The evolutionary history of anthropoids, the simian primates that include monkeys, apes, and humans, is marked by a deep split into two major lineages. This divergence separated the ancestors of all living New World monkeys from the ancestors of all living Old World monkeys and apes, a process that began on the African continent. The central questions for scientists are precisely when this division occurred and what geological mechanism caused the two groups to become permanently separated. Answering these questions requires examining both the physical characteristics of the resulting groups and the geological evidence of the ancient world.
Defining the Split: Platyrrhines and Catarrhines
The two groups resulting from this ancient split are classified as the Platyrrhines and the Catarrhines. Platyrrhines, meaning “flat-nosed,” are the New World monkeys, found exclusively in Central and South America. Their defining physical trait is their broad, flat nose with nostrils that face sideways, set far apart. They also possess an ancestral dental formula that includes three premolar teeth in each quadrant of the jaw.
Catarrhines, or “downward-nosed,” are the Old World monkeys, apes, and humans, native to Africa and Asia. Their noses are narrow, and their nostrils face downward. This group also has a different dental formula, possessing only two premolar teeth per quadrant. While many Platyrrhines evolved prehensile tails, no Catarrhine species possesses a tail capable of grasping or supporting the body.
Establishing the Timeline of Divergence
The chronological timing of the split is determined through a combination of molecular and paleontological evidence. Molecular clock estimates calculate the time elapsed since two species shared a common ancestor by measuring the accumulation of genetic mutations. These genetic studies have placed the split between Platyrrhines and Catarrhines at approximately 40 to 46 million years ago.
The fossil record, however, provides a minimum age for the divergence, based on the oldest identifiable remains of each lineage. The consensus view places the actual split closer to the boundary between the Late Eocene and the Early Oligocene epochs, roughly 30 to 35 million years ago. The discovery of New World monkey fossils in South America confirms that the divergence had to have occurred by at least that time. Scientists rely on both methods to anchor the minimum date for the presence of the two distinct lineages.
The Geographic Isolation Mechanism
The reason for the permanent split was an extreme geographic isolation event driven by trans-oceanic dispersal, a theory often called the “rafting hypothesis.” The split occurred on the African continent, and the ancestors of the Platyrrhines then made a remarkable journey across the proto-Atlantic Ocean to South America. This voyage was made possible by unique geological and climatic conditions during the Oligocene epoch.
Africa and South America were significantly closer, separated by an Atlantic Ocean that was narrower by an estimated 600 to 1,000 miles. A drop in global sea levels, caused by the buildup of Antarctic ice sheets, would have effectively shortened the distance even further. The key mechanism involved large, naturally formed “rafts” of vegetation, which were essentially floating islands of tangled debris, earth, and uprooted trees. These rafts, ripped from the shores of large African rivers by powerful storms, were carried westward by strong, persistent equatorial paleocurrents. Models suggest the journey from the coast of Africa to South America could have taken as little as two to three weeks, ensuring permanent separation from their Old World relatives and leading to independent evolutionary paths.