We are actually worse off than other mammals because traffic control in our throat is further compromised by modifications to facilitate speech. Did you ever watch a horse drinking? It keeps its mouth in the water and drinks without interrupting its breathing. It can do this because the opening from its nasal region can be precisely lined up with the opening into the trachea. The respiratory passage forms a sort of bridge across the digestive passage, so that when the horse swallows, it can make use of space to the left and right of the bridge. Unfortunately for us, our tracheal opening has slipped further back in the throat, so that the bridge connection can no longer be made. At least not for adults; babies, for the first few months of life, can swallow liquids and breathe simultaneously, like many other mammals. Once they start making the babbling that is the precursor of human speech, however, they can no longer drink like horses. The human capacity for choking represents an ancient maladaptive legacy aggravated by a much later compromise.
FIGURE 9-1.
Diagram of respiratory and digestive passages of a larval tunicate, and of the extinct ancestor of all vertebrates, as seen in a horizontal section through the forward end of the body.
FIGURE 9-2.
The lungfish stage of the evolution of respiratory and digestive systems of higher vertebrates, as seen in a vertical section to one side of the midline. The dotted lines show the later shift of the nostril connection to the crossing in the throat, as is found in mammals.
OTHER MALFUNCTIONAL DESIGN FEATURES
Many other serious design flaws make us susceptible to medical problems. Perhaps the most often recognized is the inside-out retina. Vertebrate eyes started as light-sensitive cells under the skin of a minute transparent ancestor. The blood vessels and nerves that served these light-sensitive cells came from the outside, as good a direction as any, for a transparent animal. Now, hundreds of millions of years later, light still must pass through these nerves and blood vessels on the surface of the retina before it reaches the rods and cones that react to the light. The nerve fibers of the retina gather into a bundle, the optic nerve, which must exit the eye to get to the brain. At the hole where the optic nerve exits the retina, there can be no rods and cones. This causes the eye’s blind spot. To demonstrate it, close your left eye and focus your right eye straight ahead at the eraser end of a pencil. Move the pencil to the right without letting the eye follow it. The eraser will disappear at a spot about twenty degrees from the forward line of vision. The left eye is similarly blind twenty degrees to the left of its midline.
FIGURE 9-3.
A. The human eye as it ought to be, with a squid-like retinal orientation.
B. The human eye as it really is, with nerves and vessels traversing the inside of the retina.
The blood vessels on the retina create another problem. They cast shadows that create a network of blind spots on the retina. To overcome this, our eyes move constantly in tiny twitches so that they scan slightly different areas every fraction of a second. This mass of information is processed in the brain, which compiles it into a coherent image. We are deceived into thinking we see something continuously with both eyes when we may only be seeing it intermittently with one. Nevertheless, the shadows, like the blind spot, are always there. To demonstrate this useful self-deception, go into a dark room, press the light end of a penlight against the side of your closed eyelid, turn it on, and gently wiggle it around. When the lineup is exactly right, you will see the shadow of the intricately branching system of parallel veinlets and arterioles that supply the retina.
The inversion of the retina is a universal defect in vertebrates that makes no functional sense. As with the unfortunate intersection between the passages for food and air, the explanation is historical, and it applies only to the vertebrates. The functionally analogous eye of a squid has a more sensibly oriented retina with the nerves and blood vessels coming from behind the retina. The squid eye does not need secondary contrivances to minimize the effect of the design flaw that plagues vertebrates, any more than it need worry about eating interfering with breathing. The squid and other mollusks have their own suites of malfunctional historical legacies.
Our inverted retina is responsible not only for slight visual impairment but also for some special medical problems. Any bleeding or minor obstruction of blood flow in the retina casts a shadow that may seriously impair the visual image. Still more serious is the ease with which the light-gathering surface (rods and cones) can lift loose from the underlying interior of the eyeball. Once this condition of detached retina gets started, it is a dire emergency that, if untreated, can lead to blindness. The more sensibly designed squid eye, by contrast, has its retina anchored securely from below by numerous nerve fibers so that it cannot become detached.
In addition to those flaws, which affect all vertebrates or all mammals, there are some that affect only humans, or only humans and our closest primate relatives. The appendix is an example. People who recover from appendectomies seem to suffer no disadvantage from not having this part of the human body. The only functional significance of the appendix, as far as we know for sure, is to enable us to have appendicitis. The appendix is the vestige of part of the caecum, a digestive organ in our early mammalian ancestors that helped to process plant foods of low nutritional value. For rabbits and many other mammals, the caecum still serves this function. The shift to a diet of foods with more concentrated nutrition, such as fruit and insects, caused the caecum to degenerate in the course of primate evolution because there was no selection to maintain it. Unfortunately, it has not yet entirely disappeared, and the vestige now makes us vulnerable to appendicitis.
