Monkeys in the New World (the Americas) have prehensile tails, strong flexible tails that grip branches and operate much like another hand. Old World (Africa and Asia) monkeys do not have such a tail. The explanation for this is:

The terms old world and the new world are used in the context of history and for the prime objective of differentiating the major ecozones found on the planet. This also helps in categorizing various species of plants and animals that originated in different places and times.  

In terms of old and new, Old World signifies to Asia, Europe, and Africa, on the other hand, the New World signifies towards the Americas that comprise South America, North America, and Central America. The New World monkeys possess prehensile tails, while the Old World monkeys don't. This suggests that the prehensile tail originated in the monkeys found in the New World, primarily after the separation of the Americas from Asia and Africa.  

a. to encourage apoptosis