The Seven Mysteries of Life
Page 14
Although specialization commits cells to particular functions and life expectancies, it does not necessarily limit them to fixed locations, as can easily be seen in the travels of blood cells. Moreover, when flesh is bruised or cut, it is now known that millions of deeper-lying body cells (other than blood cells) apparently eagerly migrate upward to replace those lost in the injury, and skin cells detach themselves from all sides to spread individually over the wound's surface, floating upon the fluid film that automatically wets it while strewing it with scarcely perceptible fibrous "guidelines," along which the free cells have been seen to move and coagulate. In a way such semi-imaginary guidelines are like faint cattle trails on a Texas range, over which the cells amble forward about as obediently as steers steered by a proficient herdsman and, like them, almost surely playing a part in some hard-to-fathom overall plan. One can even imagine them enjoying their share in this mystic migration of mercy that is the healing of a wound.
Although muscle cells (like many other specialized ones) cannot replace themselves by dividing, new muscle cells are constantly being assembled from microscopic old fragments of muscle floating about in cell fluid. It is a process curiously like crystallization, in which the flowing bits manage to combine into filaments, which somehow bundle together into fibrils, which by the hundreds align to form fibers of striated muscle which, although more dynamic than any vegetable cells, turn out to look (under the microscope) remarkably like a piece of wood. This crystalline structure evidently is vital to the sophisticated coordination that enables many millions of muscle cells simultaneously to contract as a single muscle, gossamer layers of filaments sliding past each other inside the tiny fibrils, in some cases (as in midge flight) going through the cycle of contraction and relaxation up to 950 times per second, each muscle exactly balanced by its opposing countermuscle for perfect control. Muscles in a real sense are the body's engines, with their filaments pumping back and forth like pistons in the fibril cylinders, fueled internally with ATP generated by the hundreds of mitochondria in every cell. Mitochondria are remarkably like the chloroplasts in leaves (page 49), especially in their alternating layers of protein and fat that synthesize ATP, but of course are powered not directly from sunlight (as are chloroplasts) but indirectly from food (originally vegetable carbohydrates) created through the sunlight. And they resemble chloroplasts also in their ability to move about fairly freely within their cells and to reproduce themselves, behavior suggesting to some biologists that they may have once been independent organisms, which somehow got domesticated into their present specialized symbiotic role.
Nerve cells are also intimately involved in muscle cells, which they control more precisely than your car's distributor controls its engine, and they regulate many other types of cells while bestowing on an organism a continuous awareness through its senses. Fact is: it takes more nerve activity than you probably realize just to maintain human consciousness, for researchers have discovered that the average of billions of nerve cells sends reports to the brain at a rate that can never drop as low as one report per second per cell while the brain's owner is alive and well.
And bone cells, blood cells, germ cells, vegetable cells, etc., all have their own special natures. Skin cells begin to die almost as soon as they are born by stiffening or "crystallizing" into a callus, which may eventually become as hard as wood, bark or horn. Heart cells, even when separated from each other, are still hearty enough to continue faintly beating as individuals, apparently never quite forgetting their chosen purpose. And, if given half a chance, they will catch and cling to their fellows again, incorporating gradually into a hollow ball while synchronizing their rhythms, from there on redeveloping as best they can their full single throb and function as one vital organ in an organism they cannot help (at however primitive a level of instinctual being) feeling part of as long as they live.
Thus does the cell exhibit its mysterious propensity for transcendence from the microcosm into the macrocosm. Even as does life itself transcend (as our Fifth Mystery will explain) in a still more universal sense.
Chapter 4
THE BODY
* * *
HERE LET US TURN the eye of perspective from life's invisible elementary units, the cells, to what cells can assemble and become: a visible body. Which is to say that, having just examined the functioning of these constituent parts, we now want nothing so much as a good look at our corporeal whole, to take its measure and see how it works.
