Plot of the rms wavenumber of the H I distribution as a function of redshift. The right axis shows the corresponding genus amplitudes ﹩G_{s,\mathrm{peak}\,}﹩ for our simulation volume (see eq. [5]). We truncate the plot around the epoch when the topology departs strongly from a Gaussian distribution, at ﹩z\sim 8﹩ (see discussion in text).