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. 2015 Nov 23;112(49):E6762–E6769. doi: 10.1073/pnas.1520492112

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Fig. 2. — Simulated cultural evolution in constant (A–D) and alternating (E–H) environments. The black line indicates the total number of tools, and a stacked histogram shows the types of tools at each time step. (A–D) Main-axis (red), toolkit (orange), and combination (yellow) tools; (E–H) tools that are useful only in environment 1 (blue), only in environment 2 (green), or in both environments (yellow). Each graph depicts a single simulation run. (A) The lucky leap innovation process is the only source of new tools. (Left Inset) The first 500 time steps, during which no loss occurred. (Right Inset) The dynamics of tool invention and loss when the number of tools is near steady state. (B) Here, main-axis tools have an associated toolkit (mean size 10.5 tools); P_toolkit = 0.5 per tool. (Left Inset) Early tool accumulation. (Right Inset) The dynamics near the steady state. In A and B, P_SpontLoss = 0.001 per tool. (C) Here, main-axis tools can combine both with one another and with existing combinations. Toolkit tools initially outnumber combinations (Inset), but combinations quickly dominate the tool repertoire. P_SpontLoss = 0.01, P_toolkit = 0.5, P_combine = 0.5, P_combUseful = 0.2. (D) Main-axis tools can combine both with one another and with existing combinations; P_SpontLoss = 0, P_toolkit = 1, P_combine = 1. There is exponential increase in the tool repertoire size, dominated by combination tools. In A–D, N = 100, P_lucky = 0.002 per individual. (E–H) Alternating environments. The bar at the top of each graph indicates the environmental state (green/blue) at each time step. Population size is 50 in E and G and is 500 in F and H; the probability of loss of tools that are not useful in the current environment is low (0.09) in E and F and high (0.9) in G and H. Additional parameters are P_lucky = 0.001, P_SpontLoss = 0, P_toolkit = 0.05, P_combine = 0.05, P_switch = 0.001, P_envUseful = 0.1. The y axes differ in all panels, and comparisons between panels should reflect relative changes and not the absolute number of tools. Full parameter lists for Figs. 2 and 3 can be found in SI Appendix, Table S1.