Summary

In Chapter 3, Bryson takes up with the other lifeforms that inhabit our bodies: microbes. The human body is home to trillions and trillions of microbes and they provide us with about 10 percent of our calories by breaking down food our bodies cannot. Microbes are “infinitesimally small” (29) and many live for only about 20 minutes, but they are key to life. The first part of the chapter deals with bacteria. Bacteria and other microbes can horizontally transfer genes, meaning they can pick up DNA from dead neighbors. They can mutate more often and, as a result, adapt more quickly. The microbes in our bodies weigh about as much as our brains. Although people say we have 10 times as many bacterial cells as human cells, the number is actually closer to equal, Bryson writes. Our microbes are specific to us. Even though we transfer about...