So why does the appendix persist at all? It does make a minor—but by no means important—contribution to the immune system. We also wonder if it might, paradoxically, be maintained by appendicitis. The long, thin shape of the appendix makes it vulnerable when inflammation causes swelling that squeezes the artery to the appendix and cuts off its only blood supply. When filled with bacteria, an appendix without a blood supply cannot defend itself. Bacteria grow rapidly and eventually burst the appendix, spreading infection and toxins throughout the abdominal cavity. A bit of inflammation and swelling is less likely to disrupt the blood supply of a large appendix than that of a long, thin one. Natural selection gradually reduces the size of the useless appendix, but any appendix narrower than a certain diameter becomes more vulnerable to appendicitis. Thus, deaths from appendicitis may paradoxically select for a slightly larger appendix, maintaining this less-than-useless trait. Selection is also almost certainly very slowly making the appendix shorter, but in the meantime the appendix may be maintained by the shortsightedness of natural selection. We wonder if other vestigial traits might also be maintained because further diminishing them increases vulnerability to a disease.
Many primates and most other mammals can make their own vitamin C, but we humans cannot. Our ancestral shift to a high-fruit diet, rich in vitamin C, had the incidental consequence about forty million years ago of allowing the degeneration of the biochemical machinery for making this vitamin. Our frugivorous close relatives share our requirement for dietary vitamin C. All animals need particular organic substances (vitamins) in their food, but different groups have different requirements.
Some of our vulnerability to mechanical damage can also be blamed on various past evolutionary developments. A sharp blow to the side of the human head may fracture the skull, damage the brain, and cause death or permanent impairment. The same blow to an ape head may result merely in a bruised temporalis muscle and temporary impairment of chewing. The difference arises from the increased size of the human brain case and shrinkage of the jaw musculature, which incidentally rob the skull of its earlier cushioning. The hard hats construction workers and cyclists wear are a technological fix for a biological deficiency. If workers and cyclists go on being careless about wearing their hard hats, perhaps in another million years we will again have a thick padding of tissue under our scalps to reduc
e brain injuries.
The same increased skull size has resulted in a fetal head that fits through a human pelvis only with difficulty. A woman’s pelvic structure is slightly different from a man’s, so as to provide a large birth passage and, as childbirth approaches, the pubic joint loosens to further facilitate the passage of the infant. Yet childbirth is still more difficult than it would be if the vagina could open outside the massive ring of pelvic bone, perhaps above the pubis on the lower abdomen. The passage of the vagina through the pelvis is a severe historical constraint on the evolution of any further increase in fetal head size. This constraint, of having to fit an oversize head through the pelvic ring of bone, explains why human babies have to be born at such an early and vulnerable stage of development, compared to, for example, ape babies.
The prevalence of maladaptive human design features has been recognized for a long time. A 1941 book by George Estabrooks, Man, The Mechanical Misfit, describes many of the structural defects and compromises in human anatomy, especially those that result from turning a horizontal four-footed animal into an upright two-footed one. The weight of the top part of the body greatly compresses the vertebrae in the lower spine, and standing upright requires more muscular effort than a horizontal posture would. The pelvis was originally designed to resist a back-to-belly force of gravity, not the fore-to-aft force that ours must resist as long as we remain upright, either standing or sitting. Elaine Morgan’s recent book The Scars of Evolution gives a readable account of these maladaptive legacies.
A long list of medical problems, ranging from minor annoyances to serious disabilities, results from the mechanical inadequacies of our adaptations for an upright posture and two-footed locomotion. Perhaps the most important is the episodic lower back pain experienced by so many people. Our knees, ankles, and feet are also extraordinarily vulnerable. How often do we hear of athletes missing games because of knee and ankle injuries? One of the authors once leaped high in a volleyball game, and when he came down only his left foot was on the court. The right landed on the foot of a teammate and turned sharply inward, seriously straining the vulnerable lateral ligament, which is usually the part that fails when an ankle is sprained. The author met his classes on crutches for the next week and was glad he was not part of a band roving over the Paleolithic savannah. He also regretted that the human ankle is not better designed.
The abdominal viscera of a mammal are enclosed in sheets of tissue designed to hang from the upper wall of the abdominal cavity. This is fine for a mammal on all four legs, but in an upright mammal the sheets of tissue may be said to hang from a vertical pole, a grossly ineffective arrangement that causes such diverse problems as digestive system blockages, visceral adhesions, hemorrhoids, and inguinal hernia. The mammalian circulatory system is also compromised by upright posture. It works fine for a dog or a sheep, but our upright posture increases the hydrostatic pressure in the lower extremities and can cause varicose veins and swollen ankles. The opposite effect, deficient blood pressure in the brain, can result in dizziness or momentary partial blackout when we suddenly stand up from a recumbent position.
Sometimes the body’s responses to problems are just the opposite of what would be adaptive. When the heart muscle is too weak to pump all the blood it receives, the blood backs up into the lungs and feet and causes shortness of breath, swollen ankles, and other symptoms of congestive heart failure. You might expect that this would cause excretion of excess fluid, but patients with heart failure retain salt and fluid, and this excess blood volume makes the problem even worse. This response is maladaptive in patients with heart disease, but, as internal medicine physician Jennifer Weil points out, the body’s response is designed for a different problem. In a natural environment, most instances of deficient blood pumping would result from bleeding or dehydration, in which the fluid retention mechanism would be useful indeed! Heart failure occurs mainly in old age and mechanisms to conserve body fluid can be useful throughout life, so this system is a fine example of a cause of senescence which is maintained because of its benefits in youth.
