Median stacked spectra in redshift bins as in Figure 6. At z < 6 (top panel) we show the median and 68% range of our template spectra in a fully ionized IGM, convolved to the prism resolution (grey line and shaded region). At z ≥ 6 we show the predicted median and 68% range of spectra assuming four IGM/CGM evolution models (grey line and shaded region, described in Appendix C): (1) an increasingly neutral IGM following the Mason et al. (2019) reionisation history; (2) DLA column densities increase with increase cosmic density, N_H I ∝ (1+z)²; (3) the same density evolution and at z > 6 a minimum log₁₀N_H I > 20.3 due to the lower UV background; (4) assuming the same density evolution and that f_cov = 1 at z > 6. The spectra are most consistent with the majority of the evolution being driven by the IGM evolution.