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After millions of years of evolution, why do our bodies still seem so flawed? Why do we get disease at all? Why hasn’t natural selection prevented heart attacks, nearsightedness, and Alzheimer’s disease?

The common answer is that “natural selection isn’t powerful enough to get rid of disease.” This is usually the wrong idea.

Instead, it’s important to realize that the body is a bundle of careful compromises. The reason we have diseases today is that the very things that cause disease were helpful for us at one point, or are still helpful for us in certain situations. In this way, evolutionary medicine seeks to understand why we have disease.

Here are four principles of evolutionary medicine.

1) Natural selection selects for reproductive fitness.

Adaptations that net promote reproductive success are selected for, even if they cause disease after the organism reproduces.

In other words, anything that kills or debilitates you after you already raise kids to independence is not strongly selected against. Further, genes that increase your lifetime reproduction will be selected for, even if they reduce your longevity or ‘happiness.’

In an extreme example, Huntington’s Disease is an intimidating disease—patients die between ages 40-60, and it’s autosomal dominant. But because it causes no apparent harm before age 40, all their kids are born, so the disease faces little selection pressure.

Genes that promote eating and fat storage may increase survival and thus reproductive fitness, even at the expense of later heart disease.

2) Behaviors that seem entirely harmful may have unobvious benefits that improve fitness.

Traits that give an overall fitness advantage, despite increasing vulnerability to some disease, can still be selected for.

  • Physical pain is useful—it signals indicating injury, causes you to withdraw from the injuring agent, and stores painful memories to avoid the behavior that caused the injury.
  • Morning sickness in pregnancy makes the mother avoid risky foods that might contain toxins.This protects the early fetus from toxins when it is most vulnerable. Then, when the fetus has grown and is less susceptible to toxins, morning sickness subsides.
  • Sickle cell allele protects against malaria.

3) Adaptations that might have been helpful in the Stone Age are maladaptive in the modern environment.

The modern environment is very different from the environment in which humans evolved over millions of years. Genes that were once helpful may cause disease today. Examples:

  • Genes that make you engorge on foods helped humans survive famine periods. But in today’s food-abundant world, this leads to nutritional excess and obesity.
  • Energy conservation and laziness helped us avoid wasteful activity, but today they get us to sit for most of the day.
  • Alcohol addiction was not really a problem until distillation made high-concentration alcohol, and mass markets made alcohol readily available. The trait for alcoholism may have beneficial effects, such as the persistence to seek rewards despite obstacles.
  • Nearsightedness—even though 20% of humans are near-sighted, this might not have been a problem in the Stone Age. It only appeared humans starting doing close reading.
  • Sadness has the function of stopping you from wasting energy on unlikely goals. For instance, during the Stone Age, you might have received feedback that you’re no good at hunting. This would make you sad and reconsider whether you might do better as a gatherer.
    • In the Stone Age, we happily compared ourselves within a tribe of 100. Within this small group of people, you were likely good at something and valued for what you did. This made your life feel purposeful.
    • Now, with the advent of mass media, we compare ourselves to the best of 7 billion other people. You find it harder to be particularly good at anything. Compared to sports stars, beautiful models, and dashing entrepreneurs, you may feel average in nearly all respects, and you see your family and friends as similarly inadequate.

4) Lack of features on our “body wishlist” often stem from tradeoffs we’re not aware of.

When thinking about human health, it’s tempting to wish we had near-superpowers, like immortality or the ability to regenerate lost limbs. However, the body is a careful set of compromises. The body needs to balance functions like reproduction, survival, recovery from damage, energy efficiency, growth, and susceptibility to disease.

Why don’t we regenerate limbs? This is a balance of utility vs. maintenance cost. Natural selection has apparently shown us that this type of repair capability is net negative.

For much of human history, losing a limb was likely fatal—a Stone Age man who lost an arm would bleed to death in minutes. If the chance of survival in such a case was low, then there was little point in having the machinery to regenerate limbs. If everyone who had limbs amputated died, then the gene to regenerate arms could not be selected for.