We have been discussing defects in the basic plan of the human body. These should not be confused with mere inadequacies of execution and random departures from optimal values. As a general rule for any readily measured physical feature, it pays to be in the middle of the pack, as we illustrated previously with the birds with longer- or shorter-than-average wings, which were especially likely to be killed in a storm. Unusually tall or short people tend not to live as long or as healthily as those of average height. Babies of average birth weight are usually better off than those who are much heavier or lighter. Everyone knows that high or low blood pressure is not as good as normal blood pressure. A high level of adaptive performance usually requires that many quantitative characteristics closely approach optimal values. While no individual is perfect, the various parameters sometimes combine to yield remarkable excellence. Yet even in near perfection there is substantial variation—as is well known to those basketball stars who played against Michael Jordan.
Many design features, while not maladaptive, are functionally arbitrary and explicable only as historical legacies. In mammals, the right side of the heart circulates the blood to the lungs, the left side to the rest of the body. In birds it is the other way around, for no better reason than that birds and mammals came from different reptilian ancestors that took arbitrarily different routes to cardiac specialization. Either way works equally well. Some arbitrary features can be advantageously exploited. Many people who are alive today would be dead except for the happenstance of everyone having two kidneys. When one fails or is donated, the other is able to do double service. By the same logic, many people die of having only one heart. The reason we have two kidneys and one heart is simply that, right from their origins, all vertebrates had two kidneys and one heart. This is pure historical legacy and has nothing to do with the advantage of having two of one organ or the disadvantage of having only one of another.
We have belabored what is wrong or arbitrary with the human body because the design flaws can cause many medical problems, but we hope that our readers will also realize that much about it is just right. Our oversize brains may be vulnerable to injury and may impede childbirth, but they make us the unchallenged leaders of the animal kingdom in cognitive capability and in all the social and technological advances that this makes possible. No other species in the history of our planet has ever controlled its environment to the extent that we have since the invention of agriculture. Similarly, our longevity is impressive in relation to that of any other mammal, except a few, such as elephants, that are far larger than we are. We can live about half again as long as any other primate.
Moreover, many of our other adaptations are equal or superior to those in other mammals. Our immune system is superb. Also, despite conspicuous design flaws and individual imperfections, our eyes and related brain structures incorporate layer upon layer of information-processing marvels that extract the maximum amount of usefulness from visual stimuli. If hawks, for example, have visual acuity that is in some ways superior to ours, this one kind of superiority must be purchased with some kind of trade-off. Animals that can see better than we can in the dark cannot see as well in the light. Normal human vision approaches a theoretical maximum of sensitivity and discrimination over a wide range of conditions. We are only beginning to understand how it is that a face, seen from one angle at a certain distance, may later, from another angle and distance, be instantly recognized. No current computer can approach such feats. Our hearing is so sensitive to some frequencies that if it were more sensitive we would not hear as well. Informative sounds would be lost in the noise of random air molecules colliding with our eardrums.
THE FINISHING TOUCH
We have been discussing mainly attributes that humans share with other vertebrates, other mammals, or other primates. Our discussions of our problems with upright stature also apply to extinct members of our genus, Homo. We now turn to more expl
icitly human legacies, with an emphasis on the evolutionary adjustments made in the period from about one hundred thousand to about ten thousand years ago. While natural selection has been changing us in many small ways in the last ten thousand years, this is but a moment on the scale of evolutionary time. Our ancestors of ten thousand or perhaps even fifty thousand years ago looked and acted fully human. If we could magically transport babies from that time and rear them in modern families, we could expect them to grow up into perfectly modern lawyers or farmers or athletes or cocaine addicts.
The point of the rest of this chapter, and the following one, is that we are specifically adapted to Stone Age conditions. These conditions ended a few thousand years ago, but evolution has not had time since then to adapt us to a world of dense populations, modern socioeconomic conditions, low levels of physical activity, and the many other novel aspects of modern environments. We are not referring merely to the world of offices, classrooms, and fast-food restaurants. Life on any primitive farm or in any third-world village may also be thoroughly abnormal for people whose bodies were designed for the world of the Stone Age hunter-gatherer.
Even more specifically, we seem to be adapted to the ecological and socioeconomic conditions experienced by tribal societies living in the semiarid habitat characteristic of sub-Saharan Africa. This is most likely where our species originated and lived for tens of thousands of years and where we spent perhaps 90 percent of our history after becoming fully human and recognizable as the species we are today. Prior to that was a far longer period of evolution in Africa in which our ancestors’ skeletal features lead scientists to give them other names, such as Homo erectus and Homo habilis. Yet even these more remote ancestors walked erect and used their hands for making and using tools. We can only guess at many aspects of their biology. Speech capabilities and social organizations are not apparent in stone artifacts and fossil remains, but there is no reason to doubt that their ways of life were rather similar to those of more recent hunter-gatherers.
Why We Get Sick Page 16
