Final answer:
After the extinction of the dinosaurs at the end of the Cretaceous Period, mammals evolved rapidly, filling the ecological niches left vacant. The Cenozoic Era saw the diversification of mammals, including the rise of primates. The Paleocene Epoch is particularly noted for having the first primate-like fossils, marking the beginning of primate evolution.
Step-by-step explanation:
Following the mass extinction at the end of the Cretaceous Period, about 65 million years ago, which saw the demise of the dinosaurs, a significant evolutionary opportunity arose for mammals to diversify and adapt to various ecological niches. During the age of the dinosaurs, mammals had remained small and ecologically inconsequential, but with the advent of the Cenozoic Era, they began to evolve rapidly to fill the vacated niches. The reduction in Earth's temperature and the spread of flowering plants during this time likely favored the endothermic (warm-blooded) mammals over the ectothermic (cold-blooded) dinosaurs, contributing to the eventual dominance of mammals.
Proto primates, or the earliest primate-like animals, began to appear in the fossil record during the Paleocene Epoch, which started around 65 MYA. These creatures, one example being Purgatorius, were significant as they bore distinguishing features shared with modern primates, such as an elongated last lower molar and an enlarged upper central incisor. Thus, the end of the dinosaur age paved the way for the advent and diversification of mammals, including the primates, with features like larger brains and eyes, and smaller muzzles, which became more pronounced by the end of the Eocene Epoch.