Relative Thumb Length
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.
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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