Thumb Opposability

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Human Uniqueness Compared to "Great Apes": 
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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.

Timing

Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Probable Appearance: 
2,000 thousand years ago
Background Information: 

Trinkaus, 1989. Olduvai Hominid 7 trapezial metacarpal 1 articular morphology: contrasts with recent humans. Am J Phys Anthropol 80:411-416. Marzke, 1997.  Precision grips, hand morphology and tools. Am J Phys Anthropol 102:91-110. Tocheri et al., 2008. The evolutionary history of the hominin hand since the last common ancestor of Pan and Homo. J Anat 212: 544-562.

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Title Certainty
Relative Thumb Length Likely

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