1Anatomy, Lincoln Memorial University-DCOM, 2Evolutionary Studies Institute, University of the Witwatersrand
March 27, 2015 , Archview Ballroom
The flexor pollicis longus muscle (FPL) allows humans to flex the thumb phalanges independently of the lesser digital phalanges. Its independence from the flexor digitorum profundus muscle, from which it is embryologically derived (Cho et al. 2012), is unique to humans amongst the Hominidae (e.g., Straus 1942). FPL facilitates precision grip and tool manufacture (e.g., Napier 1962). However, because its insertion upon the distal pollical phalanx is difficult to assess on bony anatomy and these are rare in the fossil record, the evolutionary origin of FPL remains unclear. Interestingly, Gantzer’s muscle, an accessory head of FPL (ahFPL), is a common anatomical variant in humans. We explore whether the pattern and distribution of Gantzer’s muscle in humans yield insight into FPL’s evolution. Novel frequency data from LMU-DCOM’s cadaver laboratory, 56% of European-Americans (n=72), contrast with observations from other populations; for example, ahFPL was found in 67% of Korean (Oh et al. 2000), 62% of Thai (Mahakkanukrauh et al. 2004), 46% of Indian (Pai et al. 2008), and 45% of English (Abrahams et al. 1997) antebrachia. We review the clinical literature to assess potential relevance of ahFPL to adaptive evolution. No correlation of ahFPL with accessory flexor hallucis longus (ahFHL) was uncovered, suggesting genetic and/or developmental decoupling of FPL and FHL, perhaps as a result of the divergent roles of the human pollux and hallux. Finally, we outline directions for future research; most notably, the need to collect additional data on the pattern and prevalence of soft tissue variants.