Chapter Four, Mammals Take Over, begins with the earliest traces of animals with mammal-like characteristics (viz. differentiation in teeth) 250 million years ago. The development of other mammalian characteristics such as our secondary palate; temporomandibular jaw joint; and stirrup, hammer, and anvil ear bones are described. The first appearance of fur and first suggestion of a warmblooded metabolism are included. All these mammalian developments were complete before the great Chicxulub meteor killed the dinosaurs to usher in the Cenozoic Period, the current geologic time period. The Cenozoic has been a time for mammals to perfect their individual mammalian characteristics in their numerous roles in the biosphere. A summary of the molecular clock adapted from a University of California, Berkeley Web site is also included.