Final answer:
During the Oligocene, primates that remained in the old world eventually developed into Old World monkeys, with primitive Old World monkeys like the Propliopithecoidea having a dental formula of 2.1.2.3.
Step-by-step explanation:
During the Oligocene epoch, the primates that stayed in the old world led to the group known as Old World monkeys. The Fayum primates are classified into two main groups: Parapithecoidea, believed to be the earliest New World monkeys, and Propliopithecoidea, believed to be the earliest Old World monkeys. These two evolutionary lines are distinguished by characteristics such as their dental formula, which for Old World monkeys and apes, including humans, is 2.1.2.3 (two incisors, one canine, two premolars, and three molars in each quadrant).
Further evolutionary developments during the Pliocene epoch showed clear evidence of bipedal hominins and primitive cultural behavior. Climatic changes, such as cooler temperatures and shifting land masses like the appearance of the Isthmus of Panama, shaped the evolutionary paths of various species.