Furthermore, the maintenance costs include not just the energy expended in maintaining the machinery to regenerate limbs, but also an increased rate of cancer. It’s dangerous to let mature, specialized tissue have more than the minimum needed capacity to repair likely injuries.

Why Do We Age?

Senescence has been stable over time. Over the past centuries, the average human lifespan has increased, but the maximum lifespan has not. Despite all our medical advances, humans cannot live past about 115 years.

Theoretically it would be a huge reproductive advantage to maintain health for more time - imagine humans who lived to be 300 and reproduced for 100 years. Why haven’t humans evolved to live longer?

Per evolutionary medicine principles, there must be a competitive equilibrium at play - living longer must confer some compensatory fitness disadvantage, and the inverse is true. The balance is between faster, more aggressive mating (which may necessarily cause decreased longevity) vs. longer lifespan (which may necessarily trade off with decreased fertility).

Animal experiments show that increasing lifespan causes lower and later reproduction. Somehow there is a tradeoff between longevity and vigor. For example, 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.

Sex and Reproduction

In many animals, the male produces sperm and the female produces eggs. This contrasts with hermaphrodite animals, which can produce both sex cells within one organism.

The small size of sperm and large size of eggs make it easier to get sperm inside females, rather than the opposite. It would be much more difficult for a woman to transmit her egg into a male.

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 the history of natural selection here.)
  • Males expend little resources when creating a child. Theoretically, a male can hundreds of offspring in a lifetime, while females can have only 5-6.
  • Males compete among each other to have the chance to inseminate the female. Females have the ability to be selective about which males to mate with. Therefore, males compete to show their genetic prowess through feats of strength (such as male elk fighting with their antlers) and showmanship (such as peacock feathers).
  • The father can’t...

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Why We Get Sick Summary 1: Why Haven’t We Evolved Disease Away?

After millions of years of evolution, why do our bodies still seem so flawed? Why do we get disease at all? Why hasn’t natural selection prevented heart attacks, nearsightedness, and Alzheimer’s disease?

The common answer is that “natural selection isn’t powerful enough to get rid of disease.” This is usually the wrong idea.

Instead, it’s important to realize that the body is a bundle of careful compromises. The reason we have diseases today is that the very things that cause disease were helpful for us at one point, or are still helpful for us in certain situations. In this way, evolutionary medicine seeks to understand why we have disease.

In thinking about disease, it’s important to distinguish between proximate causes and evolutionary causes of disease.

  • Proximate questions ask “what caused the disease? How did the disease start and progress?” For heart attacks, the proximate cause is atherosclerosis, and its associated causes like high blood pressure and cholesterol.
  • Evolutionary questions ask, “why hasn’t natural selection eliminated the genes that lead to heart disease? Why hasn’t it eliminated our craving for fatty foods and for overeating?”

In Darwinian medicine, evolutionary explanations of disease require explaining more than just the current function - they should explain aspects of the evolutionary adaptation that leads to the disease:

  1. how the adaptation gives an advantage
  2. why lacking this adaptation causes a disadvantage
  3. what was gradually shaped by natural selection to arrive at the current form

Evolutionary medicine is not just philosophizing. It’s also useful in predicting what to expect when treating disease. For example, infections usually cause low iron levels. The proximate viewpoint would suggest that giving iron as a treatment is an obvious choice. However, the evolutionary viewpoint would question, “is lower iron perhaps a defense against infection? Perhaps the body has adapted to lower iron during an infection, because it helps fight the infection? If this is true, wouldn’t giving iron...

Why We Get Sick Summary 2: How Natural Selection Works

Natural selection is often thought of informally as “survival of the fittest,” but this is a confusing phrase. People often have the wrong idea of what “fitness” means. It doesn’t mean fitness in the everyday sense of good health and long life.

In Darwinian terms, a higher fitness means more reproduction and a greater number of viable offspring. If a gene leads to production of more viable offspring in future generations, that gene will be enriched in the population. Likewise, if a gene codes for characteristics that result in fewer viable offspring in future generations, that gene is gradually eliminated.

Taking the phrase “survival of the fittest” again, we now see that “survival” is important for natural selection only insofar as it increases reproduction. If a gene helps an individual survive longer, but at the expense of having fewer offspring, that gene will show up less often in the gene pool.

