The impact of microbes on evolution and the passing on of important microbes to our offspring have been recurring themes in I Contain Multitudes. The examples that have been cited in previous chapters have been more focused on relationships between animals and bacteria that have been working in tandem for a long period of time. As with every long-term relationship, what Yong calls “the long waltz” between the parties began with an initial contact.

The chapter opens with the story of man whose hand was pierced by a piece of a tree branch as he was cutting it down (143). His hand eventually became infected, and a cyst formed. After the cyst was removed, scientists sequenced the DNA and found unusual bacterial DNA among the sequences. The sequences matched a bacterium called Sodalis, which up until this point was known to live only in the bodies of insects (143). This story is one of coincidences: A branch happens to cut someone, and a bacterium happens to begin to grow within the person.

Most symbioses most likely started with similarly random event that just so happened to turn into a long-term relationship (145). Microbes can be passed along during sex or by eating food containing a new bacterium that then takes hold in the gut.