The mind’s emotional abilities are equally astounding. They connect us to our partners, flooding us with love when with them, longing when they are away, sympathy when they suffer, and grief when they die. When they betray us, the mind fires us with rage. When we betray them, it racks us with guilt and motivates reparations. The mind works day and night, planning, ruminating, fantasizing, dreaming. Was Joe’s apparently friendly josh really a subtle insult? Could I really have great sex with . . . Who was that in my dream? The mind is the most extraordinary device that we know of in the universe.
The mind’s vulnerabilities are as extraordinary as its abilities. It goes awry so often in so many ways that any hosannas for the designer would soon be transformed into fury and lawsuits. Some failures occur early. Just when love is cementing emotional bonds with parents, some children pull back into an autistic shell, never to reemerge. Some three-year-olds learn the word “No” and never look back, defying every parental instruction. Most parents make extraordinary sacrifices for their children’s welfare, but a few lock their children in closets, hold their hands over gas burners, or force them to perform sexual acts. Such experiences are awful beyond measure, but why, thirty years later, do they often still have more influence than all subsequent events?
The elementary school years offer a respite. Energies go mainly into growth and learning; conflicts and new onset mental disorders are rare. Then puberty jolts the mind/brain with the force of a fist coming down hard on a laptop keyboard. Social sensitivity blossoms in sync with acne that amplifies it. For some children, social fears make dating difficult, and assignments to speak in class cause nightmares and school dropout.
Other minds ruminate on an infinite sequence of dire “What if?” scenarios. What if I come home from school and my parents have moved? What if I catch HIV from a toilet seat? Some have the opposite problem, anxiety deficits that cause risky behavior, including adventures with alcohol and drugs that result in addiction. Some addicts can stop completely. The lives of others circle around drugs or alcohol like moths around a flame, in ever narrower spirals down to death. Some young people, mostly women, go on diets that spin out of control; they see rolls of fat where others see protruding ribs. Others, mostly men, cannot understand how others get sexually aroused by people; they are turned on only by shiny black rubber.
Six Reasons Why Natural Selection Left Us Vulnerable to Disease
If the mind had been designed, we could ask if its flaws resulted from incompetence, carelessness, or malevolence. But the mind is not a machine. There was no designer. There was never a plan. There are no blueprints for the brain. There is not even one exactly normal version. Like every other part of the body, the brain was shaped by natural selection. Genetic variations among our ancestors caused differences in brains that caused variations in behavior that influenced how many children they had. The result is brains that have extraordinary capabilities—and many vulnerabilities.
The annual Italian science festival Festa di Scienza e Filosofia in Spoleto is a cultural gem that glitters each July in a small town in Umbria. The theme of the 1998 festival was evolutionary medicine. As the applause died down after my talk on evolution and mental disorders, I stepped off the podium to see another speaker approaching, the famous biologist Stephen Jay Gould. He was notoriously critical of evolutionary applications to human behavior, so I was apprehensive. When he said, “Nice talk, Randy,” I was elated, but he quickly went on, “Of course, they had no idea what you were talking about.” I protested, and he explained, “Most people have no idea about how natural selection works, and most of the ideas they do have are wrong. There is no point in talking about how evolution applies to something like mental disorders without first explaining evolution.” Then, in the most engaging talk at the festival, he did just that. Lesson learned. Following Gould’s advice, here are a few fundamentals that are crucial to our enterprise.
Do you put random coins from your pocket into a jar? If you do, the jar is soon a mélange of copper and silver. As you take out mostly silver coins, the glint of silver coins fades until it is hardly worth fishing for them in the sea of copper. The principle of selection explains the change in the jar. Natural selection is the same process occurring in living organisms over the generations. If genetic variations influence the number of offspring who survive to reproduce, then a species will change over the generations, and the average individual will become more and more like those who had the most offspring. This is not a theory; it is a deduction that must be true if the assumptions are satisfied.
Natural selection shapes traits that work well. The beaks and tongues of individual woodpeckers vary slightly. Those that extract insects more efficiently from trees get more food and fledge more chicks. This process shaped sharp-edged beaks that slice swiftly through wood and long barbed tongues that extract wriggling insects. Dogs provide a more familiar example. Choices people made about which individuals to feed and, more recently, to breed have, in just a few thousand years, shaped dogs that are remarkably adept at herding sheep, retrieving birds, digging out rodents, attacking intruders, and cuddling on laps while looking adorable.
Some behaviors that seem stupid turn out to be smart. In a lunch conversation, a neurosurgeon said he didn’t think animals behaved adaptively at all based on his recent observation of seagulls feasting on scores of turtles that had all hatched at once on a Florida beach. However, hatching en masse gives at least some turtles a chance of making it to the ocean safely, the same way that a military charge is more likely to get some soldiers to an enemy line than if soldiers marched forward one by one.