PHYSICAL LIMITS
I was surprised to discover that, among adult humans, some weigh 200 times more than others and that variations in the human species cover as wide a scope as in the dog species, which includes both Chihuahuas and Saint Bernards. In evidence, the generally accepted heaviest human who ever lived was Robert Earl Hughes of Monticello, Illinois, who died in 1958 at the age of thirty-two a few months after being carefully weighed before witnesses at 1069 pounds, which is considered about normal for a six-year-old elephant. While the lightest human on record seems to have been Zuchia Zarate, an emaciated nineteenth-century Mexican midget 26 1/2 inches tall, who weighed 4.7 pounds at the age of seventeen yet managed to survive until she was twenty-six. Literally, she could have stepped out of a balloon a mile in the sky without much risk of injury if she had had an ordinary umbrella in her hand and knew how to open it up as a parachute. The tallest human ever reliably measured was Robert Pershing Wadlow, credited with 8 feet 11 1/2 inches, who died in 1940 when he was twenty-two, and the shortest, a Dutch midget known as Princess Pauline, who was only 23.2 inches tall at the age of nineteen. As for the human capacity for drastic physical change, the record probably goes to an American circus "fat lady" known as Dolly Dimple, who at the age of fifty-eight weighed an almost lethal 555 pounds but, to foil the undertaker, quit her job and in 14 months under close medical care slimmed down to 120 pounds, the while reducing her "vital statistics" from a formidable 84"-84"-79" to a fetching 34"-28"-36".
If extremely large or small people do not seem entirely normal to you, I must point out that total humanity includes a complete spectrum of monsters that are definitely even less normal, though at least forty kinds of them occur often enough to have standardized names. Indeed I hardly think it would be possible to imagine any sort of creature, even one (or is it more?) with multiple or fused heads, extra limbs, tails, horns or other appendages, that is not on record somewhere sometime as having actually been born to some real animal or human parents. And that verifiable fact raises the interesting abstract question of whether any meaningful boundary can ever be drawn between one individual and two or, for that matter, between 0 and 1 or i and 1 1/2 or any other numbers.
It is in the supposedly empty gap between a single whole human and two whole humans, I notice, that most monsters find their logical designation, which may turn out to be anything from a single inoffensive individual with a slight suggestion of an extra person about him (a sixth finger, say, or some small appendage) to two individuals who are barely merged by a narrow peduncle of flesh (the bond connecting Siamese twins), which hardly needs make them unattractive. But midway between these borderline cases there recurrently appear all sorts of genetic nightmares that represent more than one person without ever becoming entirely two - obvious mutations or mistakes in development that express themselves in scrambled multiple features, sometimes repeating heads or eyes sufficient for several people while omitting legs or other parts entirely.
In duration of living also, human beings have a remarkable record, even without swallowing the claims of Methuselahs or accepting centenarians who can't quite find their birth certificates. I mention this hazard, because there has probably been more self-deluded bragging and deliberate fibbing in the allegations of longevity (sometimes for patriotic or political reasons) than in any comparable branch of statistics. In fact scientific researchers, I am told, have discovered a suggestively stable ratio between the numbers of illiterates and of claimed centenarians in the major countries of Earth, which seems
to be due to the tendency of very old people, particularly ones who keep no written records, to pad their ages at a rate that fairly consistently averages 17 years per decade. Notwithstanding all such foibles, humans very likely live longer than any other mammals (including whales, one of whom with identifying markings was sighted in an Australian bay over a period of 90 years), but they certainly have serious warm-blooded rivals among vultures and possibly condors and certain eagles, the only doubt stemming from the fact that no one is known to have kept systematic birth records of animals long enough yet to prove precisely how long the longest-living ones can live. The oldest irrefutably documented age attained by a human being appears to be 113 years 124 days by an eighteenth- and nineteenth-century Quebec bootmaker named Pierre Joubert; and the rarity of verified centenarians in general is suggested by the failure of the most reliably pedigreed large group of people anywhere, the British peerage, to produce a single 100-year-old peer in ten centuries - until Lord Penrhyn "proved the rule" by dying in Towcester, England, on February 4, 1967, at the ige of 101.
