Humans have an opposable thumb, meaning that they are able to simultaneously flex, abduct and medially rotate the thumb (pollex) so as to bring its tip into opposition with the tips of any of the other digits. This ability is facilitated by a sellar (saddle-shaped) joint between the trapezium (the wrist bone that supports the thumb) and the first metacarpal, which allows an approximately 45° range of rotation of the thumb about its own long axis. Humans share pollical opposability with most other catarrhines (old world monkeys and apes). However, humans differ from other primates in having a relatively longer and more distally placed thumb (see Relative Thumb Length) and in having larger thumb muscles (the thumb muscles constitute about 39% of the mass of the intrinsic hand muscles in humans, as compared to only 24% in chimpanzees). These differences, especially with respect to relative thumb length, make it difficult for non-human primates to employ tip-to-tip precision grips when manipulating small objects (such that small objects must generally be pressed by the thumb against the lateral side of the index finger). The greater mobility of the human thumb, and our enhanced ability to manipulate small objects with thumb tip-to-finger tip precision grips, likely evolved for finer manipulative abilities in the context of increased dependence on, and elaboration of, technology.
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