The thumb is the first finger 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.
In fact, 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).
Typical 'interdigital' kinds of grip are, for example, 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 (of course, not all people hold things in exactly the same way).
The opposable or, as it is sometimes called, the 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 via a series of intermediate anthropoid[?] stages, and is therefore a much more complicated link.
The most important causes leading to the habile hand (and its thumb) is on the one hand, 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 on the other hand the simultaneous development of a larger anthropoid brain in the later stages.
However, it may come as a surprise that many animals, primates and others, do in fact have some kind of opposable thumb without having reached the level and the attainments of other anthropoid beings.
Animals which have opposable thumbs or (big) toes:
- Panda -
opposable thumbPanda 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). It is really 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.
See also anatomy.