Thumb

In human anatomy, the thumb is the first digit on a hand. The human thumb is fully opposable to the tips of the other fingers in that it may position itself, and be folded inward, toward the rest of the hand and fingers, if so required. It rotates at the carpometacarpal joint and so can complete the sometimes quite delicate task of grasping objects by pressing them against the rest of the hand or finger(s).

Grips

Typical interdigital grips include the tips of thumb and second finger (forefinger/index finger) holding a pill or other small item, or thumb and sides of second and third fingers holding a pen or pencil.

Evolution

The opposable or prehensile thumb is usually associated with the evolution of homo habilis, the forerunner of homo sapiens (the human being of today). This, however, is the result of evolution from homo erectus (around 1 million years ago) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.

The most important factor leading to the habile hand (and its thumb) is the freeing of the hands from their walking requirements - still so crucial for apes today, which in its turn was one of the consequences of the gradual pithecanthropoid and anthropoid adoption of the erect bipedal walking gait - and the simultaneous development of a larger anthropoid brain in the later stages.

Other animals with opposable thumbs or digits

Many animals, primates and others, also have some kind of opposable thumb or toe:

• Panda - Panda paws have five clawed fingers plus an extra bone that works like an opposable thumb. This "thumb" is not really a finger (like our thumb is), but an extra-long wrist bone that works like a thumb.
• Koala - opposable toe on each foot, plus two opposable digits on each hand
• Opossum - opposable thumb
• Cebids (New World primates of Central and South America) - some have opposable thumbs
• Bornean Orangutan - opposable thumbs so that its forefeet are really like hands. The interdigital grip gives them the ability to pick fruit. They also have an opposable big toe.

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