Animal longevity seems to vary roughly in direct proportion to body size and in inverse proportion to reproductive capacity, with cold-blooded creatures usually outliving warm-blooded ones, but there are so many deviations from these rules that zoologists have hardly begun to explain the phenomenon. Cats live longer than dogs, for example (known limits being 35 for cats to 27 for dogs), while zebras outlive lions in zoos (38 to 35), but presumably not in the wild state where they serve as lion food. In accord with the fact that a 580-pound giant clam has been estimated (from growth layers) to have lived about 100 years, you might think animals with the fast metabolism of birds would burn themselves out and die young, but something about the flying life must give them durability, for birds live much longer than most other animals their size, including cold-blooded ones, with the exception of a few insects (queen ants have lived 19 years, cicadas regularly 17), many small reptiles (30 years or more) and toads (up to 54 years). Even bats live three or four times as long as mice their size. But it is the large tortoises that evidently hold the record for age among animals, the oldest documented patriarch of all being Tu'imalila, the tortoise reported to have been given to the Queen of Tonga by Captain James Cook in 1772, which died in Tonga in 1966, probably after living more than 200 years.
Trees, on the other hand, usually outlive animals, and some of them last more than 20 times as long, evidenced by the ring count of a recently cut bristlecone pine in the White Mountains of California that proved it had lived over 4600 years, having started from a seed just before the pyramids of Egypt were begun. Of course if one regards the resproutings from old stumps or roots as a continuation of the life of the same organism, vegetables can be considered practically immortal. And this is even more true of most animal and vegetable cells when you count the twin offspring from a divided cell as continuations of the old cell that formed them. By the same reasoning, a strain of bacteria recently revived from a Permian limestone formation in Germany is probably 250 million years old, while similar pre-Cambrian cells, if they prove viable, could have lived two billion or more years, roughly the age of such a star cluster as the Pleiades, and evidently only possible because (on Earth at least) there seems to be no longrange aging process in the successive generations of cells.
If bodies are far exceeded in longevity by the cells that compose them, the bodies can take heart in the fact that they are now gaining relatively. The life expectancy of prehistoric man is estimated to have been only 18 years, but under the civilizing influence of ancient Rome it rose to 22 years, in England in the Middle Ages to 33, in America in 1900 to 47. While India and other poor countries are still floundering under a life expectancy in the 30s, western Europe, Japan, Australia and the United States by now have raised theirs to 70 years, mostly by improving the art and science of resisting disease. Even dogs in America have doubled their expectancy (from 7 years in 1930 to about 14 now) at a rate faster than man's, theirs having been comparatively much lower when man finally acquired an easy means of helping them.
BONES AND MUSCLES
So far medicine has discovered little about how to control the seemingly inexorable process of aging and biologists do not even agree on its definition. But just as metal is said to "fatigue" while its molecules gradually collect in knots, so does a similar coagulation transform bone, muscle and flesh as the years go by. Body cells tend to lump together, while dying off faster than they can be replaced. And, after the age of about thirty, water and reserves noticeably decrease, bodily efficiency declines roughly one percent a year, tissue wastes away, enzymes disappear, mutations damage genes and organs wear out, one after another. Typically, in the body of an eighty-year-old man 50 million of whose cells are dying off each second, while perhaps only 30 million new ones replace them, muscle has lost 30 percent of its former weight, the brain has shriveled 10 percent, nerve trunks have shed 25 percent of their fibers, each breath uses 50 percent less air, each heartbeat pumps 35 percent less blood, the blood absorbs oxygen 6o percent more slowly, and the kidneys (curiously like loyal members of a team) have sacrificed their efficiency by half to help other organs worse off than themselves.
Yet the fact that a body will continue functioning through all such adjustment speaks for the wonder of this corporation of 100 organs, 200 bones, 600 muscles, trillions of cells and octillions of atoms that are physically you, a human cosmopolis, a mysterious going concern that is so adaptable that a contortionist recently squeezed himself into a 19" X 19" X 19" box, and an emergency specialist on the New York Police Force commented that "the average human body can be compressed into a space 4 to 6 inches wide without serious harm." And I'd say the roots of this resilience permeate the body's microcosmic structure with such elegant perfection that no one could critically examine it without a respect amounting to awe.
