Final answer:
Since the Miocene, apes have declined possibly due to environmental changes and competition, whereas monkeys have flourished as a result of their diverse adaptations to different ecological niches and separate adaptive radiations in the New World and Old World.
Step-by-step explanation:
The decline of apes and flourishing of monkeys since the Miocene epoch is attributed to a number of evolutionary and ecological factors. Apes, which evolved approximately 25 million years ago from catarrhines in Africa, typically have larger bodies and brains, no tails, and differ in locomotion compared to monkeys. Apes, such as gibbons, orangutans, and gorillas, tend to be more intelligent but have faced challenges such as cooler climates and environmental changes that may have impacted their survival.
On the other hand, the successful adaptation of monkeys can be seen in their diverse habitats and variations. The New World monkeys of South America and the Old World monkeys of Africa and Asia have undergone separate adaptive radiations allowing them to exploit a range of ecological niches. Some possess prehensile tails and are strictly arboreal, while others include ground-dwelling species, showcasing a versatile approach to survival.
Evolutionary pressures, such as climate change, habitat alterations, and competition for resources, play significant roles in the success or decline of a species. With monkeys adapting various strategies to navigate their respective environments, they have been able to thrive in regions where apes have seen a decline since the Miocene.