In which epoch did the first definitive primates appear?

The first definitive primates emerged during the Eocene epoch, which lasted from about 56 to 33.9 million years ago. This period is characterized by significant evolutionary developments among mammals, with the appearance of early primate forms known as "euprimates." These ancestors of modern primates exhibited traits such as larger brains, forward-facing eyes, and adaptations suitable for an arboreal lifestyle, which are vital characteristics of primates.

During the Paleocene epoch, preceding the Eocene, early mammalian forms were diversifying, but true primates had not yet developed. The Oligocene epoch followed the Eocene and saw significant evolution and diversification of primates, but it was in the Eocene that the foundational traits of primates first emerged. The Pleistocene epoch, much later in Earth's history, is notable for the emergence of modern human ancestors and other large mammals but is not relevant to the origin of primates.