Final answer:
The evolutionary split in question is not specified for the Oligocene era in the provided resources. However, during this period, there was significant diversification in mammal groups, including primates, leading to the early ancestors of modern monkeys and apes.
Step-by-step explanation:
The question refers to an evolutionary split that occurs in the Oligocene era. It's important to clarify that the reference provided for the Pliocene epoch, which occurred from 5 MYA to 1.8 MYA, indicates a period slightly later than the Oligocene, which was approximately from 34 MYA to 23 MYA. No specific evolutionary split is mentioned for the Oligocene in the provided resources. However, significant evolutionary developments did occur during this time, including the radiation of many mammal groups following the extinction of the non-avian dinosaurs in the preceding eras. If the question pertains to primate evolution, it's worth noting that during the Oligocene, there was significant diversification within the primate group, leading to the rise of different families and the early ancestors of modern monkeys and apes. Biologists consider speciation events as instances where one ancestral species gives rise to two or more descendant species. These events are a significant part of evolutionary biology and contribute to the diversity of life through the branching called clades. The Pliocene epoch, while a bit later, shows evidence of bipedal hominins and primitive cultural behavior, highlighting an important period of evolutionary splits and speciation within the human lineage.