[In] The Rise and Reign of the Mammals, paleontologist Steve Brusatte’s [recounts the] sweeping history of the animals that have, for the moment, inherited the Earth. Moving generally forward in time, the book describes how the mammalian line progressively acquired a range of features that have come to define what a mammal is.
Some of the moments of evolutionary invention that led to what we now think of as a mammal are remarkably subtle. There’s the hard roof of the mouth that created a dedicated airway to the lungs, allowing mammal ancestors to eat and breathe at the same time. There’s the change from a spine that bends from left to right (which produces the classically reptilian side-to-side gait) to one that enables bending up and down, which ultimately allowed mammals to take in more oxygen as they moved, helping them run faster. And there’s the variety of tooth shapes — incisors, canines, premolars and molars — that made it possible for mammals to eat many kinds of food.
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Some mammalian characteristics are very familiar: milk production, warm-bloodedness, hair. But there’s one less–well-known evolutionary advance that was in its humble way quite profound, setting “us apart from amphibians, reptiles, and birds,” Brusatte writes. It’s a joint in the jaw that makes chewing possible (SN: 8/17/19, p. 8). The ability to chew was “a major evolutionary turning point,” he writes. “It triggered a domino chain of changes to mammalian feeding, intelligence, and reproduction.”
