Final answer:
The ancestors of modern New World Monkeys likely reached the Americas by drifting from Africa on vegetation rafts, after the two continents had separated, leading to separate evolutionary paths.
Step-by-step explanation:
The ancestors of modern New World Monkeys reached the Americas through mechanisms that are still a subject of debate among scientists. One prevalent theory suggests that these primates could have traveled from Africa to South America by drifting on mats of vegetation across the Atlantic Ocean. This migratory event occurred after the continents of South America and Africa had drifted apart. Owing to their separation, the New World Monkeys in South America and the Old World Monkeys in Africa and Asia underwent distinct adaptive radiations, leading to significant differences in their physical characteristics and behaviors. For example, New World Monkeys have prehensile tails for arboreal living, while Old World monkeys do not. This divergence took place during the Oligocene epoch, approximately 40 million years ago.