Genes that increase lifetime reproduction will be selected for, even if they reduce the individual’s longevity.

This is one reason that we haven’t evolved away common diseases—the genes that cause gout and dementia later in life may actually increase our reproductive fitness earlier in life.

Look for Alternative Explanations

A key takeaway: when we find something that seems like an error in natural selection, more likely we are missing some important function that compensates for the deficit.

There’s a parable of Henry Ford when he asked a car engineer, “Is there anything that never goes wrong with any of these cars?” The engineer responds, “Yes, the steering column never fails.” Ford responds, “Redesign it. If it never breaks, we must be spending too much on it.”

Likewise, the body has evolved to be a set of compromises. Some traits might increase vulnerability to some diseases, but still give an overall fitness advantage.

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....

Why We Get Sick Summary 3: Infectious Disease

The war with bacterial and viral pathogens has gone on for millions of years and continues today. Our body has evolved defenses to combat infections, and in turn the pathogens evolve ways to overcome these defenses.

As previously described, the body is a collection of compromises. Maintaining the defenses at all times would be too costly—there’s no need to raise defenses when there are no pathogens around.

When the defenses do activate, they sometimes cause symptoms that appear to be the disease. In reality, the symptoms are the defense mechanisms at work. If this hypothesis is true, then treating these symptoms can counter-intuitively aggravate and lengthen the infection. Here are a few examples:

  • Fever increases effectiveness of the immune system. While it might superficially seem like the pathogen is causing the fever directly as a result of its havoc, in actuality the fever is our defense against infection.
    • In fact, syphilis was once treated by infecting the patient with malaria, which would induce a fever. This discovery won the 1927 Nobel Prize in Medicine!
    • Why don’t we have a fever all the time? Because fever depletes nutrient reserves 20% faster, causes temporary male sterility, and it may cause tissue damage.
  • During infections, the body depletes iron to limit bacterial reproduction. Bacteria need iron to reproduce.
    • The body circulates proteins (called leukocyte endogenous mediator) to decrease blood iron availability. Furthermore, the protein transferrin circulates in blood and binds iron, sequestering iron away from bacteria.
    • This even affects our behavior—when sick, we tend to dislike iron-rich foods like ham and eggs.
    • There’s more suggestive evidence—egg white protein is 12% conalbumin, which binds iron, and thus keeps eggs fresh. Similarly, human milk protein is 20% lactoferrin while cow milk is 2% lactoferrin; breast-fed babies have fewer infections than bottle-fed babies.
    • Counter-intuitively, giving iron...

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Why We Get Sick Summary 4: A Never-Ending Arms Race

As discussed, hosts and pathogens adapt to each other in continuous cycles. One strategy is quickly defeated by a counterstrategy. This gives rise to the Red Queen Principle, named after the statement in Alice in Wonderland: “It takes all the running you can do, just to keep in the same place.”

  • The arms race can escalate to the point where the host organism expends so much on defense that it’s hard put to meet other basic needs, but the cost of losing the arms race is so great that it must continue defenses.
  • Bacteria and viruses have the edge in numbers. Bacteria can have 300 generations in a week (each time generating novel mutations that may help it infect), and viruses may have even more. Furthermore, they have vast numbers, with trillions in count for one human host. Humans therefore have a huge handicap in natural selection, and instead must compensate with adaptable mechanisms, like an antibody production system that can generate millions of unique antibodies.

Case Study: Bacterial Resistance to Antibiotics

Antibiotics were one of the great medical successes of the 20th century. However, due to widespread use, increasingly bacteria are evolving resistance to even our most powerful antibiotics. The multi-drug resistance strain of tuberculosis causes a 50% mortality rate.

Bacteria don’t become resistant to antibiotics through gradual tolerance. Instead, rare mutations occur that confer resistance. In an environment where an antibiotic is present, these resistant bacteria replicate more readily and take over the population.

Once bacteria evolve drug resistance, they can pass these genes to other bacteria through plasmids.

If the environment changes and the antibiotic is removed, the ancestral strain slowly replace the resistant strains. This suggests that maintaining the resistance causes a fitness disadvantage (once again, natural selection is a collection of compromises).

