Final answer:
The increase in size of catarrhine primates across the Eocene-Oligocene boundary is linked to evolutionary changes favoring larger brains and eyes, smaller muzzles, and in response to environmental pressures like cooler temperatures and interspecies competition.
Step-by-step explanation:
Primate Evolution Across the Eocene-Oligocene Boundary
The increase in size of catarrhine primates across the Eocene-Oligocene boundary is often attributed to various evolutionary pressures. During the Eocene epoch, primates had adapted to a variety of niches, with many exhibiting arboreal characteristics suited for tree-dwelling. Starting around 34 MYA, the Oligocene epoch witnessed the emergence of the first true fossil monkeys at sites like the Fayum in Egypt. These primates, which had larger brains and eyes, and smaller muzzles, possibly due to the increased reliance on sight over smell, represented a continuation of primate evolution.
The end of the Eocene saw cooler temperatures and possibly competition from these first monkeys, which led to the extinction of many prosimian species. North America and Europe, once host to diverse primate populations, experienced primate extinctions due to these environmental changes and potentially due to the competitive edge of newly evolved primates with better adaptive features. These extinct early prosimians were largely replaced by emerging catarrhine primates, who continued to adapt and eventually gave rise to the diversity of Old World monkeys and apes we see today.