Chapter 4 describes the biological development of teeth through Shubin’s encounters with fossilized teeth on various expeditions. One of Shubin’s first fossil digs was during graduate school under his mentor, Farish A. Jenkins, Jr. Shubin joined Jenkins’ team on a dig in the Arizona desert, though he initially struggled with finding fossils. However, Shubin finally spotted the fossil of a tooth lying on the surface of the desert.

Inspired by the discovery, Shubin planned his own expedition to Nova Scotia to search for early mammal fossils. In his first expedition, Shubin discovered a jaw that shows characteristics of both reptile and mammalian teeth. One key characteristic of mammal teeth is their precise “occlusion” (94), the upper and lower sets of teeth fitting exactly together. While the jaw Shubin discovered mostly shows reptilian characteristics, it also shows a version of the occlusion typically found in mammals. Shubin’s fossilized jaw belongs to the early animal tritheledont, and significantly shows “that some kinds of reptiles already displayed our mammalian kind of chewing” (96). The development of occlusion is important because it gave humans the ability to consume a “diverse diet” (98).

The rest of the chapter explores the origins of another key characteristic of teeth: “their hardness” (99), which comes from a molecule called hydroxyapatite, found in the teeth of all living creatures.