However, the disadvantage of the resistant mutation can itself be mutated away. Therefore, antibiotic resistance can persist in a population, even when no...

Why We Get Sick Summary 5: Injury and Fear

Pain and fear are often considered negative conditions to be avoided. But both pain and fear serve vital functions in survival. Pain signals that tissue is damaged; fear signals that danger might be nearby, and so caution is warranted.

Both pain and fear are useful to avoid injury. Blocking either can worsen damage. People who lack the capacity to feel pain almost all die by age thirty; people born without fear tend to end up in the emergency room or the morgue.

How Do We Learn Fear?

Fear of some things is innate, since that mistake even once is very costly. A rabbit is born to be afraid of foxes, since slipping up once can cost a rabbit its life. But innate behavior is inflexible—it can’t mold to new situations that would merit more appropriate responses.

More flexibly, other fears can be learned so as to be situationally useful. A fawn might stare blankly at an approaching wolf until its mother flees. Then the fawn learns that wolves are probably bad and should be run away from, and the fawn passes this lesson to her children. These types of fears are not hardwired; they can be unlearned, and they can be extinguished when the cue is removed.

Humans have the benefit of reasoning and memory, so we can learn indirectly. We know that fire is dangerous and we install smoke alarms without personally knowing anyone who died in a fire.

Avoidance can be conditioned more easily to some cues than others.

  • Researchers found it easier to train dogs to avoid a peppermint smell associated with stomach illness than a noise associated with illness. Smell and toxic food are more strongly linked than sound and toxic food, which makes sense—in everyday life, smells are more likely than sounds to be reliable indicators of bad food than sounds are.
  • Likewise, dogs learn to avoid electric shocks more readily when paired with a sound, than when paired with a smell. Sounds more reliable indicators of acute injury than smell are.
  • Some phobias are learned more readily than others. Monkeys can learn a fear of snakes by watching a video faster than fear...

Why We Get Sick Summary 6: Toxins

Just like animals, plants undergo natural selection for reproductive fitness. In the wild, plants develop defenses from eating, like hard pods and toxins.

Plants have developed a wide range of toxins, including tannins (in wine), alkaloids, cyanide, glycosides (from foxgloves), diazepam (from potatoes), and solanidine (nightshades, potatoes). While the amounts of toxins in typical plants aren’t enough to damage humans, consider how much they might deter a hungry mouse weighing 1/3000th of a human’s weight.

Using Darwinian Thinking to Forage

If you’re ever foraging for plants in the forest, you obviously would like to know which plants are toxic and not. Darwinian thinking is helpful here: consider the natural competitive equilibrium that would result in the phenotype observed.

  • Plants that are readily accessible, with little seeming defense around them (such as tubers, mushrooms, and leaves) must have selected other defense mechanisms, like potent toxins. In contrast, those that have clear defenses (like hard shells around nuts, or thorns on berry bushes) are less likely to be toxic.
  • Plants create fruits for animals to eat and disperse the seeds in their excrement. Therefore, seeds are more likely to be toxic if destroyed or chewed—a disincentive for hungry animals to chew seeds. Further, immature fruits without ready seeds are wasted if eaten, so they’re also more likely to be toxic.
  • The same applies to easily accessible animal prey, such as caterpillars and insects. Eat the camouflaged frog, not the bright yellow one sitting sleepily on a branch.
  • Toxins are metabolically taxing for plants to produce, so rapidly growing plants and the first leaves of spring are less likely to be toxic.
    • Some plants might adopt the extreme strategy of fast growth with no toxins (such as bamboo).
    • 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...

Why We Get Sick Summary 7: Genes and Disease

We now return to a question asked earlier in the summary: Why haven’t we evolved away genes that cause disease?

You now know a few reasons: the disease-causing gene may have benefits that are not as obvious. Also, the gene may have been beneficial during the Stone Age, but only cause disease in today’s environment (we’ll cover this explanation more later in the chapter).

