62 pages • 2 hours read
Daniel LiebermanThe Story of the Human Body
Nonfiction | Book | Adult | Published in 2013A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Part 3, Chapters 10-11Chapter Summaries & Analyses
Part 3: “The Present, the Future”
Part 3, Chapter 10 Summary: “The Vicious Circe of Too Much”
Obesity, or excess stored energy, is the most pressing modern health concern; however discussions on obesity are inconsistent. Obesity stems from a mismatch between energy consumption and usage. When energy is balanced, a person maintains their weight, and if it is not, they gain or lose weight. Negative balance, or weight loss, hinders reproduction, so humans evolved to avoid weight loss through resting and eating.
When a person eats, food is digested into component parts—proteins, fats, and carbohydrates. Excess fats are stored as subcutaneous or visceral fat, with subcutaneous fat sitting under the skin and visceral fat lying between abdominal organs. Carbohydrates are broken down into either glucose or fructose sugars. Glucose prevents cell death, but excess glucose is toxic, so it is regulated by insulin. Some is used for energy, some is processed into glycogen in the liver, and the rest is stored as fat. Fructose is metabolized in the liver, and excess fructose is stored in the liver or transferred into the bloodstream, both of which have health consequences.
Bodies store and withdraw energy as needed, allowing people to function when they have a negative energy balance. However, positive energy balances are now more common. Low fiber levels in processed food speeds digestion, and the excess energy is often stored as visceral fat, which can cause metabolic syndrome, a medical condition characterized by digestive and cardiovascular symptoms. Metabolic syndrome increases one’s risk of developing other noncommunicable diseases, like type 2 diabetes or liver disease.
Experts have proposed hypotheses to explain human’s susceptibility to obesity, but Lieberman asserts that all humans are prone to obesity because they evolved to store excess energy. Put simply, obesity is rising because people are eating more and moving less. Modern diets, which are high in glucose and low in fiber, promote high blood sugar and fat production. Lieberman identifies other factors—genetics, sleep, stress, intestinal bacteria, and physical activity—that impact weight. Scientists have identified genes associated with obesity, but their expression is largely dependent on environmental conditions, and insufficient sleep can stimulate hunger. Inactivity, too, is associated with obesity, and a person’s weight is, in part, dependent on their gut bacteria.
Type 2 diabetes is caused by excessive sugar intake, visceral fat, and inactivity. It develops when the body becomes desensitized to insulin. Early type 2 diabetes is reversible with diet and exercise changes, but as it progresses, the pancreas, which produces insulin, eventually exhausts itself and stops functioning; at this point, the disease is irreversible. Type 2 diabetes is generally preventable and sometimes reversible; however, modernity makes it difficult for many to eat healthfully and exercise. Type 2 diabetes demonstrates an example of dysevolution: Its prevalence is increasing because its causes are not being addressed.
Cardiovascular diseases are also mismatch diseases. Atherosclerosis, or blood vessel inflammation, is usually the first sign. Cholesterol can adhere to inflamed arteries, exacerbating the damage and creating a feedback loop, as the immune system attacks the cholesterol and triggers more inflammation. Coagulated cholesterol hardens into plaque, narrowing arteries and increasing the risk of blockages and clots. Atherosclerosis is worsened by smoking, drinking, and stress, but can be prevented with regular physical activity, particularly aerobic activities, and a healthy, low-salt, high-fiber diet. Some experts suggest people should mitigate their consumption of saturated fats to avoid high cholesterol levels, while others suggest avoiding simple and processed carbohydrates.
Cancer, which has long occurred in humans and other species, has become more prevalent. They begin as mutated cells which then replicate and spread, causing organ failure. Environmental conditions, age, genetics, and some pathogens can increase one’s risk of developing cancer. Reproductive cancers are more common in women who have not had children, suggesting they are sometimes a mismatch disease. Evolution favored higher reproduction rates; thus, women are not adapted for having so many monthly menstrual cycles. This mismatch is intensified by obesity, as positive energy balances increase reproductive hormone levels. Similarly, high testosterone levels in men are linked to higher risks of prostate cancer. In some ways, cancer treatments are dysevolutionary, as they often neglect to prevent the causes, but Lieberman argues that treatment for existing cancers—surgery, chemotherapy, radiation, medication—are not dysevolutionary because they are sensible and do not perpetuate cancer.
Lieberman reiterates that mismatch diseases, or “diseases of affluence,” are noninfectious and have complex causes and limited impacts on reproductive rates, and their causes are connected to culture. He writes that humans have created environments that make them sick, and he questions what improvements can be made. Being overweight does not necessarily make someone unhealthy, so Lieberman turns the discussion to physical activity.
Part 3, Chapter 11 Summary: “Disuse”
Many illnesses and injuries, like osteoporosis and its related injuries, are mismatch diseases that arise through inactivity or disuse. Biological traits like bone thickness are impacted by the environment—e.g., active children develop thicker bones. This is called phenotypic plasticity; it serves to adapt people to their surroundings, and it often takes place during childhood. Modernity can cause phenotype mismatches, such as might occur if a pale person travels to a hot climate and gets a sunburn.
