Final answer:
Propliopithecids are ancestral to all later catarrhine primates and are considered the earliest relatives of Old World monkeys and humans due to their dental formula. Despite originating in Africa, they may represent the earliest New World monkeys. Their evolution set the stage for the diverse adaptations in the primate lineage.
Step-by-step explanation:
Propliopithecids are considered to be the ancestors of all later catarrhine primates, which means they are potentially the earliest evolutionary relatives of Old World monkeys, and, by extension, humans. Having evolved during the Oligocene Epoch, they are closely related to the primates found in the superfamily Propliopithecoidea. This group of primates is significant due to their dental formula of 2.1.2.3, which is a trait found in all catarrhine primates, including those in the Catarrhini infraorder, like Cercopithecoidea and Hominoidea.
The Propliopithecids are thought to represent the earliest New World monkeys despite having first evolved in Africa. They exhibit physical and anatomical traits that would become more pronounced in primates, such as larger brains and eyes, which were likely adaptations to their social behavior and arboreal lifestyle. Over millions of years, these traits have emerged as distinguishing characteristics of advanced primates, setting the stage for the vast evolutionary diversification that would follow, ultimately giving rise to the variety of species in the primate order today.