Here are even more reasons that we haven’t evolved away genes that cause disease:

  • Natural selection may not be able to completely eliminate disease genes due to insufficient adverse selection.
    • A harmful recessive gene that confers no disadvantage for heterozygotes has a low rate of adverse selection, even if the unfortunate homozygous recessive die without reproducing.
    • The math shows that if a lethal recessive gene that is created in one out of a million pregnancies will stabilize in frequency at one out of a thousand individuals. In other words, mutation can create the defective gene as fast as natural selection eliminates it.
  • Genes that cause serious diseases may have some less obvious benefits that allow persistence.
    • Consider a gene that is lethal if homozygous. Maintaining such a gene at a prevalence of 3-11% in the population requires a reproductive advantage of 6%.
    • For example, bipolar disease causes manic periods that may promote risky feats leading to success (and thus social stature), as well as sexual aggression. This can increase reproductive fitness even at the cost of mental illness.
    • The gene for cystic fibrosis may reduce death from diarrhea.
    • The gene for Tay-Sachs disease may protect against tuberculosis.
    • The mutated DR3 gene that causes childhood-onset diabetes decreases miscarriage. By the Punnett square, a mating of Dd x DD suggests that half of babies should be heterozygous Dd, but the observed rate is actually 66%!
  • 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...

Why We Get Sick Summary 8: Why We Still Age

What people call aging is biologically termed senescence, the bodily deterioration accompanying age. Human organ systems tend to wear out at the same rate, on average. Maintenance and repair systems lose their efficacy, and old people become progressively more vulnerable to diseases and injuries.

Senescence has been stable over time. Over the past centuries, the average human lifespan has increased, but the maximum lifespan has not. Despite all our medical advances, humans cannot live past about 115 years.

Theoretically it would be a huge reproductive advantage to maintain health for more time - imagine humans who lived to be 300 and reproduced for 100 years. Why haven’t humans evolved to live longer?

Early theories suggested senescence was necessary to make room for the young. But as we’ve learned with lemmings in Chapter 2, natural selection doesn’t occur on the group level. It’s also prone to exploitation by individuals who don’t follow the same strategy—the gene that caused individuals to reproduce rapidly and live longer would overtake the population.

Later theories, in line with our other explanations of why genes persist, suggest that genes that promoted early survival and reproduction would be selected for, even if they cause disease later in life. In a Stone Age world where most people die for reasons other than old age, there wouldn’t be adverse selection against senescence.

Again, per evolutionary medicine principles, there must be a competitive equilibrium at play - living longer must confer some compensatory fitness disadvantage, and the inverse is true. The balance is between faster, more aggressive mating (which may necessarily cause decreased longevity) vs. longer lifespan (which may necessarily trade off with decreased fertility).

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...

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Why We Get Sick Summary 9: Legacies of Evolutionary History

Evolution makes incremental changes on what came before. It does not totally scrap a current design and start from scratch. This can lead to some historical artifacts that cause problems today, and that we might redesign should we have the choice.

For example, in all vertebrates, the esophagus (leading to the stomach) and the trachea (leading to the lungs) have the same input (the mouth). This can lead to choking. This arrangement came from an early wormlike ancestor that used the same tube for both respiration and digestion. All vertebrates descended from this ancestor and inherited this design.

In contrast, insects and mollusks have complete separation of the breathing and eating structures. They evolved on a different lineage and aren’t beholden to this evolutionary artifact.

Here are more examples of evolutionary legacies:

  • In our eyes, the retina is inside-out—it’s covered by a layer of blood vessels and nerves. Light entering the eye has to pass through this layer before hitting the layer of rods and cones. This is an artifact of how blood vessels and nerves were arranged in transparent animals—since they were transparent, it didn’t really matter where how the two layers were arranged.
    • The nerves causes a blind spot. To overcome the blind spot, our eyes need to perform tiny twitches constantly to form a complete picture.
    • The inverted eye arrangement promotes a detached retina. In contrast, squid have eyes where the retina is anchored from below by nerve fibers, and detachment is much more difficult.
  • The appendix was previously used for digestion as a caecum, used to digest plant foods of low nutritional value. Today, the appendix has little function except to cause appendicitis.
    • Interestingly, if the appendix gets too small, it is more prone to bursting, since swelling is more likely to burst a long thin appendix than a large one. Thus, there is a natural selection against reducing appendix size.
    • **The lesson: some vestigial traits might persist because further diminishing them increases vulnerability to...

