Opposable thumbs marked a turning point in primate, especially human, evolutionary history for their disruptive improvement of manipulation abilities. This project uses the lens of “socio-cultural evo-devo” to shed light on the opposable thumb's evolutionary and developmental role beyond prehensility, focusing on its understudied contribution to human decision-making. The phrase “rule of thumb” is used in decision making to indicate any fast-and-frugal decision rule (heuristic) that individuals use beneficially in specific environments. The thumb itself can be used, out of metaphor, to make accurate spatial estimations in the wild. Here, I plan to study how opposable thumbs have morphed into rules of thumb as a process of resource reuse (exaptation). The argument is that thumbs have represented “plastic” resources in the evolution of judgment and decision-making abilities. A parallel will be drawn with the cognitive science concept of “affordance”: the thumb’s morphology specifies a set of potentialities/affordances for organisms engaging in decision-making tasks. At the same time, using the thumb in these tasks may affect its anatomy, behavior and use, and hence modify the affordance space. Such reciprocal form of causation will give me the chance to explore the connection between affordances and evolvability