the number of identified species of animals and plants grew astronomically. By 1800 it had reached 70,000. Today more than 1,500,000 million different species—two-thirds animal and one-third plant—are known, and no biologist supposes the count to be complete.
Even fairly large animals remain to be found in odd corners of the globe. The okapi, a relative of the giraffe and the size of a zebra, became known to biologists only in 1900 when it was finally tracked down in the Congo forests. Even in 1983, a new kind of albatross was recorded on an island in the Indian ocean, and two new kinds of monkey were found in the Amazon jungles.
Undiscovered varieties of organisms are sure to be hidden in the ocean depths where investigation is more difficult. The giant squid, the largest of all invertebrates, was not proved to exist until the 1860s. The coelacanth (see chapter 4) was discovered only in 1938.
As for small animals—insects, worms, and so on—new varieties are discovered every day. A conservative estimate would have it that there are 10 million species of living things existing in the world today. If it is true that some nine-tenths of all the species that have ever lived are now extinct then 100 million species of living things have been found on Earth at some time or other.
CLASSIFICATION
The living world would be exceedingly confusing if we were unable to classify this enormous variety of creatures according to some scheme of relationships. One can begin by grouping together the cat, the tiger, the lion, the panther, the leopard, the jaguar, and other catlike animals in the cat family; likewise, the dog, the wolf, the fox, the jackal, and the coyote form a dog family, and so on. On the basis of obvious general criteria, one can go on to classify some animals as meat eaters and others as plant eaters. The ancients also set up general classifications based on habitat and so considered all animals that live in the sea to be fishes and all that fly in the air to be birds. But this standard made the whale a fish and the bat a bird. Actually, in a fundamental sense, the whale and the bat are more like each other than the one is like a fish or the other like
a bird. Both bear live young. Moreover, the whale has air-breathing lungs, rather than the gills of a fish, and the bat has hair instead of the feathers of a bird. Both are classed with the mammals, which give birth to living babies (instead of laying eggs) and feed them on mother’s milk.
One of the earliest attempts to make a systematic classification was that of an Englishman named John Ray (or Wray), who in the seventeenth century classified all the known species of plants (about 18,600), and later the species of animals, according to systems that seemed to him logical. For instance, he divided flowering plants into two main groups, on the basis of whether the seed contained one embryonic leaf or two. The tiny embryonic leaf or pair of leaves had the name cotyledon, from the Greek word for a kind of cup (kotyle), because it lay in a cuplike hollow in the seed. Ray therefore named the two types respectively monocotyledonous and dicotyledonous. The classification (similar, by the way, to one set up 2,000 years earlier by Theophrastus) proved so useful that it is still in effect today. The difference between one embryonic leaf and two in itself is unimportant, but there are a number of important ways in which all monocotyledonous plants differ from all dicotyledonous ones. The difference in the embryonic leaves is just a handy tag which is symptomatic of many general differences. (In the same way, the distinction between feathers and hair is minor in itself but is a handy marker for the vast array of differences that separates birds from mammals.)
Although Ray and others contributed some useful ideas, the real founder of the science of classification, or taxonomy (from a Greek word meaning “arrangement”), was a Swedish botanist best known by his Latinized name of Carolus Linnaeus, who did the job so well that the main features of his scheme still stand today. Linnaeus set forth his system in 1737 in a book entitled Systema Naturae. He grouped species resembling one another into a genus (from a Greek word meaning “race” or “sort”), put related genera in turn into an order, and grouped similar orders in a class. Each species was given a double name, made up of the name of the genus and of the species itself. (This is much like the system in the telephone book, which lists Smith, John; Smith, William; and so on.) Thus the members of the genus of cats are Felis domesticus (the pussycat), Felis leo (the lion), Felis tigris (the tiger), Felis pardus (the leopard), and so on. The genus to which the dog belongs includes Canis familiaris (the dog), Canis lupus (the European gray wolf), Canis occidentalis (the American timber wolf), and so on. The two species of camel are Camelus bactrianus (the Bactrian camel) and Camelus dromedarius (the Arabian camel).