Why We Get Sick Summary 11: Allergy

(Shortform note: The ideas in Chapter 10 were integrated into previous chapters, so our summary is skipping to Chapter 11.)

Allergies show such a strong reaction, are so inconvenient, and form a system of such complexity that, per evolutionary medicine principles, it seems unlikely they have no useful function to compensate, else they’d have been selected out.

Briefly, how do allergies work? They are the consequence of a series of steps upon exposure to a foreign substance:

  • The substance is consumed by macrophages.
  • Macrophages present the foreign proteins to helper T cells, which in turn present them to B cells.
  • B cells secrete IgE antibodies, which bind to basophils or mast cells.
  • Mast cells release chemicals that cause the symptoms of allergic reactions—histamine, defensive enzymes, platelet activators, amd smooth muscle stimulators.

IgE makes up just 1/100000 of the total antibody in blood, but it produces an outsized response. In an allergy, 10% of IgE may be specific to the antigen, such as pollen;.

The function of allergy is unclear, though there are a few leading hypotheses:

  • Defense against internal parasites and bacteria
    • Worm parasites stimulate the host to produce IgE antibodies, and IgE is protective against parasite infection.
    • Yet the inverse may be true: worms may stimulate IgE for their benefit, since it increases blood supply.
    • Some people with low levels of IgE have recurrent infections of lungs and sinuses.
  • Defense against ectoparasites (parasites that live outside the host, like ticks and lice)
    • An inflammatory response may prevent ticks and lice from getting their blood meal.
    • This explains the concentration of mast cells on the surface of the skin why they stimulate itching.
  • 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...

Why We Get Sick Summary 12: Cancer

Cancer arises when normal mechanisms for cell growth go awry.

The human body has 10 trillion cells, many of them replenishing themselves. With each division of a cell, mutations in genes are introduced. Given all this activity, it’s really a wonder that we’re typically protected against cancers for decades at all.

Cells have a number of mechanisms to prevent cancers:

  • They repair DNA mutations.
  • Tumor suppressor genes inhibit cell growth when they detect problems with the cell.
  • For cell replication to proceed, multiple checkpoints need to be active. If any one is defective, perhaps because of mutation, replication stops.
  • Mechanisms internal to the cell detect a cancerous state and induce apoptosis, or cell death. (The p53 pathway is an example.)
  • Immune cells recognize the target cell is cancerous from the outside, and they trigger apoptosis or inhibit growth in the cancerous cell.

The relationship between cancer and host operates in some ways like that between virus and host. Cancer can be considered a parasite that appropriates resources for its own gain at the expense of the host. But there’s one big difference between virus and cancer—the cancer is noncommunicable and thus dies with the host.

Modern Times and Cancer

Why have cancer rates increased over the past centuries?

  • As we previously discussed, our bodies didn’t evolve to keep us alive for 80 years, so the normal protective measures may senesce with age. As our average lifespan increased, we also gave more time for cancer to develop.
  • We’re also exposed to certain carcinogens that didn’t previously exist (tobacco, meat cooking, plastics, and ionizing radiation).

Menstruation and Cancer

There is one clear linkage between the modern environment and cancer in women: more menstrual cycles means more cancer in female organs (breast, ovaries, and...

Why We Get Sick Summary 13: Sex and Reproduction

There are perhaps few mysteries as intriguing and perplexing as the courtship between females and males. Darwinian thinking has explanations for this too.

Sex and Mating

Why Sexually Reproduce?

Some animals reproduce asexually, with mothers essentially giving birth to a clone of itself. Why do mammals and humans go through all the trouble of sexual reproduction?

The classical explanation is that sexual reproduction promotes genetic diversity, through the combination of genetic material from the mother and father. Having more genetic diversity avoids over-optimization to one genome, and it promotes survival in changing environments.

