Chemical evolution of the abundances of CO, CN, CS and HNC over 10⁷ yr predicted by our best-fit model with the parameters (n(H), A_V, T_gas) = (300 cm⁻³, 3 mag, 40 K). The colored horizontal bands correspond to the observed abundances towards the cloud with v_LSR =−17 km s⁻¹, including the inferred uncertainties. The vertical dashed line indicates the time of best agreement (t = 6.2 × 10⁶ yr) between observations and model results.