Relative Thumb Length

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Human Uniqueness Compared to "Great Apes": 
Absolute Difference
MOCA Topic Authors: 

Humans have thumbs that are long relative to the other digits of the hand (as compared to apes and most other primates). This relative elongation is derived in part by an elongation of the thumb bones themselves (the first metacarpal and the proximal and distal pollical phalanges), by a shortening of the other digits, and by a more distal positioning of the thumb on the hand. The greater relative length of the thumb compared to other apes can be seen by comparing the opposability indices (= 100 x thumb length/index finger length) of humans (60) with champanzees (43) and orangutans (40). The relative elongation of the human thumb is central to our ability to employ tip-to-tip precision grips when manipulating small objects (see Opposability of Thumb), and 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.

Related MOCA Topics
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Title Certainty
Thumb Opposability Likely
Tool Making Likely

References

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