Final answer:
The Oligocene epoch saw the appearance of the first fossil monkeys due to global climate changes. Cooling temperatures and the emergence of grasslands led to the extinction of early prosimian species and the rise of monkeys with specific adaptations.
Step-by-step explanation:
The Oligocene epoch, which lasted from approximately 34 million years ago (MYA) to 22 MYA, marks the appearance of the first fossil monkeys. The global climate changes during this period had an impact on primate evolution. The cooling temperatures and the emergence of grasslands due to the spread of flowering plants led to the extinction of many early prosimian species and the rise of monkeys with larger brains and eyes, and smaller muzzles. The broader Tertiary Period, which encompasses the Oligocene, was characterized by a warm and humid climate that supported diverse mammalian evolution and the proliferation of modern rainforests and grasslands. However, the end of the Eocene epoch preceding the Oligocene saw cooler temperatures and competition, leading to the extinction of many early prosimian species. This scenario sets the stage for the bifurcation of primate evolution towards the emergence of simian primates—monkeys and apes that would eventually give rise to hominins—due to ecological niches vacated and new environmental pressures.