Part of Hall of Human Origins. As the grasping hand evolved, claws disappeared. Today, most primates instead have flat fingernails and larger fingertip pads, which help them to hold on. The hands of many higher primates can grasp and manipulate even very small objects. The human opposable thumb is longer, compared to finger length, than any other primate thumb.
Other animals with opposable thumbs include gorillas, chimpanzees, orangutans, and other variants of apes; certain frogs, koalas, pandas, possums and opossums, and many birds have an opposable digit of some sort. Many dinosaurs had opposable digits as well. Granted, most of these are primates, as are we. I wonder if rationalization is something unique to humans. The ability to ponder may be as well.
Hold your hand out in front of you and look at it carefully. The human hand is made up of four fingers and one thumb. Have you ever thought about how much you use your thumb? Examine your hand again. Move your thumb and fingers to find out how your thumb moves differently than your fingers.
It is often said that the defining trait which separates man from the animals is an opposable thumb. While many may take that remark in jest, there is in fact much truth in it. Without the opposable thumb, human beings might never have attained the highly civilized, sophisticated, and technological lifestyles that many of them enjoy today. Opposable thumbs are required for efficient gripping of objects as well as performing fine manipulations upon them. After all, virtually all of the early accomplishments of human civilization were built through sheer manual labor!