Step 1: Understanding the Problem:
In primate evolution, monkeys are broadly classified into two major geographical and morphological groups: New World monkeys and Old World monkeys.
Step 2: Approach and Formula:
Use morphological features and standard evolutionary classification examples to distinguish between the two groups.
Step 3: Detailed Explanation:
New World Monkeys (Platyrrhines) inhabit the tropical regions of Central and South America.
They are characterized by broad, flat noses with sideways-facing nostrils and, very uniquely, most possess long prehensile (grasping) tails for tree climbing.
Spider monkeys, along with squirrel monkeys and marmosets, fit perfectly into this classification.
Old World Monkeys (Catarrhines) inhabit Africa and Asia.
They possess downward-pointing nostrils and their tails are strictly non-prehensile.
Baboons, macaques, and langurs all belong to the Old World monkey classification.
Step 4: Final Answer:
Spider monkeys belong to the New World monkey group.