Top: evolution of the molecular gas mass density obtained from ALMACAL-22 (purple) in comparison with predictions from simulations. We include the results from Illustris TNG (Popping et al. 2019b), EAGLE (Lagos et al. 2015), SHARK-2 (Lagos et al. 2018), UniverseMachine (Guo et al. 2023), and COLDSim (Maio et al. 2022). Bottom: we compiled several measurements from previous CO surveys, as is shown by the grey points, including ALMACAL-22 (see text). We present a new fit of the functional form used in Walter et al. 2020, including the ALMACAL-22 estimate, shown by the purple line, with the 1σ uncertainty shown by the shaded region. For reference, we add the estimates of ρ_H2 at the highest redshift measured up to date by the REBELS survey as the dark red point (Aravena et al. 2024), which is in excellent agreement with the extrapolation of our fit.