PDF Summary:Why We Get Sick, by Randolph Nesse and George Williams
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Why is it, after millions of years of evolution, that our bodies are still so flawed that they break down and wear out despite our best efforts to increase our longevity? Why do we get disease at all? Why hasn’t natural selection prevented heart attacks, nearsightedness, and Alzheimer’s disease?
In Why We Get Sick, Randolph Nesse and George Williams write that our bodies have evolved over millions of years as a set of compromises, largely in pursuit of reproductive fitness. Put concisely, whatever enables you to have kids will persist in the gene pool, even if it causes you disease and pain later in life. In this guide, we explore their theories, examining the evolutionary roots of all kinds of disorders, from obesity to nearsightedness to depression.
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Once-Beneficial Adaptations Can Cause Modern-Day Disease
Our modern environment is very different from the environment in which humans evolved over millions of years, and many adaptations that may have been helpful in the Stone Age are maladaptive in our modern world. For example, the authors note:
- Genes that make you binge on foods helped humans survive famine periods. But in today’s food-abundant world, this leads to nutritional excess and obesity.
- Our genes also work against us during weight loss. Because we evolved to survive famine, when our bodies detect a deficit in calories, they cut our metabolism to conserve calories, making further weight loss difficult.
- Energy conservation and laziness helped us avoid wasteful activity, but today they get us to sit for most of the day.
- Our eyes evolved to see far-away objects in bright sunlight clearly, but they’re not well-suited to the close, indoor-lighted reading we do today. This has led to a large proportion of our population developing nearsightedness.
- Genes that lead to alcoholism and other forms of addiction may have been beneficial in the past, increasing our ability to pursue rewards despite difficulties.
- 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.
- Genes for sickle cell disease protect against malaria, an endemic pathogen in regions where sickle cell disease is prevalent.
- 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.
- 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. Such tendencies today, however, can lead to depression.
Complexity Increases Chances for Mistakes
Some disease is due not to outdated adaptations like in the list above, but instead, to adaptations that have become immensely complex over time as our bodies have become more advanced.
Cancer
Cancer is one example of this. Cancer arises when normal mechanisms for cell growth go awry. The human body has 10 trillion cells, many of them constantly replenishing themselves. With each division of a cell, mutations in genes are introduced. This level of activity is necessary to keep our bodies functioning at a high level, but it opens up the potential for errors at many different points in the process.
Allergies
Allergies are another disorder that may be due to the complexity of otherwise beneficial processes. They prompt such strong reactions in people and are so complex that it seems unlikely they have no compensating function, or they’d have been selected out. However, scientists aren’t clear on what factors cause them.
Leading theories hold that they stem from defenses against a variety of substances that would cause us harm, such as parasites, bacteria, or toxins. It’s likely that they’re the unintended consequences of an overly robust immune response to these types of things.
Evolutionary Compromises Affect Physical Health
According to Nesse and WIlliams, many of our diseases and physical disorders can be credited to compromises our bodies had to make as we evolved. Some adaptations that were highly advantageous also brought some disadvantages, but the advantages outweighed them. For example, becoming bipedal—learning to walk on two feet instead of four paws—brought immense advantages for our species. We were able to carry food, hunting gear, and other survival supplies—as well as our babies—over long distances, allowing us to travel to areas with better nutrition sources and to hunt and gather more effectively.
However, the adaptation also made us vulnerable to back problems and injuries to our knees, ankles, and feet. In addition, 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, it causes problems like digestive system blockages and hemorrhoids.
Another evolutionary compromise has to do with our intelligence. Larger brains have allowed our species to thrive, but it complicates childbirth, since the size of the female pelvis limits the size of the skull that can pass through it. This not only makes childbirth more difficult and dangerous for women, but it also means children are more helpless upon birth, requiring a long period of care before becoming self-sufficient. Contrast this to other animals like baby deer, who are much more self-sufficient when born.
Some of Our Body Parts Are Legacies of Previous Species
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, according to the authors.
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.
Another example is our appendix. 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.
Notably, if a person’s appendix is unusually small, it’s 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. This suggests that some vestigial traits might persist because further diminishing them increases vulnerability to disease.
Defense Mechanisms Against Infections Can Harm Us
Nesse and Williams write that throughout human history, infectious diseases—both bacterial and viral—were probably the most common cause of mortality, and much of our genetic selection would have favored genes that protected against infectious diseases.
When those defenses get activated, they sometimes cause symptoms that appear to be disease. In reality, the symptoms are the defense mechanisms against disease. However, even though these defenses are intended to protect us, they can sometimes do us great harm, and can even be fatal.
Some examples of symptoms that our bodies use to protect us are:
- Fevers: 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, the fever is actually our defense against infection.
- Mucus: 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.
- Diarrhea: During illness, diarrhea causes ingested pathogens to be expelled more quickly.
Evolution Limits Our Ability to Heal
Some animals can regenerate whole body parts when lost, such as starfish regenerating arms and lizards regenerating tails. So why can’t humans do this? And aside from limb regeneration, why can’t we heal our less-severe injuries more quickly than we currently do?
The authors explain that, as always, any trait requires balance. When repairing an injury, the body needs to balance scarce resources with fast healing: Faster healing requires taking more resources from the rest of the body. We also have to balance allowing use of the injured body part with providing protection and healing. Using a burned hand can give a person more capability, but it will delay healing.
In the case of regenerating limbs, 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.
We can see the balance evolution has chosen when we observe that less accessible body areas, such as the brain and heart, have less regenerative capacity. It seems natural selection has decided that infections of the brain or heart are usually fatal, so maintaining regenerative capabilities in these areas would have little benefit.
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