Temporal variability in flaking stone has been used as one of the currencies for hominin behavioural and biological evolution. This variability is usually traced through changes in artefact forms and techniques of production, resulting overall in unilineal and normative models of hominin adaptation. Here, we focus on the fundamental purpose of flaking stone—the production of a sharp working edge—and model this behaviour over evolutionary time to reassess the evolutionary efficiency of stone tool technology. Using more than 19,000 flakes from 81 assemblages spanning two million years, we show that greater production of sharp edges was followed by increased variability in this behaviour. We propose that a diachronic increase in this variability was related to a higher intensity of interrelations between different behaviours involving the use and management of stone resources that gave fitness advantages in particular environmental contexts. The long-term trends identified in this study inform us that the evolutionary efficiency of stone tool technology was not inherently in advanced tool forms and production techniques, but emerged within the contingencies of hominin interaction with local environments.