Sesamoids of the pollical metacarpophalangeal joint and the evolution of hominoid hands

Abstract

Sesamoids of the metacarpophalangeal joint are tiny bones located in the volar plate and articulated with the metacarpal head. Almost all living humans have radial and ulnar sesamoids in their pollical metacarpophalangeal joints. These bones protect and guide the tendon of the long (= extrinsic) pollical flexor. Whereas this condition is considered to be primitive for primates, living great apes have a tendency to lose these pollical sesamoids. Susman (Science 1994; 265: 1570–1573) correlated the loss of the pollical sesamoids in living great apes with the remarkable reduction/loss of the tendon of the long pollical flexor. However, the prevalence of these pollical sesamoids in chimpanzees drastically differs among previous studies. Thus, we CT-scanned cadavers of 12 chimpanzees, four gorillas, and two orangutans, and investigated the frequency of pollical metacarpophalangeal joint sesamoids in these apes. Combining our findings with previously reported data gave updated frequencies of 21% in chimpanzees (n = 24) and 0% in gorillas (n = 7) and orangutans (n = 6). This result is in accordance with the purported independent reduction (or loss) of the tendon of extrinsic flexor of the pollex in great apes (Diogo et al., Journal of Human Evolution 2012; 63: 64–78) and underscores the view that living great apes independently lost this tendon–sesamoid complex. Given that a reduction (or loss) of the tendon of extrinsic pollical flexor in great apes is a trade-off between emphasis on hook grips and pollical reduction, human hands have not experienced specialization for hook grips and retain the primitive condition in this regard. Orangutans and chimpanzees independently specialized for hook grips. The case for gorillas, whose hand proportion is similar to that of humans, is equivocal. Gorilla hands may have attained their current state secondarily or they may have lost the powerful extrinsic flexor of the pollex for reasons other than specialized hook grips.