Mark W. Hamrick*, Steven E. Churchill², Daniel Schmitt² & William L. Hylander² *Department of Anthropology, Kent State University, Kent, Ohio 44242, U.S.A. E-mail: mhamrick@kent.edu ² Department of Biological Anthropology and Anatomy, Duke University Medical Center, Durham, North Carolina 22710, U.S.A. Received 11 March 1997 Revision received 26 June 1997 and accepted 6 July 1997 Keywords : thumb morphology, grips, stone tools, A ustralopithecus , Paranthropus . EMG of the human flexor pollicis longus muscle: implications for the evolution of hominid tool use Modern humans possess a distinct and well-developed flexor pollicis longus muscle, an extrinsic thumb flexor which is ‘‘either rudimen- tary or absent’’ in great apes ( Straus, 1942 , p. 228). Previous workers (e.g., Napier, 1962 ; Susman, 1988 ) have related the origin of a well-developed flexor pollicis longus muscle to the acquisition of precision grasping and stone tool making capabilities in early homi- nids. The proposed functional association between flexor pollicis longus activity, precision grasping, and stone tool manufacture has, however, never been tested experimentally. This study uses electro- myographic techniques (EMG) to investigate the role of flexor pollicis longus during a variety of tool making, tool using, and manipulatory behaviors in order to determine the functional and evolutionary significance of the human flexor pollicis longus muscle. Our results indicate that flexor pollicis longus is recruited during forceful tool using and stone tool making behaviors, regardless of the power or precision grip used to hold the tool. In particular, both stone tool use and stone tool making employing three- and four-jaw chuck precision grips elicit consistently high levels of FPL activity. Flexor pollicis longus activity increases most when resistance is increased to the thumb’s volar pad during these hammering, cutting, and knap- ping behaviors. In contrast, we observed relatively low levels of flexor pollicis longus activity during the fine manipulation of food items, the making of slender wooden probes, and the use of these probes as tools. The paleontological, archaeological, and experimental data suggest that a well-developed flexor pollicis longus muscle functioned initially in the hominid lineage to stabilize the terminal pollical phalanx against loads applied to the thumb’s apical pad during the frequent and forceful use of unmodified stones as tools.  1998 Academic Press Limited Journal of Human Evolution (1998) 34, 123–136 Introduction Napier (1956 , 1962 a, 1980 ) concluded from his comparative studies of primate hand morphology and prehensile behavior that the humanlike capacity for making and using stone tools is facilitated by our ability to produce a variety of ‘‘power’’ and ‘‘preci- sion’’ grips. These power and precision grips are in turn facilitated by several derived musculoskeletal features of the human thumb. These derived features include, among others, a well-developed and fully independent m. flexor pollicis longus (FPL; Napier, 1962 a; Tuttle, 1992 ). The extrinsic thumb flexor of most non-human primates is, in contrast to that of Homo sapiens , either absent or part of the m. flexor digi- torum profundus mass. Straus (1942) , for example, found that among great apes the long flexor tendon to the thumb was ‘‘either rudimentary or absent’’ in 96% of orang-utans, 72% of gorillas, and 52% of chimpanzees. Furthermore, when present in these taxa, this tendon was never observed A ddress all correspondence to: Dr Mark W. Hamrick, Department of Anthropology, Box 5190, Kent State University, Kent, OH 44242 U.S.A. E-mail: mhamrick@kent.edu 0047–2484/98/020123+14 $25.00/0/hu970177  1998 Academic Press Limited