The Story of the Human Body

Lieberman mentions that primitivists often use Henry David Thoreau’s Walden to support a return to pre-modern lifestyles, but Lieberman argues that rejecting modernity is oversimplified and ineffective. Humans are relatively successful; their population has drastically expanded, and despite humanitarian issues, many humans have plenty of resources and are healthy, and capitalism provides them with numerous opportunities and amenities. However, modernity has consequences, including the prevalence of infectious epidemics and noncommunicable diseases, and many people are subjected to the consequences of excess and convenience.

Human evolution is less noticeable because the process is slow and human lives are short. Little time has passed since humans first evolved, and cultural buffering has prevented adaptation. Cultural evolution surpasses natural selection and has biological impacts, as gene expression often depends on environmental conditions. Traits impacted by the environment are called “culturally heritable.”

Evolution is rarely considered in medical science, which makes humans less effective in mitigating diseases and sometimes intensifies diseases or causes subsequent conditions. For instance, prescribed antibiotics can trigger Crohn’s disease. Evolutionary medicine recognizes that some symptoms, like fevers and muscle pain, are adaptations and that human evolution prioritizes reproduction over health. It also recognizes the mismatch hypothesis, or the concept that many modern diseases arise from a mismatch between evolved traits and the environment. Most people will die from mismatch diseases.

Inherited genes are impacted by the environment, and inherited traits are only adaptations within certain contexts. All mismatch diseases are caused by the biological implications of environmental factors. They are strongly influenced by cultural evolution, and they often develop slowly and become more prevalent with age. Identifying mismatch diseases and their causes and determining whether a trait is maladaptive are difficult and sometimes controversial. Lieberman includes a partial list of potential mismatch diseases which includes: Acne, anxiety, ADHD, bunions, cavities, coronary heart disease, depression, diaper rash, eating disorders, endometriosis, flat feet, glaucoma, insomnia, lactose intolerance, back pain, obsessive-compulsive disorder, osteoporosis, scurvy, and stomach ulcers. He stresses that such diseases might arise outside of mismatch conditions and that the list is very limited, completely excluding mismatch infectious diseases.

Mismatch diseases can be studied through cultural responses. Lieberman examines this through scurvy and cavities. Scurvy, caused by vitamin C deficiency, has been mostly eradicated since vitamin C is incorporated into processed foods. Cavities, which are caused by bacteria that feed on starchy and sugary foods, are prevalent because the cultural responses—dental hygiene and cavity fillings—do not sufficiently address the underlying cause. Cavities represent a negative feedback loop where the cultural responses create conditions that intensify the disease’s prevalence.

Lieberman refers to this type of cultural evolution phenomenon as “dysevolution,” or harmful cultural evolution. Treating symptoms can exacerbate dysevolution, as it reduces the need to address a disease’s causes. Mismatch diseases that cause dysevolution share common traits: They are noninfectious diseases with causes that are difficult to identify and address, they don’t interfere with reproductive success, and their causes have cultural benefits.