In this chapter, Lieberman explains how the body is able to adapt to the environment.  The ability of the body to respond to environmental factors is called phenotypic plasticity, and some examples of this include tanning in the summer and gaining thicker bones with exercise.  However, if humans move to a different environment, these adaptations can become mismatched.  Additionally, the body requires stresses, such as exercise or sunlight to gain these adaptations.  This “use it or lose it” situation can create mismatch if the necessary amount of stress is not met.  One example of this is osteoporosis.  Bones require regular exercise to cause small deformations, which heal and cause the bone to grow back stronger.  Osteoporosis can occur in the absence of regular exercise and calcium deficiencies.  Another mismatch is wisdom teeth that do not fit in the jaw.  Lieberman explains that farmers often had awful teeth with cavities, and 25% of them with impacted wisdom teeth.  However, hunter gatherers have no problems with impacted teeth.  Lieberman suggests that during childhood, repeated chewing on tough foods is necessary to align the teeth correctly and strengthen the jaw.  However, foods today do not require strenuous chewing, so modern humans must rely on orthodontists and oral surgeons to fix the resulting dental problems.  Additionally, Lieberman explains that early exposure to microbes is important to stress the immune system properly.  If this does not occur, humans are more at risk for allergies and autoimmune diseases.  Lieberman suggests that it is important to learn the cause of mismatch disease such as allergies, to treat the source of the problem.  He explains that all of these diseases are a result of dysevolution, where we treat the symptoms but not the cause.  He argues that many mismatch diseases can be prevented with regular exercise and a healthy diet.