While other primates have opposable thumbs, humans are unique because they can bring their thumbs across the hand to their ring and little fingers. Humans can also flex the ring and little fingers toward the base of the thumb. This allows humans to have a powerful grip and dexterity to hold and use tools.[3]
https://en.wikipedia.org/wiki/New_World_monkey
New World monkeys are the five families of primates that are found in the tropical regions of Central and South America ... Unlike humans and most Old World monkeys, their thumbs are not opposable.