Bone growth depends on physical activity: “[C]hildren who chew harder, tougher food develop thicker jaw bones” (299). Growing thicker bone requires more energy, so the body only grows thicker bones in response to consistent, strenuous use. Regular movement causes minor deformations in bones, and healthy bones readily repair these deformations; however, bones weakened by disuse are more prone to injury. This is seen in astronauts whose bones are weakened after extended missions. Two common skeletal issues arising from disuse are osteoporosis and impacted wisdom teeth.
Osteoporosis afflicts one-third of women and 10% of men who are 50 years old or older. Archaeological records show osteoporosis-related injuries were rare in the past, suggesting the condition is a mismatch disease. Osteoblast cells build bone, and osteoclast cells dissolve old bone; osteoporosis occurs when osteoclasts outperform osteoblasts. This leaves bones porous and weakened. This process is exacerbated in women by low estrogen levels, which is why osteoporosis often follows menopause. Insufficient diets, inactivity, and earlier onset of menstruation all contribute to the prevalence of osteoporosis. While osteoporosis is a by-product of increasing lifespans and earlier onset of puberty, it is considered a mismatch disease because it is influenced by diet and physical activity.
Impacted wisdom teeth occur when there is not enough room in a person’s mouth for their third molars to properly erupt. The teeth in skulls of farmers and modern humans are full of dental issues, while hunter-gatherer skulls contain relatively healthy teeth, including wisdom teeth. As with osteoporosis, impacted wisdom teeth arise from disuse. Stone Age hunter-gatherers chewed tougher foods for longer periods of time, resulting in stronger and larger jaw bones which had the capacity to hold wisdom teeth. Chewing also impacts tooth alignment, and individuals who chew less are more prone to misaligned teeth. Modern diets consisting of soft processed foods that are easy to chew perpetuate these dental conditions; however, Lieberman notes that “it would be absurd to abandon orthodontics and force children to chew mostly hard, tough food” (308). Instead, he suggests alternative solutions, like gum chewing.
The modern sterilization trend is causing health problems by damaging humans’ microbiome—the trillions of symbiotic microorganisms and worms living on and in the human body. Damaging the microbiome can cause immune system dysfunction. The hygiene hypothesis proposed by David Strachan suggests that humans need exposure to pathogens in order to develop healthy immune systems, and the idea was developed to explain the increasing rates of allergies, which are heightened immune responses to otherwise harmless substances. Another proposed cause of this increase—the “old friends” hypothesis—is that human microbiomes are “seriously abnormal” due to modern lifestyle differences. Both hypotheses suggest modern treatments for allergies and asthma are dysevolutionary because they do not address the underlying causes of the diseases.
After another reiteration of what constitutes a mismatch disease, Lieberman suggests such diseases can potentially be avoided by proper public health measures, such as encouraging childhood physical activity.
Part 3, Chapters 10-11 Analysis
In the first half of Part 3, Lieberman identifies and describes multiple mismatch diseases, and in doing so, demonstrates the complexity of the mismatch hypothesis. Liberman both directly and indirectly addresses various conditions which make preventing or managing mismatch diseases more complicated; this, in turn, reinforces the idea that mismatch diseases are not the fault of the individual. Some of the complications mentioned in the text include cultural influences and a lack of scientific consensus, and Lieberman mitigates some of the complexity through structural and literary devices.
Lieberman repeatedly draws attention to the powerful role of cultural norms. Not only do people tend to modify their behavior to match cultural norms, but also culture often controls what options individuals have access to. For instance, Lieberman writes, “Paradoxically, it now required more effort and money to consume food that has fewer calories” (290). The increased price of healthy foods acts as a barrier that makes healthy choices less accessible, which complicates the prevention of several diseases, like type 2 diabetes, that are impacted by diet. Medical non-consensus adds another layer of difficulty.
Lieberman directly addresses the conflicting messages regarding obesity; some view obesity as a sign of ill-health, while others, including Lieberman, disagree. Medical professionals also disagree on the health value of fats—“[W]e may have overdemonized saturated fats, which are possibly not as detrimental as the consensus view would have it” (282). These social, economic, and scientific barriers make it more difficult to identify, prevent, and treat mismatch diseases both on an individual and a societal level. It also develops the theme of Evolutionary Implications for Health Policies and Practices by demonstrating the need for cultural change, which is explicitly discussed in the final chapter.
To mitigate some of the complexity in his discussions and to create a more digestible reading experience. Lieberman uses repetitive structures and literary devices like analogies to emphasize critical points. He uses an analogy to compare the concept of energy balances to bank balances. However, this can be considered a flawed analogy, as a positive energy balance is associated with obesity and accompanying health risks while a positive bank balance is not generally associated with negative consequences. Along with literary devices, Lieberman uses repetitive structures to create consistent patterns and to highlight important takeaways. The pattern of using rhetorical questions to transition between topics, which was established in the first chapter, is continued through the end of the book. This increases the predictability of the text, as well as incorporating immersive aspects.
The conclusions of Chapters 10 through 12 also end with reiterative summaries that reflect on the mismatch diseases covered and explain how the respective conditions fit the definition for mismatch diseases. By repeatedly stressing the common features of such diseases—their complexity, their irrelevance to reproduction, and the cultural value of their causes—Lieberman makes it easier for readers to independently identify potential mismatches that are not discussed. This effectively extends and personalizes the discussion by allowing readers to apply the mismatch hypothesis to their individual experiences.
