Final answer:
During the Oligocene epoch, monkeys thrived because of environmental shifts and the decline of prosimians. The rise of rainforests provided new ecological niches, and traits like larger brains and smaller muzzles favored their success.
Step-by-step explanation:
During the Oligocene epoch (34-23 million years ago), monkeys thrived due in part to major environmental shifts. The earlier Eocene epoch saw a decline in the early prosimian populations, giving rise to the first monkeys with more advanced features such as larger brains, smaller muzzles, and postorbital bars composed entirely of bone. Evidence found at the Fayum site indicates that the Parapithecoidea and Propliopithecoidea primate groups had dental formulas resembling those of New World and Old World monkeys, signifying their evolutionary success during the Oligocene. The transitional climate of the Tertiary period fostered the growth of rainforests, which provided a rich environment for arboreal life and a potential factor in primates' success due to the expanded ecological niches available.
The fossil records from the Fayum site suggest that these primates adapted well to their environments, and as the Eocene epoch ended with cooler temperatures, they faced less competition, allowing them to explore and occupy various ecological niches. The Oligocene epoch saw the rise of the first anthropoid monkeys, marking a key period in primate evolution where environmental factors such as lush forests provided abundant food resources and living spaces conducive to their arboreal lifestyle.