A key feature of Fig. is the reddish region along the main diagonal, which indicates continuity in the evolution of the periodic system, as the most similar periodic system is always one of an adjacent year (SI Fig. ). Two further aspects of this plot are central for our discussion: its rows and columns indicate, respectively, evolutionary preservation and anticipation capacity of the periodic system. The former quantifies the degree of preservation of the periodic system of the past into the future, while the latter how much of the periodic system of future years is contained in systems of the past. The extension of red regions towards the right indicates high evolutionary preservation contributing to the convergence of the periodic system. Remarkably, Figure : Evolution and convergence of the periodic system. Each value quantifies the similarity between the periodic system of year i (column) regarding that of year j (row) (z(i → j), Methods 6.5). Lower triangle indicates how much of the system of year j (past) remains in the system of year i (future) -evolutionary preservation. Upper triangle quantifies how much of the periodic system of year i (future) is in the system of year j (past) -anticipation capacity. Divisions on the right indicate periods on the evolution of the periodic system, which are determined by patterns in the evolutionary preservation of the periodic systems they house. The unusually high values of anticipation of the periodic systems of 1800 are caused by the low number of elements and compounds, which led to very few (13) templates (Fig. ) to draw similarities from, thus rendering statistics unreliable. This improved in subsequent years. Further details on high similarity values among periodic systems in SI Fig. . this preservation does not vary monotonically, indicating the presence of some periods in the evolution of the periodic system, which we discuss below. In contrast, the short red regions extending upwards from the diagonal indicate that despite its convergence, the periodic system has undergone several updates across history making the systems of the future largely differ from those of the past. As the variation of the periodic system is mainly governed by similarity relations among chemical elements, rather than by their ordering , the interplay of small anticipation capacities with high evolutionary preservations indicate that element similarities have been actively updated over the history of chemistry but that, nevertheless, a core of stable similarities has been often found throughout history, leading ultimately to the current periodic system.