Selection shapes brains that maximize the number of offspring who survive to reproduce themselves. This is very different from maximizing health or longevity. It is also different from maximizing matings. That is why organisms do things other than having sex. Especially humans. Having the most offspring requires allocating plenty of thought and action to getting resources other than mates and matings, especially social resources, such as friends and status. Everyone else is doing the same thing, creating constant conflict, cooperation, and vast social complexity whose comprehension requires a huge brain.2
While the principle of natural selection is simple, its process and products are unimaginably complex. Genes interact with one another and environments to create bodies and minds that maximize Darwinian fitness, but this is not as simple as it sounds. Individuals sometimes make drastic sacrifices that benefit others. When honeybee drones sting, they die, giving their lives to protect the hive. The mystery preoccupied the genius British biologist William Hamilton. He finally recognized, in 1964, that genetic variations that decrease an individual’s survival and reproduction can nonetheless become more common if they benefit relatives who have some of the same genes.3 His discovery was presaged in the biologist J. B. S. Haldane’s succinct reply to the question “Would you sacrifice your life for your brother?” “No,” he said, “but I would for two brothers—or eight cousins.” Genes that induce individual animals to help relatives can nonetheless become more common over the generations if they give sufficient benefits to the relative compared to the costs to the actor.
Hamilton proposed a simple formula that transformed the study of behavior: C
Within a year of Hamilton’s discovery of kin selection, and not knowin
g about it, George Williams wrote a slim book entitled Adaptation and Natural Selection.8 Before its publication, biologists routinely assumed that selection acted for the good of groups and species. Williams explained why this is a mistake. Biology has not been the same since.
The idea that selection works to benefit groups was illustrated in the 1958 Walt Disney film White Wilderness. It showed scores of lemmings jumping into a fjord, as a mellifluous narrator explains that the self-sacrifice by some is necessary to ensure that there is enough food that the species can survive. A 1962 book by the zoologist V. C. Wynne-Edwards described examples of animals that stopped breeding when food supplies were short, to support his thesis that such tendencies evolve to prevent the demise of the entire group.9
Williams pointed out that this makes no sense. Genetic variations that induced an individual to stop breeding would be selected out, even if they benefited the group, even if they could save the species from extinction. Individuals that stop breeding for the good of the group will have fewer offspring than those that carry on, so such sacrifices must have other explanations. As for the lemmings, the Disney film crew could not find any lemmings jumping into fjords. So they bought brooms, paid locals to trap lemmings, then secretly but literally swept them into the sea.10
Evolutionary theory was transformed by the recognition that group selection is weak and that kin selection offers a powerful explanation for altruistic behavior. These are only two of many evolutionary reasons why natural selection didn’t do a better job of making organisms resistant to disease. Traits that make us vulnerable are legion. Why do we have an appendix? Why do we have wisdom teeth? Why is the birth canal narrow? Why are the coronary arteries prone to blockage? Why are so many people nearsighted? Why haven’t we evolved immunity to influenza? Why is there menopause? Why does breast cancer occur in one woman out of eleven? Why are so many of us obese? Why are mood and anxiety disorders so common? Why do genes for schizophrenia persist? Every trait or gene that makes an organism vulnerable to disease poses an evolutionary mystery.
The old answer was that there are limits to what natural selection can do—for instance, eliminating all mutations. That is one important kind of explanation, but the central insight of evolutionary medicine is that there are also at least five other evolutionary reasons why we are vulnerable to diseases.11,12,13,14,15 Evolution explains not only why the body works so well but also why some parts are prone to failure. Brief examples provide illustrations for disease in general, as well as mental disorders.
SIX EVOLUTIONARY REASONS WHY BODIES/MINDS ARE VULNERABLE TO DISEASE
Mismatch: our bodies are unprepared to cope with modern environments.
Infection: bacteria and viruses evolve faster than we do.
Constraints: there are some things that natural selection just can’t do.
Trade-offs: everything in the body has advantages and disadvantages.
Reproduction: natural selection maximizes reproduction, not health.
Defensive responses: responses such as pain and anxiety are useful in the face of threats.
1. Mismatch
Most of the chronic diseases that plague us now result from living in modern environments.16,17,18,19 This does not mean we would be better off in the environments of our ancestors. Life back then was worse than just nasty, brutal, and short. Imagine having an infected impacted wisdom tooth in a time without dentists. Even minor infected wounds caused death or the slow loss of a limb. The standard treatment, pouring boiling oil on the wound, was only sometimes effective. When steel tools made amputations possible they were swift, because there was no anesthesia. Pregnancy with a large baby meant an agonizing death. Then there was simple starvation. We are far healthier than our ancestors were.
Many of our current health problems result, nonetheless, from the environments we have created to satisfy our desires.20,21,22,23,24 Most people in developed societies live better now physically than the kings and queens of just a century ago. We have a surfeit of delicious food, protection from the elements, time for leisure, and relief from pain. These accomplishments are spectacular, but they also cause most chronic disease.
