What is one of the most uncertain aspects regarding how New World primates reached South America?

The uncertainty surrounding how New World primates reached South America largely stems from the method of rafting on vegetation islands being a plausible yet ambiguous explanation. This theory suggests that primates could have floated on large masses of vegetation that broke off from the mainland and drifted across the ocean. This scenario is supported by evidence of plant and animal dispersal through oceanic currents, but it remains uncertain how frequently such events could have enabled species to successfully travel long distances to reach new habitats like those found in South America.

In contrast, migration by land bridges is less applicable due to the geographical changes that occurred, especially with the separation between North and South America, while the concept of direct flight across the ocean is implausible, as primates do not possess the capability for long-distance flight. Hybridization events do not pertain to the original question of how New World primates arrived in South America but rather relate to species intermingling once they are present. Hence, the rafting hypothesis remains the most widely recognized yet uncertain aspect regarding the origins of New World primates.