The manual pressures of stone tool behaviors and their implications for the evolution of the human hand

Abstract

It is widely agreed that biomechanical stresses imposed by stone tool behaviors influenced the evolution of the human hand. Though archaeological evidence suggests that early hominins participated in a variety of tool behaviors, it is unlikely that all behaviors equally influenced modern human hand anatomy. It is more probable that a behavior's likelihood of exerting a selective pressure was a weighted function of the magnitude of stresses associated with that behavior, the benefits received from it, and the amount of time spent performing it. Based on this premise, we focused on the first part of that equation and evaluated magnitudes of stresses associated with stone tool behaviors thought to have been commonly practiced by early hominins, to determine which placed the greatest loads on the digits. Manual pressure data were gathered from 39 human subjects using a Novel Pliance manual pressure system while they participated in multiple Plio-Pleistocene tool behaviors: nut-cracking, marrow acquisition with a hammerstone, flake production with a hammerstone, and handaxe and flake use. Manual pressure distributions varied significantly according to behavior, though there was a tendency for regions of the hand subject to the lowest pressures (e.g., proximal phalanges) to be affected less by behavior type. Hammerstone use during marrow acquisition and flake production consistently placed the greatest loads on the digits collectively, on each digit and on each phalanx. Our results suggest that, based solely on the magnitudes of stresses, hammerstone use during marrow acquisition and flake production are the most likely of the assessed behaviors to have influenced the anatomical and functional evolution of the human hand.

Biometric variables predict stone tool functional performance more effectively than tool‐form attributes: a case study in handaxe loading capabilities

“…() and Williams‐Hatala et al . () further emphasizes the high muscular recruitment and loading required by the thumb and index finger during effective flake and handaxe use. Other work has examined fossil evidence with the aim of identifying the influence that stone tool production and use likely had on hominin manual anatomy (Kivell ; Marzke , ).…”

“…; Williams‐Hatala et al . ). Previous investigations into the role of loading on stone tool performance have principally been focused on projectile velocities and the consequences of impact (Hutchings ; Milks et al .…”

“…; Williams‐Hatala et al . ), and the use of force‐sensitive cells beneath portions of cut material (Key and Lycett , ). To date, however, the selective pressures acting on tool‐form and biometric/biomechanical attributes have been investigated in isolation and their relative strength of influence have never been compared directly.…”

Extraordinary grip strength and specialized myology in the hyper‐derived hand of Perodicticus potto?

“…Williams‐Hatala et al. ), as opposed to relying upon the presence of an oxygen‐regulating vasculature structure alone.…”

“…Leischner et al 2018), or the collection of manual pressure distributions during gripping behaviors (e.g. Williams-Hatala et al 2018), as opposed to relying upon the presence of an oxygen-regulating vasculature structure alone.…”