If you get a chance to join a doctor making hospital rounds, ask which patients would be there if they had been living in an ancestral environment. Those with cancer and heart and lung diseases caused by smoking would not be there; neither would those whose diseases are caused by alcohol or drugs. Most patients with diabetes, high blood pressure, coronary artery disease, and obesity-related diseases would not be there.25 The majority of breast cancer patients would never have gotten cancer.26,27 There would be few if any patients with multiple sclerosis, asthma, Crohn’s disease, ulcerative colitis, and other autoimmune diseases that have become epidemic only in recent years.28,29
The greatest boon of modern life is also the greatest villain: the availability of plentiful food.30,31,32,33,34,35 Or, rather, foodlike substances manufacturers concoct with the exact combinations of sugar, salt, and fat that we most desire. Those desires were helpful on the African savanna, where sugar, salt, and fat were scarce; now our preferences make us obese and ill. Addiction to tobacco was not much of a problem until the breeding of milder strains and the invention of cigarette papers; now smoking causes a third of all cancers and much heart disease. Fermented beverages were sometimes available, but now readily available beer, wine, and spirits cause alcoholism worldwide. Advances in chemistry and transport make concentrated drugs such as heroin and amphetamine available everywhere; in combination with novel means of administration such as needles, they cause massive modern epidemics.36,37,38,39,40
Better nutrition makes children mature faster; many women now start menstruating at ages eleven or twelve, half a decade before their bodies and minds are fully prepared for pregnancy, to say nothing of being prepared to care for infants.41 More subtle aspects of our environment also increase the burden of disease. Exposure to light at night blocks the normal release of melatonin and increases cancer rates.42 Birth control results in modern women having four times as many menstrual cycles, and proportionally higher hormonal exposure and cancer rates, than women living in ancestral environments.43
Living in modern environments explains the prevalence of some mental disorders. Substance abuse, eating disorders, and attention disorders are problems mostly in modernized societies. Depression and anxiety disorders are often blamed on modern life, but their prevalence in earlier times remains unclear. Schizophrenia and obsessive-compulsive disorder do not seem to be much more common now. Mismatch is the first of six reasons why we are vulnerable to disease. It is an important explanation for some mental disorders.
2. Infection
When most people think about disease, they think about infection. It is a wonderfully simple schema. When germs get into the body and grow, they cause disease. Doctors prescribe antibiotics to kill them. The reality is far more complex, interesting, and discouraging.
A human generation is about twenty-five years. A generation for bacteria is just a few hours—about 30,000 times faster. From this perspective, it is amazing that large, slow-evolving organisms such as humans have survived. Life on a tiny scale existed on earth for 3 billion years before anything bigger evolved. We may well discover other planets where larger organisms never evolved because they were soon consumed by smaller organisms that evolved much faster.
The threat of antibiotic resistance is now familiar. Those few bacteria that survive exposure to an antibiotic soon take over. This is an ordinary evolutionary process, but interestingly, medical journals rarely use the “e-word.” Instead they use euphemisms such as “emerge” or “arise” or “spread.”44 This circumlocution matters. Well-meaning would-be evolutionary doctors have sometimes tried to prevent antibiotic resistance in their hospitals by all agreeing to use the same antibiotic as their first choice, shifting to a new one every few months. While intuitively attr
active, sequential exposure to different agents may speed the evolution of multiple drug resistance.45 Many doctors also tell their patients that they should take every pill in the bottle of antibiotics to prevent resistance; however, recent studies show that if pneumonia is already under control, taking an antibiotic longer increases selection for resistant strains without shortening a person’s illness.46,47 The lack of evolutionary knowledge in the medical profession harms health.
Bacteria and hosts coevolve; every time the host evolves new defenses, the pathogen evolves ways to get around them. Streptococci, the bacteria that cause strep throat, disguise themselves as human cells.48 So the antibodies our immune system creates to attack them are likely to damage our own cells. Damage to the kidney causes glomerular nephritis. Damage to joints and heart valves causes rheumatic fever. Damage to neurons in a part of the brain called the basal ganglia causes abnormal movements called Sydenham’s chorea and some cases of obsessive-compulsive disorder.49
Sometimes hosts and bacteria help each other. The old idea that bacteria are usually bad is being replaced by an evolutionary view of a complex microbiome that is essential to health. Disruptions of the microbiome are strongly implicated in our modern epidemic of obesity as well as autoimmune diseases such as multiple sclerosis, type 1 diabetes, and Crohn’s disease.50,51,52 Something about modern environments is causing excesses of inflammation that cause these diseases and atherosclerosis. Is antibiotic disruption of our microbiomes responsible?53 If so, our ability to avoid and kill bacteria comes at a high price.
Good Reasons for Bad Feelings Page 5
