PDF Summary:Why We Get Sick, by Randolph Nesse and George Williams

...

This still doesn’t completely answer the question. If natural selection is so powerful, then why do we still get disease? There are a range of causes:

• Defenses confused as disease
  • Many defenses during disease seem like diseases themselves. A common one is a cough, which is often confused as a cause of disease, when it’s actually a defense mechanism.
  • If you incorrectly believe a defense is a disease and you treat it, you can cause catastrophic damage.
• Infection
  • These are external agents like bacteria and viruses that cause disease. We’ve adapted defenses to them, but they continuously adapt to our defenses. It’s an ever-lasting arms race.
• Novel environments
  • We evolved over millions of years living in small hunter-gatherer tribes on the plains of Africa. Today, we live in a world of constantly available food, cars, dense cities, and artificial lights. Our bodies did not evolve for this environment, and these new environments can cause diseases that were uncommon in our prehistoric past.
• Genes
  • Genes that cause severe damage are selected against by natural selection. Therefore, defective genes with no...

...

Nuances of Natural Selection

Fitness Depends on the Environment

A gene’s contribution to fitness is not determined in isolation. It’s measured in a particular species in a particular environment. Change the environment, and the gene may no longer increase fitness.

For example, imagine a population of rabbits in the forest. Half of them have a gene that makes them more timid at venturing out in the open, which decreases their chance of being eaten by foxes. The other half doesn’t have this gene. The timid half spend more time hiding in their dens, and so they tend to be less well fed than the bolder half.

The environment changes, and a very harsh winter arrives. The timid half stay in their rabbits’ dens, and most of them starve to death. The bolder half venture out and get food; while the bold rabbits are more likely to be eaten by foxes, they also eat more in the winter, and so a greater fraction survives than the timid half. Over a series of harsh winters like this, the timid gene may be eliminated from the population.

Change the environment, and the gene may no longer increase fitness. As we’ll discuss later, this may be the case for a variety of genes...

...

*   Counter-intuitively, giving iron supplements in famines can _increase_ fatal infections, since the body hasn’t had a chance to develop transferrin.

• When a local body part is infected, we feel pain. This reduces the use of damaged tissue that can compromise reconstruction and defensive mechanisms. The pain also causes us to remember the event that caused the infection, and to avoid the same situation in the future.
  • There’s suggestive evidence of the use of pain—a rare genetic disease prevents some people from feeling any pain, including pain from injury or illness. Nearly all of these people die by age 30.
• When sick, we secrete mucus. The mucus traps pathogens, then coughing up mucus dislodges it to be swallowed, so that the digestive system kills pathogens and recycles the protein in mucus.
• During illness, diarrhea causes ingested pathogens to be expelled more quickly.

In all these defense mechanisms, treating the symptom for the sake of comfort might compromise recovery and worsen the illness.

But not all defenses are adaptive or essential, and sometimes there isn’t a need to suffer without reason. The authors aren’t suggesting never relieving...

...

However, the disadvantage of the resistant mutation can itself be mutated away. Therefore, antibiotic resistance can persist in a population, even when no antibiotics are present for a long time.

Why Don’t Parasites Help Us?

Here’s an interesting thought—if parasites use host resources to survive and reproduce, why don’t they help the host thrive and get more resources? The longer the host lives, the more the parasite can reproduce, and the more they can spread offspring to new hosts. This would suggest that the natural evolutionary path of parasites is to gradually become more useful until it becomes indispensable to host survival.

The authors argue this thinking is flawed and ignores two critical points:

• First, this ignores competition between different parasites within the host, and even within the same parasite species.
  • Say that a liver fluke and Shigella are competing for the same resources in the host. The one that most effectively exploits those resources will win.
  • Within the same species, if one parasite selects for cooperation and less virulence, other aggressive members of the species will overtake it in reproduction. The...

...

• Some phobias are learned more readily than others. Monkeys can learn a fear of snakes by watching a video faster than fear of flowers.

Our modern environment has changed a lot in the past thousands of years, but we haven’t yet evolved visceral responses to new environmental dangers.

