Around 4 Ga, the acidic seawater contained high amounts of HS and thus created a reducing environment with a potential of around −0.2 V. So any element that had a large negative value with respect to the reduction potential of the environment was available in its free ionic form and can subsequently be incorporated into cells, i.e. Mg has a reduction potential of −2.372 V, and was available in its ionic form at that time.