Consider collagen, the little-known main ingredient of animal protein. It is the tough, fibrous connective tissue of body parts as diverse as skin, ligament, tendon, gristle and bone, and it accounts for 40 percent of all human protein, including the webby material between muscle fibers and around cells in many organs. It has the texture of leather and the guts of glue. In a tendon it has the tensile strength of a light steel cable. In the eye's cornea it is stacked like plywood and appears as transparent as glass. In bone its fibrils are arranged like bridge girders for maximum resistance to expectable stress. Its basic molecule is a left-handed polypeptide helix and three such helices are twisted around each other to form a right-handed superhelix, whose three parts are evidently held together by hydrogen bonds while their wholes lock into various larger crystalline patterns according to their function. Among other qualities, collagen can adapt to changing requirements while being dissolved and recrystallized, a capacity to transform that makes it particularly effective in healing wounds. However it increases its cross-links with age, presumably through normal molecular movement, and the result. is obvious in the stiffening of joints, the hardening of arteries and the cracking of leathery skin with advancing years.
The only thing that may hold the body together more than collagen is the skeleton with its muscular covering. The skeleton of course is a living engineering structure: the pelvis an arch into which the lower spine wedges like a keystone, while the vertebral column stands above it, supported by muscles as a ship's mast is secured by shrouds, the foot a small cantilever bridge, the knee a pulley wheel on a crane that swivels in the ball-and-socket joint of the hip, the elbow and knuckles simple hinges...
At first thought bones seem not really alive, but this illusion is quickly dispelled by a microscopic study of their growth, which turns out to be a highly organized activity comparable to the building of a subway in a bustling city without disturbing it. All the time a young bone is lengthening, it is sealed at its ends by disks of collagen-reinforced cartilage that correspond to the shields behind which men drill a tunnel. The disks grow steadily ahead at their front surfaces, while cells bearing such
minerals as calcium, phosphate and carbonate arrive in a continuous stream behind them to convert their rear parts into bone, almost as if men were coming up with concrete to replace the dirt excavated by the tunnelers. The construction is in no way crude, but follows microscopic specifications in a plan of exquisite design, for not only is the cartilage composed of parallel columns of cells in a kind of granulated honeycomb, but the much harder bone turns out to be a hydroxyapatite crystal of still finer grain whose mosaic of tiles is laid in a carbonate and citrate mortar with collagen for a binder. And while the bone grows in length it also grows in girth, accomplishing this through the combination of bone erosion (by wrecker cells inside the marrow cavity) and bone building (by mason cells on the outside), the latter accretion serving also as a kind of mineral bank in which reserve calcium, phosphorus, trace metals, etc., are conveniently stored for the body's future need.
Muscle seems so different from bone that it is hard to believe at first that it too is basically crystalline. Yet recent work at the molecular level has established the fact beyond question. The common muscles that men and animals use to move their limbs and bodies at will are called striated, because under magnification they show cross stripes at right angles to the fibers that run the length of them, like the grain in wood, also crystalline in form. Indeed these cross stripes exist because the fibers are not only perfectly parallel to each other but all their parts are exactly matched crosswise, giving the whole a carpetlike warp-and-woof texture that is vital to its function. If you doubt this, you should see a muscle fiber under a series of microscopes. First, in low magnification, it looks like one of hundreds of silken violin strings that are bundled together while stretching from the tendon at one end of the muscle to the tendon at the other several inches away, and it is usually less than one thousandth of an inch in diameter. Then, under higher magnification, you see that the fiber is made up of a thousand or so smaller and parallel fibrils, generally aligned into striped ribbons. Finally, under still higher magnification, each fibril turns out to have a grain of hundreds of even tinier parallel filaments of two distinct kinds, one of them twice as thick as the other, arranged in a beautiful hexagonal pattern reminiscent of cartilage and other honeycomb forms. But the special thing about the filaments in muscle fibrils is that the thick ones (made of long, golf-club-shaped myosin molecules) are chemically very different from the thin ones (made of chains of round, golf-ball-like actin molecules) and the two kinds, lying alternately side by side, can be made to react powerfully upon each other in such a way that they are forced to slide past each other.