• We’re closely attuned to the dangers of fire, and we pull our hand quickly from a hot pan on the stove. But we’re not as sensitive to extreme cold—we don’t have the same visceral avoidance of liquid nitrogen and dry ice.
• Light-skinned races are vulnerable to sunburn because they evolved over hundreds of thousands of years in less sunny areas. But now, one race can live in all sorts of environments, and pale people aren’t adapted to sunny areas. They overexpose themselves to sun, having no visceral avoidance of sunlight. Hours later, a sunburn appears and causes pain. But the sunburn is so far removed from the sun exposure that it doesn’t evoke as strong a lesson.

Injury

When repairing an injury, the body needs to make a series of balances:

• Between using scarce resources vs. speed of healing. Faster healing requires taking more resources from the rest of the...

...

*   A local area devoid of herbivorous predators would have plants less likely to be toxic.

Toxins and People

Why We Crave Variety in Diet

In Stone Age times, eating too much of one thing increased the risk of consuming an unhealthy amount of toxins. Eat a diet consisting entirely of wild potatoes, and you might actually be poisoned.

Therefore, we’ve evolved a preference for variety in food. We prefer to eat a buffet of options, rather than stick to a single diet daily. This is yet another example of how a trait was helpful in the Stone Age, but a bad adaptation for today’s food-plenty world.

The Modern Age’s Toxins

Today’s environment likely exposes us to fewer toxins than ever in history. However, the body builds tolerance to toxins upon exposure. Therefore, today we might be the most susceptible to toxins, and the most incapable of handling large toxic insults, than ever before.

At the same time, the modern world has introduced a variety of new toxins to which we haven’t evolved an instinctual avoidance of. This includes heavy metals, antifreeze, cleaning products, and radioactivity.

Cooking Reduces Toxins

**Cooking neutralizes many...

...

• Genetic diseases that occur after reproductive age are less selected against.
  • Huntington’s Disease causes little harm before age 40. By this time, people will have already had children, so there is little selection pressure against the gene.
• Genes may have benefits only in certain environments.
  • In regions with malaria, G6PD deficiency is protective, since it causes the red blood cell to burst when the parasite uses the cell’s oxygen, thus limiting the parasite’s reproduction.
• Some genes may give heterozygotes an advantage, even if homozygous recessives suffer.
  • The sickle-cell gene causes sickle-cell disease when inherited as homozygous recessive. However, in malarial areas, heterozygotes have less severe malaria.

Stone Age Genes in Modern Times

As we’ve learned, a major reason that disease-causing genes still exist is that they may have been beneficial during the Stone Age, but only cause disease in today’s environment.

Consider how vastly different the survival conditions of early humans were, compared to the modern age.

• Mortality was high. In fact, humans reproduced at a rate just enough to balance mortality....

...

Here are examples of genes that confer advantages early in life but cause later disease:

• Hemochromatosis—excess absorption of iron avoids anemia in early life, especially in women and menstruations, but causes disease later.
• Pepsinogen I—a mutation causing excess production increases gastric acid and thus the chance of peptic ulcers, but it may protect against infections.
• Our immune system is age biased—it protects from infection, but it also damages tissues and promotes cancer. It helps us survive in our early days, at the expense of long-term damage.
• Alzheimer’s Disease is absent in primates. It produces abnormalities in recently evolved regions of the brain. Therefore, it’s possible the genes causing it may confer some advantages in intelligence.

Here’s more suggestive evidence that lifespan and fertility trade off with each other:

• Caloric restriction: Mice on caloric restriction extend their lifespans, but they don’t reproduce. They stay suspended in a pre-reproductive state waiting for adequate food supply.
• When beetle populations undergo artificial breeding for earlier reproduction, they show shorter lifespans and earlier...

...

*   **The lesson: some vestigial traits might persist because further diminishing them increases vulnerability to disease**.

• Humans have relatively larger heads than our primate relatives to provide for higher intelligence. However, this complicates childbirth, since the size of the female pelvis limits the size of the skull that can pass through it. Were it not for evolution, we might otherwise design the uterus to be below the pelvis to avoid these problems.
• Becoming bipedal gave humans a host of issues:
  • We get lower back pain from compressive forces.
  • Our knees, ankles, and feet are subject to different forces than quadrupeds and are vulnerable to injury.
  • Our abdominal viscera is designed to hang from the upper wall of the abdominal cavity. This works fine for animals on four legs, but in bipedal humans, this causes problems such as digestive system blockages and hemorrhoids.
• We have two kidneys and one heart, like all vertebrates do, because of evolutionary history.
• If the heart muscle is too weak to pump fully, congestion of excess fluid happens. In these times, we’d wish for the body to excrete excess fluid. However, historically,...

