Abstract
Unique anatomical features of the human hand facilitate our ability to proficiently and forcefully perform precision grips and in-hand manipulation of objects. Extensive research has been conducted into the role of digits one to three during these manual behaviours, and the origin of the highly derived first digit anatomy that facilitates these capabilities. Stone tool production has long been thought a key influence in this regard. Despite previous research stressing the unique derived morphology of the human fifth digit little work has investigated why humans alone display these features. Here we examine the recruitment frequency, loading magnitude, and loading distribution of all digits on the non-dominant hand of skilled flintknappers during four technologically distinct types of Lower Palaeolithic stone tool production. Our data reveal the fifth digit to be heavily and frequently recruited during all studied behaviours. It occasionally incurred pressures, and was used in frequencies, greater or equal to those of the thumb, and frequently the same or greater than those of the index finger. The fifth digit therefore appears key to >2 million years of stone tool production activities, a behaviour that likely contributed to the derived anatomy observed in the modern human fifth ray.
Highlights
Unique anatomical features of the human hand facilitate our ability to proficiently and forcefully perform precision grips and in-hand manipulation of objects
The fifth digit appears key to >2 million years of stone tool production activities, a behaviour that likely contributed to the derived anatomy observed in the modern human fifth ray
Forceful precision grips and proficient in-hand manipulation underpin many of the behaviours considered unique to modern humans and our hominin ancestors[1,2,3,4]
Summary
Unique anatomical features of the human hand facilitate our ability to proficiently and forcefully perform precision grips and in-hand manipulation of objects. Due to the human thumb’s robust skeletal morphology[12,13], unique muscular anatomy[14] and representation as a “defining” human feature[15], it has been the focus of research into the evolution of hominin manual capabilities[16,17,18,19,20,21] This has often been to the exclusion of the other four digits, in particular the fourth and fifth, whose derived traits are not as well explored and explained. The human fifth digit is relatively independent, unlike the fourth which during flexion and extension can unintentionally recruit the third and fifth[26,27] This independence is in part due to the saddle- shaped articulation for the fifth metacarpal base on the hamate, which allows the fifth metacarpal to flex and rotate at the same time, and better oppose the thumb and the rest of the hand[4,28]. Combined with ulnar expansion of the palmar aspect of the fifth metacarpal head the unique morphology of humans at this joint, allows for effective opposition of the fifth digit to the thumb[28]
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