Microbes have been around longer than multicellular species, and they shaped the earth in a way that allowed us to thrive on the planet. Since they were established long before us, it is apparent that they have been responsible for our evolution from single-cellular species to human beings. We relied on bacteria as cues for food, shelter, and stability; now, bacteria rely on us for food and a home, while we rely on their genes to help digest food. We are in a tight symbiotic relationship with our bacteria that we still don’t completely understand. It is clear, though, that bacteria have directed our evolution since the beginning.

Yong uses examples ranging from squid using bacteria to glow and keep predators away to humans microbiomes taking up seaweed-eating genes from marine microbes. Bacteria play into the evolutionary actions of natural selection rather than going against the theories of Darwin like many believe. 

Yong emphasizes frequently that symbiosis is neutral, even though it may seem that the relationship between host and microbe can be net positive. This neutrality comes from the fact that microbes are also living creatures that have their own interests in mind; they, too, need food and shelter and will put those directives above their hosts.