...

• A defense against toxins
  • The responses of allergy seem useful to guard against toxins—shedding tears, mucous secretions, vomiting, diarrhea.
  • This function fits the rapidity and severity of allergies—if exposed to a toxin, you want to get rid of it quickly and violently.
  • This may help explain why people differ in sensitivity to different antigens—people are allergic to the corresponding toxins to which they are especially vulnerable.
  • Like a smoke alarm, the many annoying false negatives make up for the one real emergency that it protects against.
  • It would help to discover the toxins associated with common antigens, such as seafood, pollen, cat hair.
• Allergies are actually incidental responses to bystander molecules.
  • A person with allergies may have had a provoked immune system at some point in her history, say during an infection. At this time, the immune system developed sensitivity to antigens that happened to be around at the same time. In some sense, the body essentially confused the antigens as the cause of disease and kicks off strong reactions upon future exposure.

**Why are allergies becoming more common...

...

Compared to modern women, Stone Age women experienced fewer menstrual cycles for a few reasons:

• They had later menarche or first periods (say, at 15 years old) and earlier menopause (around 47 years old). This was in response to scarce resources and plentiful infections.
• They had more children, and breastfeeding meant many fewer menstrual cycles.

Today’s environment promotes an early menarche and late menopause. Furthermore, women have fewer children, and they breastfeed less. All of this cause modern women to have 2-3x the number of menstrual cycles of Stone Age women.

The underlying mechanism could be that the hormonal responses promoting reproduction cause increased vulnerability to some cancer.

Optimistically, this also provides an angle of attack for finding a way to simulate Stone Age menstruation patterns to reduce cancer risk. For example, blocking menstruation through long-term hormonal birth control might decrease the rate of cancer.

...

Human sperm are much smaller than eggs, which contain most of the resources for the developing embryo. This size disparity arose through natural selection. Consider a situation where eggs and sperm started out being the same size. The sperm that carries fewer nutrients travels faster and wins the race to the egg. To compensate for the lower-nutrient sperm, the eggs that contain more nutrients end up producing more viable offspring. Both these forces continue, with the sperm getting smaller and the egg getting larger, until we arrive at today’s equilibrium.

The size of the egg and sperm have huge consequences on the mating relationship between males and females:

• Sperm enters the female, and the fetus develops in the female over 9 months. This requires a much larger commitment for females than males.
• Females know for sure the child is theirs, while males don’t. The female could only have produced the egg that led to the embryo inside her, but multiple males could have contributed sperm to the female, so no male is completely sure that the child is theirs. (Shortform note: Of course, this is no longer true with today’s genetic testing, but we’re discussing...

...

Let’s cover a range of conditions considered mental illness, question the function of the root emotion, and consider why mental illness might be so common today.

Anxiety

Anxiety causes the fight or flight physiological response that is useful in aiding an escape from danger—a rapid heartbeat, faster breathing, sweating, and an increase in blood glucose.

Anxiety is triggered in the face of danger or an emergency. Justifiable modern day triggers include hearing a gunshot or having a paper due.

But the anxiety-provoking system may be overly sensitive, just like a smoke alarm. A false positive (being anxious when you don’t need to be) isn’t very costly, but a false negative (not being anxious when you should be) can be fatal. As a Stone Age person, imagine you hear a stick break in the forest. It’s better to be overly cautious and assume it’s a tiger, than to have too little anxiety and assume nothing is wrong.

If anxiety is protective, why not be anxious at all times? The stress from anxiety uses extra calories, makes us unable to perform many everyday activities, and damages tissues. Like driving a car to its absolute limit, anxiety is a costly defense mechanism that...

...

• For instance, some patients understand heart disease is proximately caused by high cholesterol, high blood pressure, and diet. They see doctors as admonishing them for avoiding foods they like.
• But if you taught them Darwinian medicine, they may understand why their dietary preferences evolved in a certain way that is maladaptive in today’s world. They may realize how food manufacturers have co-opted this primal eating drive to produce deceptively tasty food. Armed with this knowledge, patients may have more ammunition in their battle against their biology.

If patients understood the evolutionary bases of their disease, they can have satisfying reasons for why the disease exists. This may prevent patients from feeling that disease is meaningless, and it may inspire hope that there are ways to circumvent the disease.