The evolution of the human body and health appears throughout the text, and the theme’s development aligns with the parts of the text. Lieberman introduces and develops the theme by exploring biological evolution, then he discusses how cultural evolution impacted human bodies and health during the Neolithic, and he concludes the theme by discussing the modern health implications of human biology.

Human evolution took place over millions of years and included innumerable changes, but Lieberman identifies several physical alterations that are more significant and relevant to the conversation on modern health—bipedalism, diet diversification, larger bodies and brains, slower physical development, and enhanced social capabilities. Each of these traits were developed through natural selection, meaning they improved human reproduction rates but not necessarily their overall health. For instance, while bipedalism allowed humans the use of their hands, it also resulted in decreased strength and climbing abilities, higher rates of back problems, and more discomfort and instability during pregnancy. Thus, bipedalism, like many biological characteristics, increased humans’ ability to procreate but did not improve their health.

In Part 2, Lieberman discusses how the Agricultural Revolution and the Industrial Revolution drastically altered human culture and environments.