Riddled With Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are - Tapa blanda

MARLENE ZUK is a professor of biology at the University of California, Riverside, where she studies parasites and behavior in a variety of animals. She is the author of Sexual Selections: What We Can and Can’t Learn about Sex from Animals. She lives in Riverside, California.

MARLENE ZUK is a professor of biology at the University of California, Riverside, where she studies parasites and behavior in a variety of animals. She is the author of Sexual Selections: What We Can and Can’t Learn about Sex from Animals. She lives in Riverside, California.

Chapter 1
 
What do we do when we get sick? The details, of course, depend on the illness, but the immediate and obvious answer is that we try to make ourselves feel better. A cold or bout with the flu usually spurs us to the pharmacy for cough lozenges, aspirin, and decongestants, while an upset stomach means finding a remedy for running to the bathroom. Alleviating the symptoms may not be a permanent cure, but it can’t do any harm, right?
Not necessarily. Although I dislike the analogies between war and disease, it does help to think of symptoms as the sounds, bullets, and urgent messages coming in from the front, with no indication of whether they are from friend or foe. Wiping them out indiscriminately risks destroying our own troops with friendly fire. In some ways, the Calvinists were right—suffering can be good for you, though the more modern application of this idea suggests that it is not the soul but the body itself that benefits from pain. The trick is to figure out when that is the case, and when there is no virtue in misery.
And what about diseases that do not result from infectious agents like bacteria or viruses, but seem to be a flaw in the factory model? Although some aspects of the human body can seem like a miracle of precision in design, our knees and backs give out and childbirth is not exactly a walk in the park. We have wisdom teeth that cause problems because there is no room for them in our jaws. Genetic diseases like cystic fibrosis persist in the population. We scientists always point to the amazing adaptations of organisms, whether it’s the design of our eye or a lowly moth’s wings that mimic leaves so perfectly that they have built-in tatters and stains from bird droppings. If nature does such an incredible job with camouflaging a mere insect, why has she dropped the ball with our vertebrae?
A relatively new field, called evolutionary medicine or Darwinian medicine, takes a radically different perspective on health and illness to answer these questions. First articulated by psychiatrist Randy Nesse and evolutionary biologist George Williams, Darwinian medicine places diseases and defects in an evolutionary framework to make sense of the apparent mismatch between the way our bodies often work and the way we would like them to: Natural selection may not have produced diseases like diabetes or arthritis directly, but it has made bodies vulnerable to them for a variety of reasons.
 
DESIGNED BY A COMMITTEE
Back and knee problems are the source of enormous amounts of pain, and they cost society millions of dollars in treatments ranging from anti-inflammatory drugs to surgery, lost work time, and insurance payments. Lower back pain alone is said to afflict nearly half of many adult populations at one time or another within a given year. It is not confined to industrialized or even modern-day populations. Evidence of arthritis of the vertebrae and joints can be found in prehistoric hunter-gatherer skeletons, and back problems are even common in less technologically advanced populations, although the prolonged sitting of a cubicle centered lifestyle seems to exacerbate the malady. In search of the root cause, many scientists point to the toll that it takes for an originally four-footed mammal to get up and spend all of its time on its hind legs. Other apes are bipedal, or moving on two legs, for short periods, but our way of walking and running exclusively on two feet is ours alone. The evolution of bipedalism, thought to have arisen over 4 million years ago, may have freed humanity’s hands for stone tools, food baskets, and mobile phones, but it also caused a shock to the skeletal system from which we have still not recovered. Anthropologists speculate that our joints would have to be too large for efficient locomotion if they were also stable enough to bear our body weight, resulting in the current compromise of aching backs and knee braces.
Bipedalism also seems to be responsible for the change in pelvic anatomy between humans and our apelike ancestors. The short, broad pelvis of humans supports the torso and the gut, which is a good thing when you are walking upright, but it also made the birth canal narrow, which is not such a good thing when you are trying to have a baby. In addition, brain and head size seem to have increased quite rapidly in humans over the last million years or so, and while much of the growth of the infant brain takes place after birth, the size of the head of a newborn baby is still perilously close to the size of the passage it emerges from, something that causes more complications at delivery in people than in other primates. Childbirth is still a challenge for other mammals, but it is virtually always more straightforward in all animals than it is in humans. Interestingly, humans also seem to differ from most mammals in their apparently universal need for emotional support from others during labor and delivery; birth is unaccompanied in most mammals, but women from many if not all cultures seek out other people during childbirth. Anthropologist Wenda Trevathan suggests that the shift in the presentation of the baby during delivery to exit the birth canal face first, another result of our bipedal skeletons, also led to an increased risk of mortality if births were unattended. Hence, she argues, early humans were safer if birth became a social, or at least a supported, affair, something she suggests modern birthing practices would profit from understanding. Regardless of these travails associated with walking upright, walk upright we do, and Darwinian medicine suggests that problems like complicated births and backaches may simply be part of an evolutionary legacy. Nesse says, “We think the body is designed for health, but it isn’t. Natural selection maximizes reproduction, not survival or health.” This is a key point. Of course a certain level of good health is requisite for functioning, so that the truly enfeebled are at a disadvantage, but as long as an organism can reproduce and pass on its genes, all of its flaws will be passed on as well. François Jacob said that nature is a tinkerer and not an engineer; in other words, bodies got that way not through goal-oriented advance planning but by a series of lurching steps. Certainly a hypothetical mutant with a good back and wider pelvic channel who also had more offspring could prosper, but if possessing such traits imposed too many constraints on locomotion, said creature might never arise, or if it did, wouldn’t make it out of the starting gate.Another problem in producing a perfect body is that evolution can only change things that happen at certain times of life. Say a bird can produce more eggs early on in life, because her body can turn calcium into eggshells more quickly than other birds can. That bird would be blessed with chicks over and above her relatively eggless relations. But what if the same gene that gave her all those eggs while she was young also caused her bones to become more brittle as she aged, resulting in the avian equivalent of osteoporosis, so that she couldn’t fly? Many genes do have such multiple effects, and biologists are extremely interested in the consequences of their actions. Surprisingly, it turns out that as long as a gene’s deleterious effects are postponed past the first peak of reproduction, and as long as that gene gives its bearer a sufficiently large advantage, such genes will be perpetuated in the population. This good-early–bad-late interaction is called negative pleiotropy, and it could be the evolutionary answer to why we grow increasingly decrepit in old age. It also means that imperfection in one body part is a regrettable but inevitable consequence of natural selection acting to perfect another. It’s not that the good die young, it’s that being good while you’re young matters most.
 
GRASSHOPPERS, ASPIRIN, AND THE BEST DEFENSE
Pain and suffering are hallmarks of disease, and while philosophers and poets can wax eloquent about their benefits to the spirit, both physicians and their patients are united in wanting to eliminate them from the body. All forms of suffering are not created equal, however, and Darwinian medicine helps us understand the consequences of assuming that they are. Pain itself is a useful signal, of course. The few individuals born without the ability to feel pain lead very complicated and controlled lives and generally die at an early age. It is easy to understand why; we rely on pain to tell us when to move our hand from a hot stove or how far to bend a joint. But what about disease symptoms that range from annoying, like the itch of a mosquito bite, to debilitating, like the cough of pneumonia? What about the general malaise, the mopiness and lethargy that accompany a wide range of illnesses? Could they, too, serve a useful purpose?
 
Copyright © 2007 by Marlene Zuk
 
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