Fig. 1.

Schematic representation of the state of tool knowledge in a population. By a stochastic process of lucky leaps, novel tools are added to the tool repertoire, indicated along the main horizontal axis of the figure (red). A main-axis tool can facilitate the invention of a number of other tools: tools that are part of a toolkit made useful by the new main-axis tool or tools that are invented by functional analogy to the main-axis tool. These two processes are grouped under the “toolkit” label (orange). Alternatively, new main-axis tools can generate new tools by combining with existing tools to make novel and useful combinations (yellow). Useful tools need not be accumulated in a certain order: here, the population tested tool “5d” before “5c.”