In comparison, a woman who could bud off offspring without sex may have a short-term advantage in producing offspring, but the entire enetically identical clan may be wiped out in one calamity. If ten thousand clones of one woman are all especially vulnerable to influenza, they may all die in one epidemic, whereas a genetically diverse population would only lose a fraction of its members.

Different Reproductive Strategies

Different animals have different reproductive strategies. They vary in these factors:

  • The number of offspring produced per birth
  • The mode of childbearing
  • How males and females choose one another
  • The split of childcare between males and females

For example, a female salmon releases hundreds of eggs into the water, and a male fertilizes the eggs. The fish then hatch from eggs with no parent supervision and forge onward into life. This is obviously very different from how humans reproduce.

Egg and Sperm

In many animals, the male produces sperm and the female produces eggs. This contrasts with hermaphrodite animals, which can produce both sex cells within one organism.

The small size of sperm and large size of eggs make it easier to get sperm inside females, rather than the opposite. It would be much more difficult for a woman to transmit her egg into a male.

Human sperm are much smaller than eggs, which contain most of the resources for the developing embryo. This...

Why We Get Sick Summary 14: Mental Disorders

The field of psychiatry has tried to codify mental illness through clear-cut symptoms, rather than as gradations of emotions that are influenced by psychology and life experiences. Patients also understand their mental illness as imbalances in brain chemicals; they might be offended if the psychiatrist insisted that their illness were just a maladaptive psychological process.

The authors argue that ignoring the underlying function of emotions is like ignoring physiology in medicine.

As an analogy to today’s approach to psychiatry, imagine if we investigated “cough disorder,” creating objective criteria for diagnosis and subtyping (like coughing more than twice per hour). We then discover a cough center in the brain and muse about what dysfunctions lead to coughing, then investigate genetic causes for people prone to coughing.

This is clearly silly, but only because we know cough is a defense. We know not to look for causes of cough in the nerves and muscles, but rather upstream in the stimulus that provokes a cough (like the common cold).

Emotions are no different. They provide a valuable function to us in everyday life. Understanding this normal function should give us insight into when emotions go wrong.

The Function of Emotions

As with everything else discussed in Darwinian medicine, our emotions are adaptations shaped by natural selection and have powerful uses.

Emotions adjust multiple aspects of our body to respond effectively in a situation—they change our cognition, physiology, subjective experience, and behavior.

Unpleasant emotions like fear and anxiety protect us from bad situations. Positive emotions like optimism and joy help us seek opportunity and seek more of what is good for us.

When we feel an emotion, we may not be conscious of its cause, but the cause likely does exist.

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 causes the fight or flight physiological response that is useful in...

Why We Get Sick Summary 15: The Evolution of Medicine

The human body has been shaped over millions of years as a well-functioning bundle of compromises. What looks like mistakes in evolution more likely continue to exist for these reasons:

  • Behavior that seems harmful likely have unappreciated benefits that outweigh costs.
  • Natural selection doesn’t punish genes that cause damage late enough in life.
  • Novel environments make some evolved genes maladaptive today.
  • Some attributes are purely historical legacies.

The authors wish to broaden discussions of disease to include these questions:

  • Which aspects of the syndrome are direct manifestations of disease, and which are defenses?
  • If the disease has a genetic component, why does it persist? What possible benefits might it confer?
  • Is the disease partly caused by novel environmental factors that didn’t use to exist?
  • If the disease is related to infection, which aspects of the disease benefit the host, which benefit the pathogen, and which benefit neither?
  • What design compromises or historical legacies account for our susceptibility to this disease?

Broadening Evolutionary Medicine

The authors call for more funding of research for evolutionary medicine.

There have been barriers to adopting evolutionary medicine in practice, including:

  • A dislike of ideas relating to adaptation and natural selection
  • A preference for the randomized-controlled-trial method of scientific inquiry. Instead, evolutionary medicine would work more like geology, which is a credible science despite not being able to run randomized trials.
  • Medical education being too expansive and overwhelming to fit in Darwinian concepts

But evolutionary medicine may help patients come to terms...

Shortform Exercise: Reflect on Darwinian Medicine

Reflect on what you've learned about the evolutionary causes of disease.

What were your most surprising takeaways from what you learned in this book?